THE STRUCTURE OF THE EASTERN PART OF THE LAKE DISTRICT. By J. F. N. GREEN, B.A., F.G.S. (Read March 26th, 1915.)
CONTENTS. PAGE INTRODUCTION 195 HISTORICAL REVIEW 196 THE TERM "ANDESITE" 201 THE VOLCANIC SUCCESSION 203 THE MOTTLED TUFFS 205 THE MIDDLE TUFFS 207 THE HAI INTRODUCTION. HE district dealt with in this paper is that lying between T Ullswater on the north-west and a line. running from Stile End to near Wasdale Head on the south-east." It is bounded to the north-east by the Lowther Valley, and on the south-west by an arbitrary line, fixed by convenience of mapping, which passes somewhat east of Angle Tarn and Hayes Water. The deep valley of Mardale and Haweswater divides it into nearly equal portions. The area is some sixty-five square miles. It lies entirely in Westmorland and covers nearly the whole of six-inch maps 13 and 20, with parts of 7, 12, 14, 21, and 27. Within these limits lies some of the noblest and most beautiful scenery of the Lake District. The splendid crags overhanging Ullswater, the maze of wild fells and dales round the head of Haweswater, the delicate form and colour of Blea Water, are known to thousands of tourists. On the other hand, the dreary moors that stretch between the Sprint and Shap are little visited, • The district is shown on the Geological Map. Plate 18, and some horizontal sections are given on Plate '9. The Scale of Colonrs on Plate 18 is to be used for both Plates IS· and '9. J. F. N. GREEN ON and the geologist may wander for days over them or the high ground north of Mardale without meeting even a shepherd. A considerable section of the district lies above 2,000 ft. above the sea, and parts of it about the heads of Mardale and Rampsgill are over 2,500 ft., culminating in High Street (2,663 ft.), Racecourse Hill (2,718 ft.), High Raise (2,634 ft.), and Kidsty Pike (2,560 ft.). Near the southern end of Ullswater, the grand mass of Place 'Fell (2,154 ft.) overlooks Patterdale j and several other less-known fells reach much the same altitude. The greater part of this mountainous region is built up of the vast group of slate and Porphyry ..• out of comparison the most remarkable physical group in the British Isles,* commonly known as the Borrowdale Volcanic Series j and it is proposed to consider only these and the adjacent Skiddaw Slates and Coniston Limestone Series. Later rocks have not been touched. HISTORICAL REVIEW. The district has been studied by many geologists, and its problems are so bound up with those of the Lake District as a whole, that a historical survey must necessarily cover a wide field. Only the more important references among the voluminous literature are mentioned below. The foundations of our knowledge of the eastern Lake District were laid by Jonathan Otley,t who first determined the threefold succession of Lower Slates, Green Slates and Porphyry and Upper Slates with a band of limestone at the base. There after Sedgwick paid great attention to the Lake District. He considered the volcanic series conformable to the Skiddaw Slates, and, though not with such certainty, the Coniston Limestone conformable to the volcanics. He treats faulting as subsidiary, except near the Duddon, where, amongst other dislocations, a great fault along the Whicham Valley is described with a good deal of detail. In 1863 Harkness t published his classical account of the Skiddaw Slate Series. He takes the volcanics and the Skiddaw Slates to be conformable, and places a certain "greenish-grey, imperfectly bedded rock" as the horizon next above the latter. He correlates it with the rocks on the south-east of the Whicham Valley, and shows the fault along that valley in section. He gives a section (p. 127) from Ullswater to Wasdale Crag, ·showing the Skiddaw Slate brought up by anticlines at Rosgill and Keld. In 1866 the "Style End Grassing beds" (now "Stile End • Sedgwick, Quart. JOU"'. Geol, Soc., vol. Iv (I~48), p. 22r. +See Phil. Mag., vol, Ivi (1820), p. 257, and successive editions of his" Concise Description of the English Lakes." ~ Quart. [our«. Geot, Soc., vol, xix, p, II3. THE EASTERN PART OF THE LAKE DISTRICT. 197 beds") were discovered by Harkness and Nicholson, «0 being regarded as intercalated in the volcanic series. They divided the faults of the Lake District into two systems, an older, north east south west and a younger, nearly north-south. In 1868 appeared Nicholson's "Essay on the Geology of Cumberland and Westmoreland," which summed up the existing state of knowledge. The main conclusions were that the Skiddaw Slates were conformably surmounted by the volcanics; that the volcanics were conformably succeeded by the Coniston Limestone Series; following Harkness, that there were certain faults parallel to the Pennine faults, and also strike faults, notably that along the Whicham Valley; that the series was strongly folded, the Skiddaw Slate being brought up here and there by the anticlines. In 1869 Nicholson t came to the conclusion that the volcanics were unconformable to the Skiddaw Slates. He. dealt with the Ullswater area among others, and stated that in Aik Beck the Green Slates dip lower than the Skiddaws. This year (1869) marks the beginning of new views with regard to the Lake District. Up to this time the Volcanic Series had generally, in accordance with Sedgwick's opinion, been supposed to rest conformably on the Skiddaw Slates. In the same volume of the Geological Magazine we find (pp. 56, II6) notes by Mr. J. R. Dakyns suggesting unconformity at various points; but on page 382 Mr. W. T. Aveline first described it as faulted. The next year Nicholson published t the results of a careful examination of the base of the volcanics between Ullswater and Keswick, and throughout the district now under consideration. He arrived at the conclusion that it was probably faulted at all points. His sections do not agree with those of Harkness. His correlations, which will be referred to again in the sequel, are of considerable importance. The Survey Memoir on Sheet 98 N.E. (now 39) appeared in 1872. It contained very little relating to the volcanic rocks. All the porphyries were held to be tuffs. In the same year Aveline, who was engaged in surveying the more southern part of the Lake District, announced § that the only un faulted junction between the Skiddaw Slates and the volcanics was that near Bootie; and that the Coniston Limestone was unconformable. Later, however, II he stated in a joint letter with J. c. Ward that the Green Slates and Porphyries were clearly interbedded with the Skiddaw Slate in Swindale. At this time Ward was perfecting the theory that most of the Lake District porphyries and amygdaloids were highly metamor phosed tuffs, and the granitoid masses merely a further phase of • Quart. Jour«; Geol, Soc., vol. xxll, p. 480. t Geol, Mag.• pp. 105, 167. t Quart. Journ. Geol, Soc., vel, xxv! (1870), p. 599. § G.ol. Mag. (,872), p. 441. II Geol, Mag. \,875), p. 95. ]. F. N. GREEN ON their alteration. On this point should be consulted his papers in the Quarterly Journal of the Geolugical Society.* The theory seems to have been accepted by the Geological Survey and is embodied in the maps, in the Memoir 011 98 N.E. and in the "Geology of the Northern Part of the Lake District," Survey Memoir on 101 S.E. (1876), a valuable work, which, however, barely touches the area under consideration. The Survey r-inch maps, 98 N.E. and 102 S.W., had been published in 187 I and 1877 respectively. The Skiddaw Slate junction was shown in them as almost entirely faulted, and the volcanics cut up by an intricate network of faults, showing little relation to the systems defined by Harkness. The Haweswater and similar strike-faults were discarded, as was the Whicham Valley strike-fault from Sheet 98 S.W. The existence of true basalts in the Lake District seems to .have first been detected by Rutley.t In 1877 Ward noted t the presence of rocks of basaltic nature at Eycott Hill, and afterwards found § many of the rocks in the main volcanic area to be of similar character. This writer seems always to have regarded the Coniston Limestone Series as unconformable. II From this time onwards references to the petrography of the eastern Lake District became frequent. In 1886 Mr. J. G. Goodchild' endeavoured to reconcile conflicting views as to the relations between the Coniston Lime stone and the underlying volcanics by the hypothesis of local contemporaneous subsidence in the latter. In 1888 a revised edition of Survey Memoir, 98 N.E., afforded valuable additional material from the hands of Dr. A. Strahan and others. In 1891 Sir A. Geikie, in his Anniversary Address to the Geological Society, gave a general account of the vulcanicity of the district, which, in opposition to previous observers, he regarded as probably submarine; and determined the rocks as andesites and basalt. In the same year Mr. A. Harker and Dr. J. E. Marr presented their account *,;, of " The Shap Granite and the Associated Igneous and Metamorphic Rocks," which, inter alia, defined and partly mapped the horizons known afterwards as the "Shap Andesites" and "Shap Rhyolites." Further information is given by Mr. Harker in "The Ancient Lavas of the English Lake District," tt and by Mr. W. M. Hutchings.fj • Vol. xxxi (1875), pp. 388 and 568; vol, xxxii (1876), p. I. t Quart. Jour... Geot. Soc., vol. xxxi (1875), p. 420. :t Trans, Roy. Micr. Soc., vel, xvii, p. 239. § Geol, Mag., (1879)p. 52. II E.g. see .• Physical History or the English Lake District," Ge»l, Mag. (r879), pp. 49, u6. 'If Proc, Geol, Assoc., vol, lx, p. 476. ** Quart. Jour». Geoi, Soc., vol. xlvli, p. 266. tt Naturalist, May (1891), p. 145. n Geol: Mag. (r891), p, 536, see also G. A. J. Cole, Geoi, Mag. (1892), p. 43. l'HE EASTERN PART OF THE LAKE DISTRICT. 199 In the next year Dr. Marr published "The Coniston Lime stone Series" in the Geological il1agazine (1892, p. 97), describ ing all the lower junctions of the observed Lake District outcrops as faulted. This appears to be the first suggestion of general faulting at this horizon. In this year also Nordenskjold, who had recently visited the Lake District, compared some of the Westmorland rocks to the "petrosilex" of Smaland, and sug gested a pre-Cambrian age '* In 1893 a further papert by Mr. Harker and Dr. Marr classed the volcanic rocks on the northern side of the Shap granite as a basic group distinct from the andesites. In 1897 the Survey Memoir on the country between Appleby, Ullswater and Haweswater (102 S.W., now 30), which covers the greater part of the area under discussion, was published. The conclusion as to the lower limit of the volcanics is stated as follows (page 16) : ". • . as elsewhere round the margin of the Lake Country the volcanic rocks appear nearly everywhere to be faulted against the older Skiddaw Slates • . . the faults, however, are of very high hade, being what are now called" thrusts.' " The volcanic rocks were described in considerable detail, but the petrological determinations by Professor W. W. Watts showed that some of the rocks mapped as altered ashes had in reality the character of lavas. In 1899 Mr. Harker gave in The Naturalist t a number of rock-determinations. In 1900 Dr. J. E. Marr gave the first comprehensive account of the views of himself and Mr. A. Harker on the Lake District sequence and structure in a lecture before the Geologists' Association.§ He held·that the Lower Palreozoic rocks had been pushed in a general northerly direction by a force acting from the south, and that the rocks moved forward at unequal! rates, the Green Slates and Porphyries lagging behind the Skiddaw Slates, and the Upper Slates lagging behind in turn, so that faults with fissures approaching the horizontal were formed between. These fissures were termed" lag-faults." The volcanic rocks were divided as follows, in descending order: Shap Rhyolites. Shap Andesites. Scawfell banded ashes and breccias = Kentmere-Coniston Slate Band. Ullswater basic lava group = Eycott group. Falcon Crag and Bleaberry Fell Andesites. A group of garnetiferous and "streaky" rocks was also described, occurring chiefly just below the Scawfell banded ashes. • Bull. Geol, Inst, Upsala, vol. I, p, 9r. t Quart. Journ, Geot. Soc., vol. xllx, p. 359. t "Chemical Notes on Lake District Rocks," pp. 53 and J49. § "Notes on the Geology of the Lake District," Proc, Geol, Assoc., vol. xvl, p, 449. PROC. GEOL. Assoc., VOL. XXVI, PART 3, 1915.J 14 200 ]. F. N. GREEN ON It was considered probable that a passage originally existed from the Shap Rhyolites into the Coniston Limestone Series. A map on the scale of four miles to an inch" indicates a fairly continuous succession of these divisions from north-west to south east. With the exception of the lowest, all divisions are indicated in the area now being dealt with, and it is considered possible that the Falcon Crag group may be found near Shap. This view of the volcanic succession is confirmed by the descriptions and collection of determinations published two years later by Mr. Harker in the Proceedings of the Yorkshire Geological Society. t In 1904 a paper by the late E. E. Walker was published by the Geological Society entitled" Notes on the Garnet-bearing and Associated Rocks of the Borrowdale Volcanic Series.t'j Unfortunately the work had not been completed, and' no clear conclusions were arrived at on the origin of the garnets and " streakiness" ; but the evidence, especially in the Haweswater district, which was described with some detail, was held to show that the group was contemporaneous, lying between the Scawfell ashes and Ullswater lavas. This conclusion was accepted by Dr. Marr and embodied in his Anniversary Address to the Geological Society in 1906,§ in which he gave these rocks the name of" Sty Head Group." Thus during the period which had elapsed since 1868 a totally new set of theories of the structure of the Lake District had been built up. The volcanic rocks were separated above and below from the fossiliferous sediments by "lag-planes." A stupendous thrust underlay the whole, bringing Skiddaw Slate over the Coniston Limestone Series in the Caldbeck Fells. The succession was, on the whole, continuous from north-west to south-east. The Whicham Valley and other strike-faults had disappeared from the maps. These hypotheses had not, however, obtained complete acceptance. Sir A. Geikie, in his " Ancient Volcanoes of Great Britain" (1897), asserted (footnote on p. 229, vol. i) that a large number of the faults inserted in the Geological Survey map to separate the Skiddaw Slates from the Borrowdale volcanic series cannot be proved, and probably do not exist; and in a discussion on a paper by Mr. PostlethwaiteII in 1903, stating the orthodox view, Mr. D. Burns declared that he could not see any evidence for a fault in Borrowdale, while Professor J. D. Kendall did not accept the lag-faults, and held that in the Ireleth district the older beds had been subjected to faulting and denudation before the deposition of the Coniston Limestone. * Pro". Geol, Assoc., vol, xvl, PI. 13. t Vol. xiv, 1902, p. 487; vol. xv, 1903, p, 59 I Quayt.Joum, Geol. Soc., vol. Ix, p. 70. § Ibid., vol. Ixii. II Trans, Inst, Mining Eng., vol. xxv, P' 325. THE EASTERN PART OF THE LAKE DISTRICT. 201 As has already been seen, Dr. Marr adhered to his views in his Anniversary Address in 1906; and he repeated them briefly in 1910 in "Geology in the Field."* In 1912, however, some indication of a return to the con clusions of Sedgwick and Harkness appeared in two papers read at the same meeting of the Geological Society, both relating to the neighbourhood of the Duddon Estuary. The first, by Mr. Bernard Smith,'] treated chiefly of glaciation, but incidentally completely restored the Whicham Valley fault. The second, by the writer, established a succession among the volcanic tuffs near the estuary, and purported to show that the lower palreozoic rocks had been thrown into complex folds, and now lay in folded sheets with a comparatively low general dip (" sheet-dip "), though the bedding dips were high. By means of the succession it was demonstrated in the region there dealt with that the andesites and basalts had little stratigraphical significance; that the volcanics were conformable to the Skiddaw Slates, but were unconformably overlain by the Coniston Limestone Series; and that the faults supposed to mark the top and bottom of the volcanic series could not be proved. The paper was declined by the Geological Society, and was afterwards published at the writer's expense. t The present paper is the outcome of an attempt to apply the results obtained from a study of the Duddon Estuary area to a wider and better-known portion of the Lake District. All geologists must be grateful for the freedom with which the landowners and farmers of the Lake District permit investigation. I must also express my thanks to Mr. A. Harker for assistance in connection with the collections preserved in the Sedgwick Museum at Cambridge; to Dr. Marr for lending me the late Mr. E. E. Walker's M.S. maps; to Mr. G. Barrow for valuable advice; and, above all, to the Geological Survey, who placed freely at my disposal their collections of specimens and slides and the manuscript six-inch maps of the district, thus saving me much time and trouble. THE TERM "ANDESITE." In the first place it is desirable to make clear what rocks are referred to in the sequel as "andesites." Numerous rocks from the eastern Lake District have been described as augite-andesites, hypersthene-augite-andesites, or hypersthene-basalts. Frequently it is not known whether they are lavas or intrusions, but in the case of the more homogeneous rocks, especially when amygdaloidal or markedly porphyritic, .... "The Lake District and N eigbbourhood. Lower Paleeozoic Times," p. 624. t Qua.t. .Tourn, Geol, Soc., vol. lxviii, P.402. t The Order Palreozoic Succession of the Duddon Estuary. Dulau and Co., 1913. 202 j, F. N. GREEN ON confusion with tuffs is not likely to arise, except when bedding is simulated hy a fluidal structure. But intercalated among them are many bands of coarse breccia. The fragments in these breccias are entirely composed of materials similar to those of the neighbouring andesites, and can often be seen to fit into each other, so that brecciation has certainly, as pointed out by Sedgwick, taken place in situ __ not infrequently, indeed, they obviously pass into rock unbroken except for a sparse network of infilled cracks, and thence into material without a trace of brecciation. They show no trace of bedding, and are only cleaved in those cases where the neighbouring solid andesites are cleaved also. Sometimes the matrix is andesite glass. When lying on sedimentary rock, the sediment usually continues up wards between the fragments. These breccias are clearly a product of flow-brecciation or crust-shattering in andesitic lavas, or, perhaps, rarely, sills. They are wholly distinct from the bedded tuffs, and, except in unusually fine-grained examples, can easily be distinguished from them in the field, unless highly cleaved, in which state the un brecciated rocks become equally indistinguishable. Plate 17.A is a photograph of a well-developed flow-breccia, composed of basic augite-andesite with scattered garnets, show ing strong fluidal structure and cut by the same material. The section, which has been described as a crush-conglomerate," is to be found 100 yards north-east of Woof Crag, near Hareshaw, Swindale Common. J. C. Ward usually mapped such rocks with the tuffs and, by a curious inversion, regarded the unbrecciated lava, into which they naturally graduate, as a further stage of metamorphism.t His colleagues appear to have followed his example. On the maps and in the Survey publications these flow-breccias are usually described as "rough ash and breccia," or "rough coarse ash." Professor 'Watts has referred some specimens from the eastern Lake District to lavas. t Such breccias must go with the normal andesites, and in what follows it should be understood that they are so included. " Tuff" on the other hand refers to sedimentary rocks only. A source of confusion is the occurrence of unbrecciated fluidal structure, weathering like bedding, in andesite. In the field it may greatly resemble bedded halleflinta, § and may, indeed, be indistinguishable without the use of the microscope. As a typical example may be noted an exposure at the point where * Quart. Jour". Geol, Soc., vol, Ix (1904), p. 100. See also Proc, Geol, Assoc., vol. xvi (1900) p, 470. t Quart. Journ. Geol. Soc., vol. xxxi (1875), pp. 4IO-4'3, and 59'-594: "Geology of the Northern Part of the Lake District," pp. 15,19,25. For instance, in his type section, d and e are the.base and top of Lava NO.1. • at the top of Falcon Crag Is a typical flow-breccia, :t See Mem. Geol, Surv.; Sheet I02 S.W., Appleby, &c., pp. 6 and It. § An example is Survey specimen E. 168, originally described as "bedded Ash," but afterwards by Professor Watts as hypersthene-augite-andesite. PROC. GEOL. Assoc., VOL. XXVI. PLATE 17. Photo by J. F.· N. Green. A.-FLO\Y-BRECCrA WITH CONTEMPORANEOUS DYKE, NEAR "'OOF CRAG. B.-BASAL CONGLOMERATE, GREAT HOWE, NEAR To face page %02. THE EASTERN PART OF THE LAKE DISTRICT. 203 the 800-ft. contour crosses Fusedale Beck, a mile south of Howtown. Fine specimens may also be found a few yards north-east of the Cairn on Arthur's Pike, and on the southern slope of Kidsty Pike. On several occasions a fluidal structure, simulating bedding, has been seen to run parallel to a bedded halleflinta, but separated from it by a band of flow-breccia. In such cases it is easy to misplace the boundary. Fortunately such difficult rocks usually occur so near the junction between lava and tuff that mistakes do not seriously affect the mapping. Further, it should be understood that when a mass of andesite is mentioned without qualification, it must be taken as including rocks sometimes described as hypersthene-basalts, or basalts without olivine. It is not intended to prejudge the question of nomenclature, but at present it is necessary to have some general term to cover this series of closely allied rocks with silica percentages ranging, so far as known, between 50 and 62. Similarly, although it is fashionable at present to give a rock occurring as a sill a different name from the same rock occurring as a lava, the effusive nature of many of the Lake District rocks cannot yet be taken as proved. THE VOLCANIC SUCCESSION. An examination of the one-inch Survey map will show at once that the outcrop of the junction between the Skiddaw Slates and the volcanics is consistent with the outcrop of an undulating surface, the general lie of which is not far from horizontal. This outcrop cannot be mapped exactly owing to the fact that the hard volcanic rocks stand up like a rampart above the soft slates, the marked feature between forming a trap for drift and scree material. This surface has been supposed successively to be a conformity, an unconformity, a series of thrusts and a lag-fault. Near the junction the Skiddaw Slate is always a soft, variably cleaved, fine-grained sediment, usually black or bluish black. Both macroscopically and microscopically it is similar to the" Blue Shale" of the Duddon Estuary. The sandy beds that are found in other parts of the Skiddaws never approach the junction; but a fossiliferous horizon" is known from Aik and Thornship Becks and Keld Gill at points a few hundred yards from the volcanics. The length of the limit of the volcanic rocks against the Skiddaw Slate (or Upper Palreozoic) from Howtown-on-Ullswater to Butterwick is seen on the Survey map to be approximately • See Miss Elles, Quart. J ourn, Geol, Soc., vol, Uv (18g8), p. 463. 2°4 J. F. N. GREEN ON six miles. On this line a few feet of tuff may be seen in the Aik Beck, where the junction with the volcanics is exceptionally well exposed, but otherwise the first rock met with on crossing the junction is invariably andesite, usually rising steeply above the flat slate country. Examining the rocks along a traverse running inwards from the Skiddaws towards the centre of the region, andesitic tuffs are always reached within a mile of the junction, but, with the exception,' of course, of the district overlapped by the Carboniferous, it is necessary along the whole of these six miles either to climb at least 250 ft., or to proceed at least a third of a mile inwards towards the centre of the region, before any rock other than andesite is met with. Still moving inwards the tuffs are crossed, and andesite is found again. There must then be, firstly, a band of andesite; and, secondly, a band of tuff, running roughly parallel to the junction of the volcanics and Skiddaw Slate, and if these bands have a stratigraphical significance, their general dip must be fairly low. But the dips in the tuffs, many of which are remarkably well bedded, are usually high, even vertical. It follows that we must be dealing with folded sheets similar to those described by the writer near the Duddon Estuary.* We have, however, obtained in this preliminary excursion two definite lines occurring, unlike the Skiddaw junction, in rocky country with plenty of exposures, which can be mapped in detail without reference to strike or dip. These are the junction of the tuffs with the first or lower andesites, and the junction with the second or upper andesites. It is soon found that close to these junctions the andesites are almost invariably flow-brecciated, and that near the upper junction the tuffs often contain much coarse felsite and variolite. These observations are of great value in mapping, and the latter indicates that the second band of andesite is not a repetition of the first. These two junctions have been laid down on the 6-inch maps, not only along the Howtown-Aik Beck-Butterwick line, but in their convolutions throughout the rest of the district as far west as a line drawn from Howtown to High Street and thence to Stile End. The great tuff-band once defined, it is easy to work up or down from it and so to determine the rest of the succession. The upper andesites are invariably overlain either by rhyolite or by the Coniston Limestone Series; the lower andesites, with equal constancy, overlie a greater or less thickness of slate-bearing tuff, which passes down into Skiddaw Slate. Place FeU and the adjacent country have also been mapped, though not in such detail as the larger area. The succession thus established will now be described in more detail. It is substantially the same as that determined in * "Older Pal. Succ., Duddon," pp. 14-16. THE EASTERN PART OF THE LAKE DISTRICT. 205 the Duddon area, * though with differences in detail; and the same nomenclature will be used, except that, as the slate-bearing tuffs, though nearly always having a mottled appearance, are rarely shaly, the miner's term "Mottled Shale" will be replaced by "Mottled Tuffs." Flows of soda-rhyolite, Top not seen, but at least RHYOLITES. often nodular at base. 500 ft. thick in places. " Variolitic " and rhyolitic HARRATH TUFFS. material, typicallyrather Variable, but usually coarse. several hundred ft. thick. Andesitic material without augite, much of it fine- Perhaps about 500 ft. as a MIDDLE TUFFS. grained. Usually well- rule: bedded. Slate-bearing tuffs with MOTTLED TUFFS. admixture of ordinary 20 to coo ft. sediment. Soft black slate (top of "BLUE SHALE." Skiddaw Slates). Thick. Masses of augite-andesite and hypersthene-basalt occur at certain horizons. Maximum thickness, perhaps 1,500 ft. THE MOTTLED TUFFS. Between Howtown and Swindale the Mottled Tuffs are thin, and for reasons already given, rarely seen. East they thicken and are well exposed. West of Haweswater Beck they do not for the most part differ from varieties found in the Duddon area, consisting of andesite and scattered slate fragments in a ground which seems to be a mixture of ordinary sediment and fine-grained volcanic mud. Near the Beck, however, quantities of fine-grained felsitic material begin to come in, and the horizon assumes a facies unlike any observed farther west, which thickens north-eastward and becomes of great importance in the Cross Fell inlier, where it is especially well shown on Flagdaw Hill, near Knock. It is, therefore, convenient to call it the Flagdaw type. This rock IS linked with the ordinary type by the scattered • "Older Pal. Succ., Duddon," pp. 4-10. 206 } . F. N . GREEN ON slate-lapilli (Up to a decimetre in diameter) and by its inter mediate appearance, which has led to its being regularly mapped with Skiddaw Slate in the Cross Fell area, but with the Borrow dales in the Lake District." No doubt owing to the mixture of ordinary sediment, this type usually resembles Skiddaw Slate in the field more nearly than normal Lake District tuffs; but rarely it may, as near Keld, pass into coarse rhyolite-slate tuff. Its folding follows that of the Skiddaw Slate rather than that of the more massive volcanics. In view of the importance of this horizon in considering the relation between the Skiddaw Slate and the overlaying volcanics, it is necessary to examine the exposures in some detail. Fourteen yards above the ford across Aik Beck, the junction of the Skiddaws and the Mottled Tuff is laid open for about eight feet on the right bank. It is sharply folded, but there is no sign of faulting or unconformity. The tuffs, which are coarse and irregular, appear to be between 20 and 30 ft. thick, and are over lain (junction not exposed) by amygdaloidal flow-breccia, passing up into fluidal andesite. In a little brook in the wood west of Hungerhill, near Bampton, about 12 ft. of Mottled Shale is seen, overlain by andesite. A few feet below, Skiddaw Slate appears, cleaved parallel to the" shale." A quarry to the south of the farm is in the same horizon and has cut a layer of ordinary Skiddaw Slate interbedded with the Mottled Tuff, which does not seem to be more than 30 ft. thick. But south of this point the outcrop widens rapidly, and north of Walmgate Head a felsitic tuff of the Flagdaw type is frequently exposed. Nothing is seen between Haweswater and Swindale, but on the southern side of the latter valley, the Flagdaw type, repeated several times by folding, is widely exposed about Tailbert. The lower junction can be seen here in a little anticlinal inlier of Skiddaw Slate west of Dog Hill. Proceeding south-east, the series is again obscured by drift up to Keld Gill, in which, as in Thornship Beck, the Mottled Tuffs are well shown. Thin interbedded bands of sediment, indistinguish able from Skiddaw Slate, occur in the latter section. To this point the series has formed a comparatively narrow strip outcropping between the Lower Andesites and the Skiddaw Slate. This main outcrop now turns northward for a mile and a half to Shap Abbey as a broad complex syncline in which the andesite is not involved. A number of smaller infolds are nipped in in the Skiddaw Slate country that stretches from Thornship Beck to Bampton. Although in these cases the full thickness of the Series cannot be determined, it is clearly thickening north-eastward (as might be expected from the importance' of the horizon in the Cross Fell inlier). About • e.e., see maps, gllart. JOlI'''' Geol, Soc., vol, xlvii, plate xvii; Proe, C.ol. Assoc., vol. xvi, plate xiii, Survey Maps 102 S. W. and S.E . THE EASTERN PART OF THE LAKE DISTRICT. 207 Keld a coarse felsite-slate tuff comes in not far from the base, and is seen arching over minor folds laid open by the Lowther. Two other interesting sections may be mentioned, Three hundred yards south of Toathrnain, near Rosgill, a small quarry has fortunately laid open the junction between the Mottled Tuff and the Skiddaw Slate, dipping sooW. The other is in Swindale Beck, 350 yards above the junction with Tailbert Gill, and exhibits the manner in which soft interbedded bands of normal sediment are squeezed into lenses between more resistant masses of Flagdaw tuff. A similar phenomenon may be seen near Tailbert. Mottled Tuffs have not been noted above the Skiddaw Slate near Sleet Fell, but further investi gation may discover them. THE MIDDLE TUFFS. As in the Duddon area, the Middle Tuffs are the most important and constant member of the sedimentary volcanic series. They are similar in general appearance, composition, bedding, and in tendency to a slaty structure, but contain more coarse material, invariably of an andesitic nature. The lower limit of the Middle Tuffs is always defined by the Lower Andesites. The ground-mass of the rocks conforms to that of the Middle Tuffs of the Duddon area, but often contains, in addition to broken felspars, numerous angular fragments of andesite-glass, not infrequently with recognisable felspar phenocrysts and laths. The laths seem always near oligoclase, and augite pseudomorphs are very rare, so that the andesite is not of a basic variety. The fragments vary much in size and quantity, giving rise to a bedded structure, best shown on weathered surfaces. This bedding is well seen about Small Water, and on the eastern slopes of Fusedale; also in the numerous slate-quarries. These slate quarries are found at all horizons in the Middle Tuffs, whereas near the Duddon Estuary they are only known towards the base, which contains coarser material. The follow ing is a list of the quarries noted in the area now under consideration :- Patterdale Quarries (numerous on the west of Place Fell); north of Place Fell summit; Low DOd, near Scalehow Beck; Brownthwaite Crag, Martindale; Lord's Seat, near Swarth Fell ; Sealhole Quarry, Cawdale; Mardale Banks; Crag's Mill, Shap; north of Small Water; west of Kentmere Pike; Wrengill Quarries; workings about Brownhowe Bottom and Selside Brow; Mosedale Quarries. The fact that the slate quarries lay on the same horizon seems 208 J. F. N. GREEN ON to have been known to Harkness, and was clearly stated by Nicholson, who. referring to the slates near Crag's Mill, wrote" : From the position of these beds there can be no doubt of their being the equivalent of the great slate-band of Honister and Borrowdale ; and they thus afford a useful guide in mapping this region. They can be traced to the south-west along the northern side of Wet Sleddale, through the higher part of Swindale, across the head of Long Sleddale (in the Gatesgarth Pass), across Kentmere (near the Reservoir), and across the head of Troutbeck ; and I have little doubt of their identity with the great band of slates which is worked between Rydal and Grasmere, and again farther to the south-west in Tilberthwaite, and in the Dunnerdale Fells. In all these localities I believe that these slates point to an horizon very close to the base of the whole series of the Green Slates and Porphyries, though not far removed from the southern limits of the great area occupied by this formation. This view, however, appears to have been discarded by later investigators, as being inconsistent with Ward's theory of an unrepeated succession. In the eastern Lake District the Middle Tuffs are on the whole less altered than near the Duddon. Sheared-up areas are rarer; and halleflintas are scarce. THE HARRATH TUFFS. Unlike the Middle Tuffs, the Harrath Tuffs present great variability in thickness and appearance. They are coarser, though fine-grained bands occur, and bedding is less developed. All the varieties noted in the Duddon area have been found in the eastern Lake District. The andesitic and variolitic fragments are quite similar. On the whole, rhyolitic material is more common, and in certain cases, for example in Riggindale, forms so large a part of the rock that it may be termed a rhyolite tuff; but the characteristic variolite is always present.t The lower boundary is often difficult to lay down with any precision, as unless the tuffs contain either distinct felsite (which is almost or quite absent from the lower beds) or well-preserved fragments of variolite at least half a centimetre broad, it is practically impossible to distinguish them in the field from the Middle Tuffs. This line has therefore not been mapped in detail, but the general distribution has been determined. Near Ullswater the Harrath Tuffs are thin with little rhyolite, and do not seem to exceed 100 ft. near the Patterdale quarries or along Swarth Fell. Indeed in certain places rocks resembling Middle Tuffs appear to continue right up to the base of the Upper Andesites, but the variolite has always been found where • Qua,t.]ourn. Geot.Soc., vel. xxvi (r870), p, 609. t In using tor convenlence the term "Variolite," for this material the writer must not be understood as asserting its identity with any rock previously described by that name. It nearly conforms to the definition given by Cole and Gregory (QUaft. [ourn, Geol, Soc., vol. xlvl (1890), p. 330), but is not variolous. Beautifully perfItlc varieties have been noted in the area under consideration, notably on Waite Howes, Swindale. THE EASTERN PART OF THE LAKE DISTRICT. 209 microscopic sections are available, so that it may be assumed that this division is always present. The horizon thickens and the proportion of rhyolite increases southward; and rock precisely like that of the type-locality is found in Heltondale, Martindale; Rampsgill, etc. In Rampsgill Beck, a little north of the entrance to the Deer Forest, a coarse variety is found, many fragments being over a decimetre long. A pumice-tuff with beautiful "bogenstruktur" occurs at Brownthwaite Crag, and may be paralleled by rocks from the neighbourhood of Arnaby, on the Duddon Estuary. The pro portion of rhyolitic material reaches its maximum about Riggindale and Upper Measand Beck, where the upper part of the Harrath Tuffs, here fully 400 ft. thick, passes into a coarse rhyolite-agglomerate, which has been squeezed and stained to a sheary purple-and-green rock in the long spindle-shaped inlier of High Kop. Towards Wet Sleddale the type rock is plentiful, and the horizon is several hundred feet thick. In the area between Mosedale and Stockdale the Harrath Tuffs are also thick, but with little rhyolite in the lower part, so that many of the beds are difficult to distinguish from Middle Tuffs. Here and in the east towards Was dale Pike many of the beds are extremely coarse, with pebbles of felsite and variolite over a decimetre in length. This texture is most marked in the magnificent section cut by the River Sprint across the nearly vertical beds below Buckbarrow, where boulders three and four decimetres long are displayed on the water-polished surfaces. The Harrath Tuffs evidently accumulated more quickly than the underlying strata, and probably their thickness was originally variable; but it seems likely that some of the variation may be due to the occurrence of flows of the Upper Andesites at an earlier date at certain points than at others, as the thin portions seem to represent the lower part only, the most rhyolitic develop ments being found where the division is exceptionally thick. The rhyolite-fragments of the Harrath Tuffs appear to be slightly more acid than those of the rhyolitic flows. Quartz hi-pyramids have been noted in a small proportion of cases, whereas only one crystal of the bleached biotite so common in the effusive rock has been met with. Ilmenite is rare. THE RHYOLITES. The rhyolites are confined to certain comparatively small areas which have the nature of infolds. Those which have been observed are (I) Silvery Crag, Ullswater ; (2) near Pikeawassa, Fusedale; (3) Wether Hill; (4) south of Cawdale, which is con nected by a neck above the head of Willdale with (5) the main 2tO l- F. N. GREEN ON infold which stretches from Kidsty Pike four miles in a north easterly direction to Pinnacle Howe j (6) the patch at Howes Beck, near Bampton, which continues the syncline of the main infold j (7) a series of patches in front of the main infold at Measand Forces, the Whelters, and Randale Beck j (8) Kit Crag, Haweswater j (9) an outcrop nearly a mile and a half long north west of and parallel to Naddle Beck j (10) about Hareshaw, overlooking Swindale j (II) between Sadghyll and Stile End (the westward extension has not been examined) j (12) on Great Yarlside. The less altered rocks resemble in all respects the rhyolites of the Duddon Estuary, but in the neighbourhood of Hawes water many of them possess a peculiar "streaky" structure, pro duced by veinlets of calcite, chlorite, quartz, epidote, sericite, pyrites, and garnet, which has been the subject of much comment." Its causation, with that of the sporadic appearances of garnet found in several horizons, will be discussed in a future com munication. For the present it will be sufficient to say that, apart from the streaks, which are clearly secondary structures, the rocks, fresh or weathered, do not differ megascopically from slightly altered felsites in the Duddon, Coniston, and Cross Fell areas j that microscopically they exhibit a rhyolitic or " felsitic " ground with phenocrysts of orthoclase (often perthitic) and a plagioclase near albite, with often the typical bleached biotite and . leucoxene of the mica-rhyolites j t that the clarifying patchy alteration of the ground, noted in the Duddon rocks t is often seen j that they pass laterally and vertically into ordinary pinkish or cream Lake District rhyolites j that most of the published analyses of " streaky" rocks, including one from Whelter Crags,§ in the Kidsty Pike infold, are obviously rhyolitic j that their stratigraphical position is identical with that of the Millom rocks j and that similar rocks are associated with series admittedly rhyolitic, as, for example, in a quarry on the south-east slope of Dufton Pike, near Appleby, where a well-marked" streaky" mica rhyolite occurs. The streakiness is not absolutely confined to the rhyolites, being found in andesites, but is commonest and best developed in them. Silvery Crag is unstreaked mica-rhyolite. The little outlier near Pikeawassa is a red flow-brecciated lava similar to those of Butt's Foot, near Millom, and Graystone House. It contains mica (and garnet), but is quite unlike any other rock in the eastern Lake District j a fact which may be connected with the unusual • See Ward, Quart .Lourn, Geol, Soc., vol, xxxi (1875), pp. 4TI'4'3; Marr, Proc, Geol, Assoc. ('900), P. 476; Walker; (!uart. Journ. Geot, Soc., vel. Ix ('004), PI'. 89-'0'. t .. Older Pal. Succ., Duddon," p. '0. Further examination has shown that in many cases the plagioclase of the Duddon rocks Is albitie, t Ibid., p. 9. § Walker, lac. cit., p, 96. THE EASTERN PART OF T H E LAKE DI STRICT. 2 I I thinness of the Upper Andesites at this point. * The limits of the Wether Hill occurren ce are quite uncertain, as it is only known from small exposures appearing in a great expanse of peat. The Cawdale infold is not well exposed. It seems to consist principally of "streaky " rhyolite, though ordinary felsites are found about The Hause. In the Kidsty Pike infold the basal bed is an interesting green porphyritic lava with a strong tendency to the develop ment of nodules. The distribution of the nodular portions, which, so far as yet actually observed, are marked on the Map, Plate] 8, has been so well described in the Survey Memoir and by Walker, that I have been able to add but few localities. There is a striking development at the top of Kidsty Pike, where the rock has been compared to a heap of petrified tennis-balls . That the nodular felsite is merely a phase of the green porphyry, is well seen at the fine exposure near Fordingdale Force, on Measand Beck, where the nodules, closely packed at one horizon, can be seen to become more and more separated until their average distance apart is a foot or more, and finally to disappear altogether. The porphyritic elements are albite and perthitic orthoclase, rare ilmenite, and biotite, which occurs in an unusual cond ition. It is brightly pleochroic, greenish-white to olive-green with strong haloes; the birefringence is low, often near that of quartz, so that chlorite is approached. And scattered sparsely in the altered mineral appear highly refractive grains which, although showing little trace of crystalline form, can be identified as anatase. On Kidsty Pike the rock is garnetiferous. A little banded tuff separates the base of the rhyolites from the upper andesites in many parts of this outcrop. Higher rocks in the Kidsty Pike infold are chiefly" streaky" rhyolite, though large patches are found without any streaks. Pyrites is common; garnets are seen occasionally, and the mica, when present, shows the same peculiarities as in the basal flow. The northern part of the Howes Beck patch consists of porphyritic-mica-felsite, the .southem of well-developed, streaky rocks. The alteration reaches its maximum in the infold at Measand Forces. The rocks are so infiltrated with various materials, especially chlorite, that their distinction in the field from the neighbouring andesite and dolerite is a matter of the greatest difficulty, and indeed time has not been available to map the boundaries with accuracy. Under the microscope, however, a felsitic ground can be seen among the patches of chlorite, quartz, etc. Unstriated porphyritic felspars are still recognisable ; and though the mica has broken down into indefinite non-pleochroic • See below, p. 214. 212 J. F. N. GREEN ON chlorite, traces of cleavage can still be detected, while the haloes and anatase grains persist. The occurrences about the Whelters and Randale Beck consist largely of streaky rhyolite with occasional garnets. The base of the latter is often nodular. The little outcrop at Kit Crag contains some normal rhyolite, but the neighbouring Naddle Beck infold has a very fine develop ment of " streaky" structure. The porphyritic rock is found in some places at the base, and possibly the specimen from a point a quarter of a mile south-east of Walla Crag, which has been described as rhyolite by Mr. Harker," is taken from that part of the succession. Streaky structure is equally well developed about Hare Shaw. The Stile End area is only part of a large mass which con tinues westward, but has not been mapped further than Wray Crag. Nodular porphyritic rhyolite comes in at the base, as ill the Kidsty Pike infold, and can be seen intertonguing with andesitic flow-breccia on Wray Crag. On Great Yarlside, the rock only occupies a small area, and is not well exposed. These soda-rhyolites would, like many of our Ordovician rocks, be put by Continental petrographers with the keratophyres. They present close resemblances to the Devonian quartz keratophyres of Westphalia,t which, like them, contain bleached biotite and ilmenite without magnetite, and show a tendency to spherulitic developments. THE ANDESITES. In the Duddon Estuary district the andesites occur irregularly at many horizons, and some are undoubtedly intrusive. On the other hand in the Eastern Lake District they are, with certain exceptions referred to hereafter, concentrated at two horizons; that is, between the Mottled Series and the Middle Tuffs (corresponding to the" lowest trap" of Nicholson); and between the Harrath Tuffs and the Rhyolites. Consequently the two bands of andesites can be treated as stratigraphical horizons, a result of enormous convenience in mapping. They may be termed the Lower and Upper Andesites respectively. Unfortu-: nately no criterion has been detected by which the rocks of the two bands, or the upper and lower parts of either band, can be discriminated. The field evidence goes to show that they are all or nearly all lavas. There are a few occurrences where an intrusive character may be suspected; but 'It has not yet certainly been proved. The * Naturalist, 1899, p. 57. t Miigge. N.]. fur Min., etc. ,893 BB. viii, pp, 558, 594. THE EASTERN PART OF THE LAKE DISTRICT. 213 base of each flow is a flow-breccia or flow-conglomerate, often dragged out while in a viscous state. (Plate J7.A) The upper surfaces present the usual cracked and shattered condition. Occasionally a lenticular patch of bedded tuff, a few feet thick, will be found separating consecutive flows, as for example in Ireland Wood, Bampton; Speaking Crag, Mardale; Crookdale Folds. A considerable band of this nature occurs in Whelter Combe and was at first supposed to be an inlier of the tuff-band j but this inter pretation seems inadmissible, unless a large thickness of andesite has been faulted out. Correlatively with the generally effusive nature of the andesites, halleflintas are not frequent, except when the base of the Upper Andesites rests on the finer grained varieties of. the Harrath Tuffs j amygdaloids are common, covering wide areas, as on The Howes, near Bampton; and the glassy base shows no great tendency to crystallisation. Indeed, no case has been met with among the numerous rocks examined showing such a highly developed lath-structure as the rocks from, for example, Duddon Bridge, Crosshouse Hill, or Greenscoe Quarry, in the Duddon Estuary District. Hypersthene-augite-andesites of basaltic affinities are decidedly more common and have been noted at many horizons. Their distribution seems to be geographical rather than stratigraphical. In the neighbourhood of Shap they compose the lower part of the Upper Andesites; about Mardale, the upper part of the Upper Andesites j about Bampton, the greater part of the Lower Andesites. I have formed the opinion that on the whole the Upper Andesites, especially their lower part, are the more basic, but it would require more detailed observation to establish such a generalisation.* The" variolite " t of the Harrath Tuffs has now been also found to occur as the fluidal edge of the basaltic rocks. The main differences are that phenocrysts are much more common in the lava, and that the direction of the fibres is more irregular. This is what might be expected on the assumption that the tuff fragments were mainly washed down from sub-aerial tuffs in which the lapilli would be the products of explosion, while the lavas, as submarine flows, would be more lithoidal. The specimens obtained have been from the bottom of the Upper Andesites in Fusedale, Thornship Beck and Atkinson's Grain. In certain cases the felspar-phenocrysts of the "variolite" show the refractive index and angle of extinction of albite. Whether this is original or secondary is not apparent. There can be no doubt of the secondary nature of the fibrous structure, •Nearly all the andesites and basalts from this district described in Survey Memoirs and in the papers of Messrs. Harker and Marr are from the Upper Andesites. An exception is a lava from Crag's Mill, Shap, mentioned hy Mr. Harker in the Naturalist (1899), p. 56. t See above, p. 208. 214 ]. F. N. GREEN ON as throughout the various fragments in each specimen of Harrath tuff, the fibres show nearly the same orientation and develop ment. This is specially well shown in a slide of a specimen from above Sadghyll, which has cut some forty fragments 'over half a millimetre long. These are pleochroic, and in all the direction of strongest colouration is identical within a few degrees. The tracing out of individual andesitic flows has not been attempted, though it would probably be quite feasible; but the occurrence of bands of certain well-marked types is constantly forced on the observer's attention. For instance, a porphyritic basalt constitutes the top of the Upper Andesites from Kidsty Pike, at least as far east as Laythwaite Crags. The greatest distance over which a particular andesite has been definitely recognised is about two miles in the case of a remarkable porphyritic amygdaloid which has been observed at intervals from near Butterwick to the neighbourhood of Haweswater Beck. This lava has a tendency to form numerous rectilinear cracks marked by ferruginous staining, which connect the vesicles and, in the best developed examples, such as those found on the Howes, near Bampton, give the rock a peculiar reticulate appearance. The Lower Andesites are thickest towards the west, and seem to be at least 700 ft. thick in Rampsgill, and may be more as the base is not seen. Along Ullswater they may be 250 to 300 ft. In the direction of Shap they are thicker,perhaps 400 or 500 ft. All estimates are, however, very uncertain. The Upper Andesites cover a larger area than any other formation in the district. About Measand Beck, Cawdale, and Selside, they seem to be at least 600 ft. thick, but the intense folding makes it difficult to Judge. The masses to the south of Mosedale, where the folding is less violent, can hardly be less, and are perhaps much more. Near Pikeawassa, Fusedale, the Upper Andesites thin down to less than 100 ft., being surmounted by a peculiar type of rhyolite which, though found in other parts of the Lake District, is quite unknown elsewhere in the district under consideration. It seems probable that this acid flow may have been a precursor contemporaneous with the effusion of andesite in other places; and this simultaneous outpouring of rhyolite and andesite may account for other rapid variations in the thickness of the Upper Andesites. About Kidsty Pike and the south-west half of Haweswater the andesites occasionally contain numerous garnets. Also in a band in Atkinson's Grain, at Tarn Crags (Long Sleddale) and else where. Taken as a whole the andesites seem to thin east or north eastward. This is what might have been expected, having regard to their subordinate development in the Cross Fell inlier. A change in their distribution is indicated in the south of the THE EASTERN PART OF THE LAKE DI STRICT. 215 district by the incoming about Wrengill of andesite between the Middle and Harrath Tuffs. As both horizons are well exposed and typically developed, and a sharply folded anticline provides good sections of both Upper and Lower Andesites, there can be no doubt that the intercalation is not deceptive. The fiows which may provisionally be termed the Wrengill andesites-die out rapidly to the north and north-east and appear to be thicken ing to the south. Their development beyond the area mapped should be of interest. THE HYPERSTHENE·DOLERITE AND OTHER INTRUSIONS.- A considerable mass of dolerite is seen on either side of Haweswater, the exposed portions covering about a square mile. The intrusion occurs as an irregular mass, over two miles in length, in the Upper Andesites. The roof is often at, or a little below, the base of the Rhyolites. The rock was originally a coarse ophitic dolerite with inter grown monoclinic and rhombic pyroxene. Much of the felspar is certainly near labradorite, but is now greatly obscured by decomposition products. The augite may be fresh, partly uralitised or quite broken down . The rhombic pyroxene is either in the form of bastite or more completely decomposed. Stout apatites occur, and a considerable quantity of ilmenite, which has been deeply corroded into sinuous shapes. Some of the specimens appear to have originally contained no oth er mineral, but others show a clear, or slightly cloudy, unstriated felspar, with a low refractive index, occurring inter stitially among the large altered felspars. This seems to be orthoclase. It may enclose numerous needles of apatite. It may be associated with quartz, and in some of the specimens at the Sedgwick Museum, as Mr. Harker has pointed out to me, in such a way as to leave no doubt that the quartz is original. But in other specimens quartz crammed with inclusions is associated with a green isotropic matter, and may be secondary. The dolerite is thus of variable character, and resembles the quartz-hypersthene-dolerites intruded into Ordovi cian rocks in Pem brokeshire and elsewhere. I t is not cleaved, but this is not a certain criterion of age in the Lake District. No exposure showing its relation to cleaved rocks has been noted. On the other hand the dolerite maps out like an intrusion keeping near a particular horizon, and app ears to be involved in the folding, as its upper surface shows in many places a rude coincidence with the lower limit of the rhyolite. It is possible, however, that this fact may be explainable by the similarity in chemical • On this dolerite see Teall's .. British Petrography," p. 225; Surv, Mem. on Sheet 102 S.W., pp. 23. 24; Walker, Quart . [ourn, Geol, Soc•• voI.lx (1904), p. 99. PROC. GEOL. Assoc., VOL. XXVI, PART 3, 1915.) IS 216 J. F. N. GREEN ON composition of the dolerite to the Upper Andesites, most of which near Haweswater are quite basic, coupled with great difference in composition of the dolerite and rhyolite. This may have determined the progress of the intrusion upwards. It is, though with some hesitation, shown as a boss in the section (Plate 19, Section I). There are certain minor intrusions, including various micaceous quartz-porphyries about Mardale, lamprophyres and microgranites connected with the Shap Granite, and the Fairy Crag quartz-felsite. to They have not been specially examined, but may be all post-folding. A badly exposed mass of rotten dolerite (?) in the Skiddaw Slate of Tailbert may be a little boss. THE CONISTON LIMESTONE SERIES. The Coniston Limestone Series near Stockdale and the Shap Granite has been the subject of a good deal of study. A most important advance was made by Harkness's discovery of the Stile End Beds and the consequent separation of the rhyolite overlying that fossiliferous horizon from the volcanics beneath it. At first it was supposed that the Stile End Beds were an intercal ation proving conformity, but the phenomena near Coniston seemed to point to unconformity, and opinion wavered for some time. In 1891 Mr. Harker and Dr. Marr described the junctions in Stockdale Beck and near Wasdale Head as faulted, and this view was afterwards extended more generally. I have not, how ever, been able to discover any but very general descriptions of the rocks underlying, or faulted against, the Coniston Limestone .Series, The Series is exposed on the spur between Sadghyll and :Stockdale farms. On climbing this from the south the first rock met with is the well-known pink fluidal rhyolite (Stockdale Rhyolite) underlying the Coniston Limestone. Then follow quarries in the Stile End Beds, here a slate with numerous fossils, lithologically resembling the same horizon near Millom and, like it, passing down into a greenish slaty sandstone of igneous material, with many quartz-grains. This is followed by a rather coarse red-and-green rock, some what like the basal bed in Butt's Foot Wood, t but more closely resembling the basal portion of the Corona Beds on Roman Fell, near Appleby. It has a considerable amount of quartz and felspar grit in addition to andesitic and felsitic material. This is underlain by a remarkable conglomerate (Plate 17.B), which is fully 200 ft. thick near the western quarry. The matrix of this. conglomerate is a detrital material similar • Mem. Geol, Survey, Sheet r02 S.W., p. 27. Ward, Quart. [o».... Geol, Soc., vol, xxxi (,875), p. 586. t .. Older Pal. Succ., Duddon,' p. '7. THE EASTERN PART OF THE LAKE DISTRICT. 211 to the overlying bed, but more vari-coloured. Its appearance is striking, owing to the concurrence of red, green, black, orange, and yellowish-white pebbles. On microscopic examination this matrix is found to consist of fragments of various lavas, often highly vesicular, with some quartz. The interstices are filled with a delicate mosaic resembling the base of acid tuffs, but with rhombs of carbonate and occasional small clastic micas. The same material fills some of the vesicles of the pebbles. The contained boulders are beautifully rounded and of all sizes (the largest measured is 47 em. long). They comprise a very miscellaneous collection of andesites and rhyolites. Black boulders and pebbles of andesite rendered opaque by grains of (?) iron-ore are conspicuous. The lower part is marked by uncleaved boulders of a banded felsite, which stand out in strong relief. The whole is obviously detrital with 110 trace of contempo raneous igneous action. Conglomerate or sandstone of this character underlying either the Stile End Beds or the rhyolite of the Coniston Lime stone Series has now been traced along the whole southern edge of the district mapped, from near Stile End to the southern slopes of Wasdale Pike, a distance of over four miles. The succession is therefore: 4. Coniston Limestone. 3. Pink fluidal rhyolite (Stockdale Rhyolite), easily distin guished from the neighbouring BorrowdaJe rhyolites by its strong colouring, marked flow, and fresh appearance. 2. Fossiliferous slates (Stile End Beds), thinning out eastwards. I. Basal conglomerate and sandstone. The basal beds often show strong current-bedding with lenticular intercalations of muddy material, and thus present every sign of being a shore deposit. STRUCTURE. The Relation of the Borrowdale Volcanic Series to the Undertying' and Overlying Beds.-From the description of the Mottled Tuffs and the map (Plate 18) it will be seen that mapping, sections, and stratigraphy all converge to prove a folded conformable junction between the Skiddaw Slates and the over lying volcanic series, as in the Duddon Estuary area. In the case of the Coniston Limestone Series, the existence of the basal sandstone and conglomerate immediately suggests unconformity. Near Stile End the Coniston Limestone Series rests on the Rhyolites." Their nodular base is crossed at a point about • See section given by Sir A. Geikie, "Ancient Volcanoes of Great Britain" (1897), vel, I, p. z3z. j, F. N. GREEN ON half a mile south-west of Sadghyll Farm and, thence to the River Sprint, the basal conglomerate is in contact with the Upper Andesites. East of Sadghyll the rock underlying the thick conglomerate already described belongs to the Harrath Tuffs, the Upper Andesites and Rhyolites being missing, though evidence of their former existence is found in the boulders in the overlying bed. On the spur of Great Howe the conglomerate (Plate I7.B) is resting on a low part of the Harrath horizon, with little felsite. Tracing north-east, the rocks are not so well exposed, but it would seem that the conglomerate thins, so that in Stockdale Beck only the sandstone is left, lying upon a basic augite andesite, which is well exposed in and about the bed of the stream for 250 yards, being then underlain by Harrath Tuff." The outcrop of Upper Andesites broadens north-eastward until at Buck Crags, a mile and a half from the Stockdale Beck exposure, it is a third of a mile across. Along this line the basal beds of the Coniston Limestone Series are never exposed, but no doubt they are present, as loose blocks of the conglomerate have been noticed above Mere Crags and near the head of Bleaberry Gill. The beds are, however, particularly well shown in Blea berry Gill and south of Crookdale Folds, still lying on the Upper Andesites. West of Yarlside Crag, the Rhyolites come in again, but are immediately heaved 200 yards north by a fault running N. 5 deg. E., which defines the western edge of Yarlside Crag. On the east side of the fault the basal sandstone, here slaty bedded red or green rock, much resembling the same horizon at Millom, has been quarried for fence material and can be traced over Great Yarlside, dividing older and younger rhyolite, to a small exposure east of the Crag, which is the last noted in this direction. The area of Rhyolites below the sandstone on Great Yarlside is small, and the basal beds rapidly cut down through the Upper Andesites, coming again on the Harrath Beds by the exposure just mentioned. The structure of Yarlside is of special interest (see Fig. 14). Thus the whole of the evidence points clearly to a strong unconformity between the Coniston Limestone Series and the underlying volcanics with an absence of faulting. This confirms the view expressed by W. T. Aveline in writing of this area, t that the volcanic rocks of the Coniston Limestone Series are of much later date than those of the Borrowdale Series. Further, as in the Duddon Estuary area, there is overlap, which, as might be expected, is indicated not by the basal beds, which continue unchanged, but by the overlying Stile End Beds, • The section along Stockdale Beck given by Dr. Marr and Mr. Harker, Quart. Journ. CeDI. Soc., vol, xlvii(t891), p, 270. represents the Stile End Limestone as fauhed against rhyo litic ashes and brer-cias (i.e.. the Harrath Tuffs), omitting the augite-andesite. The latter has, however, been noticed by Sir A. Geikie, Ope cit., p. 231. t Mem, Geol. Surv. Sheet 98 N. E .• 2nd ed .• p. 8. THE EASTERN PART OF THE LAKE DISTRICT. 219 which thin steadily eastwards from the name-locality till they die out at some point not far east of Stockdale Beck. The Folding.-The oldest folding is that indicated by the unconformity between the Borrowdale Volcanics and the Con is ton Limestone Series. It is gentle, the strongest tilt being probably less than 10°. In the Duddon district, the axes of the pre-Bala folding appear to have a direction between north to south and north- 6 v " 5 v " - Law!:Jer:S .0'" " q",,"'00.4 \oOUOD IiJ " &J U . . . J1r I 2 7. Alluvium. 3. Rhyolites. 6. Coniston Limestone, etc. 2. Upper Andesites. S. Stockdale Rhyolite. I. Harrath Tuffs. 4. Basal Sandstone. FIG. 14.-MAP OF GREAT YARLSlDE, SHOWING FAULTED UNCONFORMITY. Y. F. N. Green, east to south-west, and the mapping along the Stile End Wasdale Head line shows that the divergence from the direction of the second folding is not great. The second folding is the powerful post-Silurian movement which dominates the structure of the Lake District. It has already been pointed out that while the outcrop of the great tuff-band, deeply indented by Fusedale, Swarth Reck, and Heltondale, clearly shows that the general dip is low, the dips in the well-bedded tuffs are usually high, and that, there fore, a strongly folded district is in question. An inspection of 220 }. F. N. GREEN ON the maps shows indeed that, as first noted by Sharpe and after wards more clearly stated by Harkness and Nicholson, the northern and southern slates lie on the same horizon at approxi mately the same orographic level. Near Ullswater the beds are folded over north-westward, a fact brought out by the dips and the mapping of major folds along Fusedale; but farther south powerful overfolding to the south-east and east is met with, which comes out grandly between Cawdale and Haweswater. ~ ~DOI'rit UAlJUV1Um LjRhYOIU··I------I ADa.lite .: FIG. IS.-MAP OF FORDINGDALE, SHOWING OVERFOLDED SYNCLINE. J. F. N. Green. A major anticline is indicated by the westward bend of the Skiddaw Slate junction between Butterwick and Bampton. At this point there does not appear to be overfolding, though the south-east limb is the steeper. The fold, however, turns over along Cawdale, and brings up squeezed Harrath Tuff in the spindle shaped inlier of High Kop, a mile and a-half long. It continues through High Raise, where the Harrath Tuffs attain a height of 2,634 ft. In front of the anticline the Kidsty Pike rhyolitic inlier is nipped in a major syncline, displayed in the deep valley of THE EASTERN PART OF THE LAKE DISTRICT. 221 Measand Reck (see Fig. 15). On the high land, at a level of about 1,75° ft., the outcrop of the Rhyolites between the basaltic rocks, which here form the top of the Upper Andesites, is a quarter of a mile broad; but it narrows down the steep slopes, till in Measand Beck, at Fordingdale Force, slightly below 1,250 ft. O.D., the nodular basal rhyolite can be seen on either limb only 40 yards apart. Obviously a comparatively small additional erosion would suffice to sever the infold into two parts. The sheet-dip of the western limb is easily calculated from the data in Fig. 15 to be 60° (reversed), and of the eastern limb 10°, both dipping a little north of west. Another sharp overfold is indicated by the infolds of rhyolite at Measand Forces and Whelter, South-east of Haweswater Come several smaller folds, riding on the north-western limb of a major anticline with an axis running from Wren gill by way of Swindale Head to Tailbert, near Shap. The close packed folding is now tending to open out, and this area is less com plex than those about Haweswater. The anticline is beautifully displayed at Wrengill, where, as already pointed out, andesite is intercalated between the Middle and Harrath Tuffs (see Plate 19, Section 2). The Lower Andesites come up in the core of the fold and, together with the Middle Tuffs and the Wren gill Andesites, can be traced right round the great amphi theatre built by Harter Fell and Kentmere Pike. As the River Sprint cuts across the strike of the overturned south-eastern limb, in which the beds dip about 75° N. N.W., a complete section can be studied in a small compass. Another large anticline, not overfolded, runs from Thornship Beck, Shap, by way of Mosedale Quarries across Sleddale Fell and through Sadghyll. Towards the Stockdale-Wasdale Head line a great fold of monoclinal character comes in, bringing in the Coniston Lime stone Series, and throwing the volcanic series well below the ground level, so that it is not brought up again in the neighbourhood. It may be added that the main outlines of this pre Carboniferous folding were sketched out correctly by Harkness as early as 1863,. but his views have received little attention. A third system of folding is that which has tilted the neighbouring Carboniferousr in a general north-easterly direction. It is so masked by the violent pre-Carboniferous movements that it does not come out in the map. But its presence is shown by the warp of the ancient "platform," which constitutes a marked feature of the scenery of the district, though disappearing * Quart. Journ. GM/. Soc., vol. xix, section on p. 127. t See Professor Garwoou's Map and section. Q"arl. [ourn, Geol. Soc.. vol, lxviii h9I2), plates liii and lvi, 222 ]. F. N. GREEN ON in the Skiddaw Slate country. On the high ground west and south of Haweswater and about Sleddale Fell the platform lies over 2,000 ft. Thence it slopes north-easterly, more and more dissected and irregular, till about the Pen and Crag Top (north of Cawdale); between Haweswater and Swindale; and near Seat Robert, its level is roughly 1,600 ft. This gives a dip of 2° to 3°. That of the Carboniferous averages between 3° and 4° near Shap.* Faulting.--'vVhen a well-defined thin bed is traced along the strike it is sometimes found to be displaced by a little dip-fault, which, as noted by Harkness, often shows a tendency to approach N.-S. rather than N.W.-S.E. Larger dip-faults, owing perhaps to the difficulty of distinguishing their effects from those of folding, have rarely been proved. A fault at Great Yarlside has already been mentioned. A fracture of considerable importance, which cuts out the main tuff. band for at least three miles, runs from Rough Hill, near Heltondale, to Swindale Foot, with a ~eneral N.N.W.-S.S.E. direction. North of Cawdale and of Haweswater it is well defined, but from the latter valley to Swindale its position is doubtful, owing to the impossibility of separating the Upper from the Lower Andesites. It appears to cross Swindale somewhat north of Swindale Foot and to run up Starling Gill, near which occurs an interesting exposure of coarsely brecciated Mottled Tuffs, with angular masses of Skiddaw Slate and Andesite. Strike-faults are probably common, but difficult to detect. A fault occurs at Pikeawassa, Fusedale, which brings in the little rhyolite inlier there, dropping it against the Harrath Tuffs. Owing to the abnormal thinness of the Upper Andesites the throw is not so large as might be inferred, being perhaps 200 ft. It has not been detected on the east of Fusedale, There is certainly faulting along Swindale, but its exact nature is not easy to settle. . The structure of this valley has not indeed been yet satisfactorily determined, since, owing to the intervention of the Rough Hill Swindale Foot fault, stratigraphical criteria for the ·andesites are uncertain. Probably the patches of tuff which appear along the north-west brow are inliers of the Harrath Beds, in which case the andesites of the slope are at the base of the Upper Andesites. As those of the south-east slope seem to be Lower Andesites, there appears to be a normal fault along the bottom of the valley. It is convenient here to consider the hypothesis of lag faulting. The writer finds it a task of great difficulty to con ceive the peculiar system of mixed pressures and tensions apparently required to produce lag-faults in association with a great thrust in rocks that are not behaving as viscous fluids; and, indeed, evidence would seem desirable that they are a physical possibility. They do not appear in the experiments of Favre, Pfaff, Mellard Reade, Daubree, Cadell, and Bailey Willis, and clear proof of such structures in areas other than the Lake District is not available, t In the eastern part of the Lake District the facts now set forth * Quart. [ourn, Geol. Soc., vo!. lxviii (1912), p, 48~. t It the "lag-slides" described by Mr. E. B. Bailey about Ballachulish are ultimately placed beyond doubt, this statement is not invalidated, as the rocks must have behaved as fluids rather than solids. ",; ~ ~ s: l ~ I ~- 1-, I i j - , I 1 I I 1- j- I ! j - J a I j- j- ,l I ~ ,- I z 1 I- V> I I i 1 1 z e, 1 i ! l I ! 1 THE EASTERN PART OF THE LAKE DISTRICT. 223 do not confirm the alleged presence of lags between the volcanic series and either the over- or underlying beds. The manner in which the strata strike towards the junctions is satisfactorily explained by folding; and the variations in the thickness of the Mottled Tuffs and of the Basal Sandstone of the Coniston Lime stone Series are no greater than might be expected if the junctions are normal. The inference is that lag-faults cannot at present be taken to be an existing, nor even perhaps a possible form of geological structure. CONCLUSIONS. The main conclusions are as follows :- I. The sequence in the Borrowdale Volcanic Series in the .Eastern Lake District is substantially the same as in the Duddon Estuary area. The andesites and basalts, however, appear to be all effusive. 2. The existence of a separate "streaky" group (Sty Head Group) or of a separate basic group (Ullswater Group) has not been confirmed. 3. As in the Duddon Estuary area, the Borrowdale Series rests conformably on the Skiddaw Slates, and is unconformably overlain by the Coniston Limestone Series. 4. The presence of lag-faults has not been proved, and the failure of the hypothesis in this particular case must throw doubt on the validity of its application elsewhere. 5. The rocks have been strongly folded. U sing Bailey Willis's nomenclature, the dominant structure is a normal anti clinorium with central axis through Wether Hill, and the folding opening out to the south-east. The volcanics are relatively competent with comparatively large folds, the Skiddaw Slates and Mottled Tuffs incompetent with smaller, more composite folds, affording nearly horizontal sheets. (Compare Sections 2 and 3, Plate 19.) 6. The outcrop of the Coniston Limestone Series marks the position of an important monoclinal feature.