Pnoc. \'1<:0£, .AssOC ..\'(l). .XXVIII. PLATE 1.

Alluvium

InlrlJsions

CDrhonif.rou~ afld Trios

Rhyolites

(Jpptr Andesites

Harroth Tuffs

Wrengill Anduites

Midtllt Tuffs !.Qwer Cold Andeaifu r~1I MoftledTuffs lofterharrowSond!

THE WATFORD ENGRAVfNG Co l!~ Skit/dow 1 J~ 0 1 .~.4 .

MAP OF THE COUNTRY BETWEEN WILTON AND BORROWDALE By J FRED K N. GREEI" --=.G.S. Jlnoc. G:F.OL .ASSOG.,VOl~.xxvrll. PLATE 2.

Alluvium

Intrusions

Corboniferous and Trios Sltules 1coniSfon Rhyolite limestone . Grit Series lower Ande$ite.~ + Hut/ooRoo Skiddow SIC/Ie

THE WATFOR:J ENGRAVINGC~LT.o Scalf!- ofNiles J 'n ~"~ "'UI...=~1=~;;-"",===>-=--~-·f 5. L...",r,s C.ONDUIT S~ ~:OLB()PI·I. w.c. I {>. , .'- ....j======it'...... {

l\'lAP OJ<' THE COUNTRY BETWl~]~N COCKERMOUTH --e'\..ND CARRICK F:f~LI,. Bv .J. FRED~ N. GREEN,F.G.S. ( NotR,. The. 1.Jount.wy 01' tlw Corboniferousi,8 fn.ken,froTrV lhb maps oft}w Ge.ologi.caL Survey) PROCEEDINGS

OF THE GEOLOGISTS' ASSOCIATION.

THE AGE OF THE CHIEF INTRUSIONS OF THE .

BY]. F. N. GREEN, B.A., F.G.S.

(Read January 6th, 1917.) PAG£~ I. THE PROBLEM [ II. HISTORICAL .•..• 2 III. THE -BoRROWDALE JUNCTION. Unfaulted character. The Sandstone. Supposed lagplanes. The purple breccia of Falcon Crag .• 5 IV. STRUCTURAL. I. The Eskdale Granite. The Thrust. 12 2. The Granophyre . 1+ 3. The St. John's Microgranite . IS 4. The Carrick Complex 17 V. STRATIGRAPHICAL. The Grits. Bearing of their composi- tion on the Carrick Fell problem. The Drygill Shales. Ordovician denudation Ie) VI. GENETIC. ••• 26 VII. THE QUARTZ-DOLERITES ••.• 27 VIII. CORRELATION OF THE BORROWDALES • 28

1. THE PROBLEM. CONDITION precedent to the useful discussion of the A evolution of igneous rocks, either within a limited area or more generally, is the determination of their succession by a study of their field relations. In the most remarkable igneous area in Britain, namely, the Lake District, the writer has endeavoured to find a clue to the sequence and structure among the volcanic beds by detailed mapping of selected portions. Naturally, the complication of intrusion and metamorphism has as far as possible been avoided; but in following up outcrops of various horizons, it was inevitable that observations should be made bearing on the relations of the larger intrusions. Some of these yielded results which wen.' important, though unexpected, and so this paper is the outcome of notes made originally with another object. PROC. GEOt. Assoc., VOL. XXVIII, PART I, [9I7.J :z ]. F. N. GREEN ON

The chief intrusions among the older Lake District rocks are acid. Basic masses, usually quartz-dolerites, occur, it is true, of considerable size, as, for example, at Carrick Fell, , Sea,thwaite, Castle Head, and Haweswater. But the large outcrops are predominantly acid, except in the case of Carrick Fell, where about half the exposure is basic. It is not easy to draw the line between major and minor intrusions. The former may be taken to be those known as the Shap granite, the Eskdale granite, the Buttermere granophyre, the St. John's microgranite, the Skiddaw granite, and the Carrick Fell complex. The total exposed area of these is about 60 square miles. The Shap granite need not be further considered, as its penetration into the Silurian, with the occurrence of boulders in the basal beds of the Upper Palreozoic, enabled its age to be determined within narrow limits half a century ago. No sort of assurance exists for the others, and the most diverse opinions have been expressed. Four come in at the same horizon, the junction of the Skiddaw Slate with the overlying volcanics; and these four show similarities in a number of other points, notably the im• portance of soda in their chemical composition, the tendency to granophyric structure, and the form of the intrusion as a sill or depressed laccolith. The fifth, the granite of Skiddaw, though, owing to the wide metamorphism accompanying it, attracting more attention than the others, is much more obscure, being in contact only with the lower part of the Skiddaw Slate Series, and of indeterminable form.

II. HISTORICAL. Sedgwick* had observed that the main granite masses were later than the Skiddaw Slates, and seems ;to have considered some of them earlier than the greater part of the volcanic series. He was quite definite on their relation to the main crust" movements, writing- I believe that the greatest elevations and conto-rtions of the slates took place after the eruption of the granite and syenite. Harkness,t in 1863, regarded the granites of Skiddaw and Caldbeck as part of the same mass. Nicholson,t in 11>69, was disposed to look upon the igneous masses in question as the roots of the ancient vents from which were derived the alternating ashes and traps.

• .. Guide to the Lakes," (Letters to Wordsworth, t842.) t Quart, Jou.... Gear. Soc.. xix, p. 124. t II Notes on certain of the Intrusive Igneous Rocks of the Lake Distdct." Quart. Jou"", Gl

Ward* however came to the conclusion, chiefly from his examination of the microscopic cavities in the quartz, that all these intrusions must be associated with the Shap Granite, and were to be assigned to the time of the great crust-movements, that is, the close of the Silurian. In 1895 Mr. A. Harker, in the course of his classical study of the Carrick Fell complex,t said;- The rocks have been intruded at the junction of the Skiddaw Slates and the Eycott Hill lavas· since these two· ·~roups assumed their present mutual relations. There is reason to believe that these relations are not the natural ones, and that the absence here of certain other groups which should intervene is a result of the great post-Silurian crust· movements. If this be 50-, the intrusions mus.t be later than these crust-mo'vements. The abs,ence of cataclas,tic structures, or any phenomena attributable to shearing in the intrusive masses, is, so far as it goes, a confirmation of this view. The faults, which have displaced the intrusive rocks. are to be referred to the later (post-Carboniferous) disturbances. In the discussiont he stated that the Skiddaw granite and greisen belonged to the epoch of the post-Silurian movements. In 1900 Dr. Marr§ held that the Skiddaw granite had been forced along the thrust-plane, and the St. John's Vale and Buttermere intrusions along the lag-plane, which, on his hypo• thesis, were required to account for the structure of the district. He suspected that the Eskdale granite was also intruded along a lag-plane, and suggested that the Skiddaw granite was De• vonian, but that the Carrick Fell rocks, regarded as lying in the same thrust as the Skiddaw granite, were more modern. In 1902 Mr. Harkerl! divided the intrusions into an older and a younger suite. In the former he included the Buttermere granophyre, the St. John's microgranite, and the Great Cockup picrite; in the latter the granites of Shap, Skiddaw, and Esk• dale, and the Carrick Fell complex. Dolerites belonged to both sets. In 1906 Mr. R. H. RastallJ~ in the course of his description ()f the Buttermere and Ennerdale Granophyre, wrote;-

The on,ty ·evidence bearing 0'11 the poinrt so far obtainedi, is the absence of cleavage on the north s,ide of the Buttermere laccolite, which seems. to show that the Skiddaw Slates on this side were pro• tected from the influence of the great thrusts coming from the south· east, which so ,strongly cleaved them in other parts of the district. The intrusion was therefore previous to the great earth-movements- of the later part of the Caledonian series. Whether it accompanied the Ordovician movements of the same series remains to be proved.

• "On the Granitic, Granitoid and Associated Metamorphic Rocks of the Lake District." Quart. Journ. Geol. Soc., Parts I-II, xxxi (1873), p. 368; Parts III·V, xxxii (1876), p. I. t Quart. Journ. Geol. Soc., Ii (1893). p. 126. t Ihid., p. 148. § II Notes on the Geology of the English Lake District." Proc. Geol. Assoc., xvi, p·449· II Proc. Yorks Geol. Soc .•. xiv, p. 489. , Quart.]ourn. GeoJ. Soc,. IxU, p. 26g. 4 ]. F. N. GREEN ON In the same year Dr. Marr, in his Anniversary Address to the Geological Society,* said:- The Buttermere granophyre and some sills and dykes, chiefly occurring in and around the central , are probably of Ordovician age; and the Eskdale granite may be of that age, though possibly newer. But he suggcstedt a Tertiary age for the Carrick Fell complex, comparing it with the Tertiary igneous rocks of Scotland. In a paper read in r908 and published in the following year, Dr. A. R. Dwerryhouset supported on the whole the current view that the Eskdale granite belonged to the Devonian period. In the discussion on this paper, however, the present writer expressed§ the opinion, as a result of his examination of the southern edge of the mass, that it was older than the main crust-movements. In 1910 Mr. RastaU,11 in a paper on the Skiddaw Granite and its Metamorphism, stated- it is clear that the Skiddaw Slates had undergone very extensive dynamic metamorphism previous to the intrusion of the granite, since the deavaJgle', £()llia.tion, and fol(iill1gare still clearly visible, alithowgh the divisional p1aIlJes hJave be<~n. sealed! up by the recrystallisation induced by heat. But he believed the Skiddaw granite to be undoubtedly older than the Carrick Fell complex, assigning it to the same phase of igneous activity as the granites of Shap and Eskdale, that is, to Late Sill.ooan or ea.t'ly Devoni:m timeg,. Finally, in 1916, Messrs. Rastall and Wilcockson,1! in their study of the accessory minerals of the granite rocks of the Lake District, say, after referring to the relation of the St. John's and Threlkeld masses to the supposed lag-plane and shatter belt- all the eviJdence points to some gen'etic connection betweeIlJ them. If this be so, their age must be late Silurian or early Old Red Sandstone, thus correlating them with the other granitic masses of the Lake District. It must be admitted, however, that there is no positive evidence on this point. The following pages treat particularly the four similar intru• sions referred to above, those of Eskdale, Buttermere, St. John's Vale, and Carrick Fell. It is proposed to discuss the more difficult question of the age of the Skiddaw granite in a later communication. It is however necessary in the first place to examine the nature of the junction between the Skidda\v Slates and the Borrowdale Volcanics, at which all four lie.

• Quart. Jou,n. Geol. Soc., lxii, p.lxxv. t Ibid., p. uiv. t Quart. Journ. Geol. Soc., lxv, p. 55. § Ibid.• p. 79. II QU4rl. J Dum. Geol. Soc., lxvi, p. 138. "ll QU4,t. Jou",. Goof. Soc., lxxi, p. 60<), THE AGE OF THE CHIEF I:-ITRUSIONS OF THE LAKE DISTRICT. 5

III. THE SKIDDAW-BORROWDALE JlJ:;\TCTION. In previous papers relating to the Lake District the writer showed (1) that in the Duddon Estuary area, with the exception {)f a short distance along the Whicham Valley, the junction be• tween the Skiddaw Slates and the Borrowdale Volcanic Series was a strictly conformable one between ordinary slate and con• temporaneous slate-bearing tuffs j (2) that over the Eastern District, about Shap and Bampton, the junction was similar, but that the slate-bearing tuffs were thinning out westward near Ulls• water, so that the base of the overlying andesite lava came within a few feet of, or perhaps at Sleet Fell rested on, the Skiddaw Slate j (3) that at Pohouse Bank, in the Whicham Valley, an andesite rested directly on the Skiddaw Slate. A reconnaissance of the remaining outcrops of junction in the Lake District proper has shown that the greater part of them conform to that at Pohouse Bank, the MotHed Tuffs being absent. Further examination has suggested that the Pohouse Bank section is that of an ordinarv flow of lava over mud, which was squeezed up into the interstices of the basal flow-breccia, and insinuated itself into the vesicles. It is, in fact, of the type which can be seen in numerous sections throughout the Lake District wherever a l

the junction of (5) and (2) when (3) and (4) are missing. Along the -Buttermere line such weldings have been collected at Matterdale End, near Ullswater; , St. John's Vale; Grange, Borrowdale; Hindscarth Edge; Honister Crag; and Scarf Gap. Along the Cockermouth-Carrick Fell junction, on the north of the Lake District, thev have been .taken at Park Wood, near Blindcrake; Whitefield Cottage, near j and Silver Gill, by Great Sea Fell. No case of a faulted junc• tion was observed. When the scoriaceous contacts are examined microscopically, the appearances are like those presented by the contacts of frag• ments in tuff with a fine-grained matrix. There is no sign of shattering, crushing, or mylonisation. On the contrary, the mud has sometimes found its way into the outer vesicles and filled or partly filled them. Two examples of the contacts between solid lava and under• lying Skiddaw Slate, from Clough Head and Whitefield Cottage, have been sliced. Both have the same general characteristics, the junction being sharp and sinuous, with re-entrant angles, and both rocks retaining their usual appearance with apparently little or no mutual action. In that from Whitefield Cottage the bedding of the mud (which contains a layer, half a milli• metre thick, of coarser tuffy material) >tends to follow the curves of the contact, and no dvnamic action other than vertical pressure is to be detected. - In that from Clough Head the andesite seems more acid, the base resembling that of some altered felsites, and shows a tendency to chloritisation for a depth of half a millimetre from the sharply defined contact. For a depth of two to three millimetres the mud contains frag• ments of slightly different colour and texture, as if the original shale had been disturbed; the line of contact between this and the normal undisturbed mud shows signs of movement with oblique slips, the appearance being as though cooled lava, with an adherent film of broken shale, had dragged slightly over the lower mud-doubtless due to the motion of overlying molten material. Examples of this dragging action on a larger scale are common in the Lake District and may usually be seen where a lava rests on bedded tuff. For a few metres below the base the bedding-planes are thrown at iIltervals into sharp irregular con• tortions, accompanied at times by thrusts and even brecciation. Such phenomena have been put down to the Devonian crust• movements,* but in the writer's experience are never seen except at a short distance above or below a lava. Thev are much more like the contortion produced in sands by the ploughing of ice than the folding due to crust-movement on a large scale, and are probably all contemporaneous. • E.g. see Marr, "Geology of tb.e Lake District," p. 84. C~~:,,:,,] Inlru~ions c::J Skiddaws IW:hiJvolcaOlcs _ Bala 1- - --J CarbonIferous. Etc FIG. l.-INDKX MAP OF PART OF TlIE LAKE DISTilICT (A and B = Plates I and z; C. D and E = Figs. z, 3 and 4). g ]. F. N. GREEN ON

l\Iapping along the junctions supports the view that they are conformable. The lower member is invariably the soft black Upper Skiddaw Slate, with occasional stony'seams; the Skiddaw sandstone and flags never approach the Borrowdales; on the contrary, the great band of sandstone and flags that passes from Mosedale over Skiddaw and , across and , to and , is rudely parallel to their base. Neither are the fossil bands known to abut against it. The highest, that of Didymograptus indentus, is regularly found at some little distance away, as in the eastern area, the Tetragraptus beds appearing farther off. With regard to the lie of the volcanics, the writer has now (see Plate I) laid down the junction of the andesites, which overlie the Skiddaws, and the bedded Middle Tuff from Watendlath via Quayfoot and the Honister quarries, over the shoulder of , past , a distance in a straight line of nine miles and three-quarters, thus proving that these ande• sites are all on one horizon, corresponding to the Lower Ande• sites of the Eastern area. * The Middle Tuffs are covered by the Wrengill Andesites, t which have thickened enormously, and are the chief mountain-builders of the Scawfel1-Pillar region; then by the Harrath Tuffs of Seatoller and Giller Combe; by a thin development of the Upper Andesites; and finally by the Rhyolitic infold of Styhead, , and Rosthwaite Fell. On the western side of ,the Buttermere granophyre the junction is more complex. The andesites appear to be in con• tact with the Upper Skiddaw Slate near Wilton Fell, but along the remainder of the line there intervene the sandstones and tuffs of Latterbarrow and the neighbouring fells. These rocks were observed by Ward, who supposed them to lie below the black sla.te of Skiddaw itself, t and noted the remarkable conglomerate of Capel Crag. § Their peculiarities have also received comment from Dr. Marr.1I When they are mapped (see Plate I) it is plain that between the andesitic lavas and the Upper Skiddaw Slates are two lenses of different material. The lower, which begins half a mile west of the River Calder, and extends in an arc eastward for two and a half miles tilI it is cut off by a fault running north-north-west across , is composed of a well-bedded homogeneous greenish• grey quartz sandstone, with rare bands of coarse welI-rounded grit. The grains of sand, commonly o. I to 0.2 millimetre in diameter, are nearly all plain quartz, but include occasional vein-quartz and in some places a small proportion of felspar.

• P.oc. Geol. Assoc., xxvi (1915), pp. 204 and 212. t Ibid., P. 215. I "Geology Northern Part Lake District," pp. 42, 47. li Ibid.. p. 23. See also G.ol. Ma~., 1879, p. 51. II "Geology of the Lake District," '9,6, p. 9 THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 9

All the larger grains are well rounded. In a slide of a coarse grit band ,the larger pebbles, over one millimetre in diameter, consist of vein-quartz, while those of intermediate size include two of hard silt. No pebbles of igneous rock have heen detected. A little clastic mica may be seen. The matrix is a silty mud, usually with much chlorite, giving the rock its greenish colour. Especially at the western part of the lens certain bands con• tain andesite lapilli, which have evidently been dropped, not washed, into place. The best example of this contemporaneous explosive vulcanicity was noticed on the eastern slopes of Lank Rigg, where a loose block of the sandstone contains an hour• glass bomb of dark altered tachylyte, now 43 em. long, but originally perhaps 6 or 8 em. longer. The greatest diameter is 15 em. The sandstone round it is slightly hardened for a thickness of 2 em. This Latterbarrow Sandstone lies conformably on, and is interbedded with, the subjacent Skiddaw Slate, as may be seen in a good section on the left bank of the Calder, at a point 300 yards south of Rowantree Sike. The Skiddaw mud was however probably suffering some erosion, as in places minute pellets of black mud, stuck all over with sandgrains, may be found in the sandstone. The upper or western lens overlaps the Lat,terbarrow Sand• stone for half a mile, and has a total length of over two miles. Along the River Calder it can be seen that there is a gradual passage from the sandstone of the lower lens by an increase of lapilli, up into a variable breccia or agglomerate with a sandy matrix. Throughout the whole of this lens of agglomerate the lapilli include rounded fragments of sandstone, indistinguish• 2.ble from the Latterbarrow Sandstone, except that they are often stained red and that epidote has developed in the chloritic matrix, presumably owing to heat. Scattered rhyolitic frag• ments appear in the upper part. The most striking development is at Capel Crag, near the top of the lens, where it passes into a conglomerate extraordinary even among the many curious conglomerates of the Lake District. It is bedded, passing up into bedded tuff, and consists largely of ellipsoidal masses of sandstone and andesite often half a metre in length. Some of the sandstone boulders contain lapilli exactly as the Latterbarrow Sandstone. There are also some large subangular blocks of rhyolite, one of which was estimated at 80 em. long. The major axes of these ellipsoids lie parallel to the bedding, so that the rock appears to be a shore deposit, presumably formed on the flank of a cone of loose agglomerate. A somewhat similar, but less coarse, material is seen in Latterbarrow Beck. 10 ]. F. N. GREEN ON

A curious feature of the upper lens is the manner in which small lapilli have at times aggregated, as if still viscous when they came into contact; they may partly wrap round one another. It is not easy to understand how this could have happened after falling into water, but the occasional presence of sandgrains caught up between contiguous lapilli seems proof that the adjust• ment took place on the sea-bed, rather than in the air. * From ,the above description it is clear that the rocks are the result of a combination of ordinary sedimentation with explosive vulcanicity; that they are conformable to the soft black upper Skiddaw Slate; that they pass up from sediment with occasional lapilli into a volcanic breccia with sedimentary matrix; and that they characteristically include lapilli of sedimentary rock. In all these respects they correspond exactly to the Mottled Tuffs as developed near the Duddon Estuary. t The only difference of importance is the sandy nature of the sediment, which may be explained by local shoaling connected with the neighbouring vents. The northern line of junction between volcanics and Skiddaws, from Cockermouth to Great Sca Fell, has much the' same features as that from Borrowdale to Buttermere. Everv• where the rocks are andesite on the one hand and Upper Skid• daw Slate on the other. The extreme top of the Skiddaw Slate in this region has a tendency to develop silty or even sandy bands, which may contain a little tuff, but the greater part is indistinguishable from that near the main southern junction. The records by Harknesst of D. lzirundo at Ellengill and Tetragraptus serra at Frozzengill (see Fig. 4) sufficiently prove that the lithological correlation is sound. The best seotion of the junction observed was in a small quarry to the east of Whitefield Cottage, south-east of Binsey Fell. The rocks, as usual along this northern edge, are over• folded from the north; and the inverted limb is cut in this quarry, so that the slate appears to rest upon andesite. The latter is highly amygdaloidal, with the common chlorite-calcite infilling. A tongue of lava has pushed into the sediment, and at the eastern end of the quarry overrolled scorire are imbedded in the slate. There is therefore everv reason to treat all the boundaries of the main area of Skiddaw Slates as ordinary folded con• formities instead of having recourse to lag-planes, which have failed in the Duddon and Eastern Districts, or to any other form of boundary fault. The acceptance of such unusual faults for over forty years, the important theories that have been grounded on them, the

• Aggregation is well shown in slide E. 2203 of the Geological Survey collection. +Older Palreozoic Succession, Duddon, pp. 5-6. t Quart. JOUNI. Geol. Soc., xix (1863), p. 138. A fine specimen of T. serra from the second locality is preserved at Jermyn Street. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. I J excellence of the exposures, and the ease with which they are accessible, leads to the expectation ,that full evidence for them must have been published. Nevertheless, in the scores of papers touching on the northern part of the Lake District hardly any evidence is to be found. The main reason assigned by Ward and his colleagues is the manner in which the bedding of the Skiddaw Slates strikes towards the junctions, an argument which has lost all significance since the folding and crumpling of the slates lias been recognised. Certainly in the Survey maps, and in some of Ward's papers, beds of tuff are shown abutting against the Skiddaws; but, in all cases \yhich the writer has tested, these slllpposed tuffs merely express Ward's peculiar views on meta• morphism, long abandoned by all geologists. The wr1ter is not aware of any place in the northern Lake District where a fault of low dip between the two series has been proved, either in an exposed section or by detailed mapping. Since the publication of the Survey Maps, Dr. Marr has suggested that the well-known purple breccia of Falcon Crag. (Ward's "a" ash) might be a fault-breccia. The rock, how• ever, has the characters of an explosion breccia. Neither in hand-specimens nor under the microscope has the writer been able to find shattering or shearing in the fragments; some of the finer bands are composed of pumice lapilli, with the typical con• cave outlines; the outer vesicles of the scorire are sometimes filled with fine matrix, showing that they were empty when they came into position; the lapilli of sedimentary rocks* are com• parable with those of the Duddon Mottled Tuffs; petrograph• ically, the purple breccia is identical with the slate-andesite tuff forming the base of the Borrowdale Series near the Duddon Estuary; and Dr. Marr has pointed out that there is interbedding with Skiddaw Slate at Cat Gill. It passes upwards into the tuffs ;; b" and"c,"a bedded division, on which lies, without any break, "d," the basal flow-breccia of Ward's No. I lava. It may therefore be taken to be, like the upper part of the Latter• banow beds, a local development of the Mottled Tuffs, which, as the product of the earliest paroxysmal vulcanicity, tend to be qriable and discontinuous. t The diameter of this lens of tuff is about three miles from Grange to the east of Rakefoot. The normal character of the Skiddaw-BorrowdaJe junction is of fundamental importance for the determination of the tectonic relations of the intrusions, which must be the first point for discussion. They will be taken in order from south to north.

* In places they are very plentiful. See the description l-y Mr. \v. M. Hutchinl:'s, Gea/. Mag., I!!gr, p. 462, which sbows that the rock is quite un'ike a crush.breccia. Also Sir A. Geikie, A"cunl Va/callO'l, vol. I, p. 234. +C/. • .Ifem. Geal. S ..",., Tertiary Rocks, Skye, pp. 'S·,6. 12 ]. F. N. GREEN ON

IV. STRUCTURAL.

(i) THE ESKDALE GRANITE.

Dr. Dwerryhouse,* in his description of this mass, which is exposed over more than 30 square miles, has noted its sill-like appearance. In the discussion on tha.t paper, the writer ex• pressed some divergence from Dr. Dwerryhouse's views, holding that the intrusion was older than the crust-movement. A sketch of the evidence for that opinion will now be given. To the east of BootIe the edge of the granite runs in contact with andesite easterly for 2 ~ miles, parallel to the junction of the Borrowdales and the Upper Skiddaw Slate. This latter junction is normal and conformable, as observed by Aveline. t He was, however, in error in supposing the components to be interbedded, since they are really folded up together, as well shown by the side of the Crookley Beck, near Cat Crags. The rocks here are within the aureole of the granite, though near the outer margin, so that some care is required in discriminating them in the field. There is no sign of faulting. Near the edge of the granite outcrop turns northward, and continues five miles in a general northerly direc• tion, always in contact with andesite; on Birkby Fell it again turns eastwards towards Devoke Water. The edge is now the roof of the mass, and on Water Crag, north of the lake, it descends below the base of the Borrowdales, and leaves a little patch of metamorphosed Skiddaw Slate,t now in the condition of a hornfels, below the altered lavas. Water Crag is bounded eastward by a fine fault-cliff marking a north-north-west-south• south-east fault of considerable importance, which heaves the granite to the south side of the water, whence the junction of granite and andesite runs in a general north-easterly direction for four miles. Now, along the whole eight miles from BootIe to Devoke Water, the andesites show strong contact-alteration for a long distance from the granite edge. The production of biotite and hornblende persists for over a mile outwards, and the black colouring characteristic of the heated volcanics is often notice• able at more than a mile and a half. But at Devoke Wa.ter (see Fig. 2) this alteration stops abruptly. For at least two miles north-eastward from the north-north-west fault no thermal effects can be seen, even within a few yards of the granite. The andesite is often much shattered and veined, but remains green

* Qua,t. ]OU1n. Geol. Soc., Ixv (,gog), pp. 55-80. t Geol. Mag., ,872, p. 44'. t Shown on the Geological Survey Manuscript six-inch Map, Cumberland 83. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 13

or grey in colour, without recrystallisation or production of con• tact minerals. Along this line, too, the granite is usualIy of the normal type, and does not exhibit the siliceous marginal modifi.· cations described by Dr. Dwerryhouse.* The junction must therefore be a fault; and the fault• plane must have a low dip, as may be proved by an examination of the cake of granite left lying on the andesite on the south side of Devoke Water. That this is a thrust outlier is plain,

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co "' ,~ c I 0"'- , , 0 0

• 0 o U b o 0 Q

• o 0 ~ " ·(> DAltered Gramte Normal Hornfels Fault Andesite AndeSIte FIG. 2.-FAULTS AT DEVOKE•WATER (Index Map. C). not only from the absence of contact alteration, but because the floor is exposed in Rigg Beck, which has cut through the thin cake, disclosing the underlying andesite. A specimen of this rock, which can only have been a metre or two below the actual sole, has been sectioned, and proves to be a porphyritic augite• anaesite without a trace of thermal action. • Loc. cit., pp. 6+'70. }. F. N. GREEN ON

The inclination of the thrust-plane at Devoke Water is easily determined to be 9 degrees north-west. Another place at which the hade is readily ascertained is near Stanley Falls, where the Birker Beck has cut a notch in the granite edge. At this point the inclination is about 7 degrees in a direction north• west bv west. The subjacent andesites belong to a much higher horizon than the andesites on the west of Devoke Water, into which the granite has been intruded. In fact, they are the Upper Ande• sites of the Eastern Lake District. This is evident from the occurrence of infolds of rhyolite at Great Worm Crag and Yoadcastle, and from anticlinal inliers of the Harrath Tuffs, which in this area pass upwards into a slate, distinguishable from the slates of the Middle Tuffs by the occasional appearance of fragments of the characteristic" variolite." The structure is, therefore, a true thrust-plane, the strike of which is nearly parallel to that of the folding, which is pre• Carboniferous. The axial planes of the folds dip in the same direction as the plane of the thrust. Thus the Eskdale Granite is earlier than a structure which can only be assigned to the pre• Carboniferous movements.

(ii) THE BUTTERMERE GRANOPHYRE. The laccolitic or sill-like nature of this intrusion is even more evident than that of the Eskdale Granite. The floor is always in the Upper Skiddaw Slate. The roof is always in the Lower Andesites (see Map 1.) except at, and to the west of, the summit of , where, as at Devoke Water, little patches of homfelsed Skiddaw Slate have been left sticking to the base of the Borrowdales. The parallelism of the outcrop of the roof to the sinuous outcrop of the junction between the slightly metamorphosed Lower Andes~tes and the Middle Tuffs was detected by Ward, who figured it in 1876,* noting that the abutment of the tuff-band against the granophyre west of Hay• cock was due to a fault which can be followed into the volcanics. The Lower Andesites can be traced from the neighbourhood of Latterbarrow right round the main outcrop of granophyre up to Glade How and . It is true that the eastern boundary of the exposure of granophyre at the foot of Wastwater is con• tiguous to andesite of a higher horizon, but there is every reason to believe that the junction is a fault, as shown on the Survey maps. The volcanics exposed near by are intensely shattered and veined, in places reduced to a fault-breccia, but show no heat-alteration j and the line runs northward into the Haycock fault, and in the opposite direction is a continuation of the fault south of the lake between the Eskdale Granite and the volcanics • Quart. fourn. G.ol. Soc., xxxii, p. 15. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 15

of the Screes,* which has a high dip, unlike the thrust-plane. It may be added that this dislocation is itself apparently a continuation of the north-north-west fault of Devoke Water (Fig. 2) and of the more southerly fracture described by Sedgwick in 1858.t The whole is probably the most important structural dislocation in the Lake District. Its approximate position is laid down on the Index map (Fig. I). The band of granophyre skirting the south-west part of the lake may be an offshoot from the roof into higher beds, but i,ts relations are not yet clear. With the exception of this detached bit, the intrusion is certainly a thin lacoolite or sill related closely to the bedding. This sheet originally extended above the present hill-tops some miles farther to the north than at present, as the slate on, and north-west of, shows contact-alteration.t These facts are incompatible with the pre-existence of the main folding. No reason can well be assigned for the flattening out of the deep eastern folds, or for the parallelism of the roof• line of the granophyre to the base of the Middle Tuffs between Pillar Mountain and Haycock, but the presence of the acid mass at the time when the crust-movements took place. It would appear, therefore, that when this western area was under compression, and the anticlinorium of the Central Lake District was coming into being, the load was insufficient to induce ready flowage in the segment stiffened by the Eskdale and Buttermere intrusions, which lifted by thrusting as well as folding. The strongest of these thrusts may well be that mapped by Mr. Bernard Smith along the Whicham Valley,§ but the Devoke Water thrust associated with the Eskdale granite must have a throw measured in miles. No doubt there are others.

(iii) THE ST. JOHN'S MICROGRANITE AND THRELKELD " GRANITE." Current maps show three separate masses of intrusive rock near St. John's Vale, namely, those known by the titles above mentioned, which are divided by the St. John's Beck, and a small outcrop at Bramcrag. On viewing the Threlkeld quarries from the north and west, they can be seen to extend for three miles at the same level, and then to continue on that level into the Bramcrag exposure, which is separated from the main mass by a small valley filled with drift, and underlies the andesite * A. R. Dwerryhouse, IDe. cit.• p. 56. t Phil. Mag., xvi, p. '55. t The writer would assign the low degree of cleava~e in this region, not to shielding fcom south-eastern pressure, as suggested by Mr. Rastall, but to the resistance to com• pression of an overlying girder of granophyre and its hardened skin of andesite and hornfels. ! {1ttart.Journ. Geor. Soc., Ixviii ('9'2), p. 406. 16 J. F. N. GREEN ON

crags of Wanthwaite. Obviously, therefore, the intrusion is a sill, and there are no grounds for separating the Bramcrag rock from that of Threlkeld in a solid map, as it must be con• tinuous with it under the drift. The quarries are all at the edge of a tabular mass resting on comparatively flat Skiddaw Slate country, and projecting in front of a rampart of lavas rising to a height of over 2,000 ft. behind the cliffs of . In advance of these cliffs,

~ ~ " .. ' '." ", '. . '. ~;--,;;::;::r:;:~;--,=;:=!~J"".u: rJ'tvH [[[[J ~n~ Bt:lrrowdales S!

FIG. 3.-THE ST. JOHN'S VALE INTRUSION (Index Map, D).

resting on the granophyre, is a cake of folded-up Skiddaws and andesite known as Clough Head, on which the rocks are well exposed and welded specimens of the junction can be obtained. They present the usual characters of a normal contact of lava and Skiddaw mud. The St. John's mass lies at a slightly lower level, projects north from the volcanic country in the same way, and seems part of the same sill, divided from the Threlkeld segment by the THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 17 erosion of the St. John's Beck, but doubtless joined to it under the alluvium of the Vale. There is, fortunately, a small patch of the roof left near Skelthwaite Crag, a quarter of a mile west of the church, which displays its relations excellently. It consists of contact-altered Skiddaw Slate lying in hollows in the roof of the microgranite. The rock is irregularly cleaved, and the cleavage extends several metres into the underlying grano• phyre, as may be seen well south of the crag, where it dips about 70 degrees in a direction 5 degrees west of north. Thus the intrusion is older than the cleavage. This area provides a good illustration of the nature of the Lake District folding. Especially when looking eastward from Low Rigg, the structure is seen to be one of successive "plat• forms" of slate, granophyre, and andesite, developed by erosion, so that the general lie is obviously nearly horizontal.* But the dips in the Skiddaws are high, and the rocks on Clough Head are strongly folded. The larger features of the scenery are controlled by the low sheet-dip, not by the high true dip.

(iv) THE CARRICK FELL COMPLEX. The general structure and relations of this series of intru• sions have been described by Mr. Harker,t who, however, did not include the western termination. The roof is everywhere in andesitic lavas considerably altered, and the remnants left on the upper surface show that it was fairly fl

sinuous lines, and the rocks correspond with the earlier stages of spilosite, as defined by Rosenbusch. * The structural position of the Carrick Fell complex in the Lake District sequence having been settled, the fault which bounds most of its southern edge becomes next in order for

:'.:.'~:: '.' . '. : ::.':'.: .::.:. /. . . ~/j\:"::'.\<: .: :.:.;..':"'-.-::;....::-...:...-::.'-'\:.:;..;~....:.....:.'::..;,:"~~--:

1500 ~D~ Polycenetk Gm Andesite Skiddaw Slate SpiJoslte Gilbbro and Granopbyra FIG. 4-.-THE NEIGHBOURHOOD OF GREAT SCA FELL (Index Map, E;)

discussion. But as a closer limit to the age of the intrusions can readily be set by stratigraphical methods, this fault will be dealt with elsewhere when considering the Skiddaw Granite. If too the Drygill Shales were once, as Dr. Marr has suggested, bounded by a thrust of Devonian age, then, in view of the absence of any alteration in the shales at the junction, the granophyre must be pre-Devonian. But this point is also best treated later. • Mik. Phys., 4th Ed. II (2), p. ,gog" Beobachtet man zunachst £in Dichterwerden und I{rossere Harte in demselben mit begleitender Abnabme der FissiUtiit, auch wohl eine Farbenanderung." THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 19

V. STRATIGRAPHICAL. No stratigraphical evidence for a lower limit of age is at present known for the Buttermere or St. John's Vale intrusions. The Eskdale Granite is of course older than the Triassic rocks which rest on it, but that adc.s nothing to the results already obtained. Fortunately, certain rocks are developed in the neigh• bourhood of the Carrick Fell complex, the study of which illuminates many Lake District problems. They form a series of patches of bedded grit and shale between Cockermouth and Great Sea Fell. The most important is that which culminates in Watch Hill, near Cockermouth, and they will be termed the Watch Hill Beds. - These peculiar polygenetic sediments attracted the attention of Ward,* who supposed them to constitute a definite horizon in the Skiddaw Slates. The published one-inch map, presumably in pursuance of this interpretation, exhibits them among an extraordinary, and, to the writer, quite unintelligible, network of faults. They do not appear to be mentioned again until 1892, when Mr. J. Postlethwaitet submitted some specimens to Professor Bonney, who described them as containing much quartz and felspar, derived apparently from the waste of a granitoid rock, and fragments of Skiddaw Slate, grit, and acid lava. He also made the important observation that in one slide a fragment exhibited Well-defined micrographic structure. In 1910 Mr. Rastall, in his paper on the Skiddaw granite,t referred to them incidentally as perhaps pre-Cambrian, com• paring them with the Ingleton Slates; and Dr. Marr has suggested the possibility of Cambrian age.§ In spite of the detail with which these grits had been mapped by the Survey, it does not seem to have been generally realised how isolated and irregular the patches are. As the map (Plate 2) indicates, the distribution is incompatible with the view that they are a definite horizon within the Skiddaw Slates. The nature of ,the folding in this northern area is easily determined. From Cockermouth to Roughton Gill and Sea Fell (see Plate 2) the junction-outcrop of the Carboniferous and the Borrowdales, and the junction-outcrop of the Borrow• dales and the Skiddaw Slates are two irregular, gently diverging lines, each about eleven miles long, which, in a general way, are not far from parallel. The average angle of inclination between the two is 10 degrees, and north of Sunderland both are pushed northward by some disturbance. These two lines of

• "Gear. Northern Part Lake District" (1876), pp. 43-47. t The Dioritic Picrite of White Hause and Great Cockup. Quart. Journ. Geol. Soc., xlviii, p. 508. ! Quart. Jour". Ceol. S"c .• lxvi, pp. J21, 122. § .. Geology ofthe Lake District," p. 13. 20 ]. F. N. GREEN ON

junction must be the traces of similar surfaces which do not depart widely from parallelism. There is no doubt about one of these surfaces. The junction• surface of the Carboniferous and the underlying rock is, apart from minor irregularities, that of a stratum dipping at a low angle, from 5 degrees to 10 degrees, swinging round from north-north-west to north. The junction-surface be• tween the volcanics and Skiddaws must be of the same type, but ,the true dips in both the latter are usually high and often quite different in direction. For instance, on Binsey, where some bedded partings between consecutive lavas are exposed, they show dips from 50 degrees to 70 degrees. At the head of Silver Gill the base of the andesites, the Skiddaw Sla.tes, and the grits are all highly inclined. Many of the Burthwaite grits are vertical. Numerous other examples could be quoted, but these are sufficient to prove that the low dip of the lower surface, i.e., that between the Skiddaws and Borrow• dales, as calcula.ted from the map, is a sheet-dip, not a true dip, and that the Ordovician rocks are lying in closely folded sheets, which are gently inclined. The grits with their shale-bands lie in irregular patches, usually higher than most of the surrounding slate, with which however their folding corresponds. The largest forms a ridge running eastward for nearly four miles from the north-east of Cockermouth via Watch Hill to the neighbourhood of Set• murthy. The junction with the underlying Skiddaw Slate is at levels from 300 to 400 ft. except about Elva Hill, where it rises much higher. East of this ridge other considerable areas are seen about Oldpark, Burthwaite Beck, and Bewaldeth, the last not cropping out below 500 feet. Still moving east• ward, there is a gap of a mile and a half in which no grit has been noted. Then a little bit of green gr~t is seen on the top of Castle How, at 1,000 ft., to the south-east of which ,are the thick well-exposed beds of Little and Great Cockup and Burntod, which do not descend more ,than a few feet below the 1,250 contour line. East of Great Cockup small patches are found on Meal Fell and Frozzen Fell at 1,7C;0 ft. The most eastern development of grit, on Great Sca Fell (Fig. 4), is of special interest, as the edge corresponds roughly with the 2,000 ft. contour line, though a western projection falls to the 1,75° ft. line just north of Ellengill. Round the grit-capped crescentic eminence of Great and Little Sca Fell the deep valleys of Silver Gill, the western Thief Gill, Frozzengill, Ellengill, and Swinburn Gill are c.ut in nothing but Skiddaw Slate and andesite, except at the extreme head of the first named, where the grits and slates are seen in contact close to the base of the BQrrowdales. The Watch Hill grits, therefore, do not lie in, but on, the THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 21

Skiddaw Slates, and it is plain that they are touching different horizons, varying from that at Silver Gill, where ,they rest against a part only a few metres from the top, to beds below the fossiliferous band of Ellengill (probably the Didymograptus indentus band), and perhaps still lower strata. They must then be unconformable to the Skiddaws and Borrowdales, occurring as remnants of a folded sheet, the base of which rises from west to east; and this interpretation is supported by their composi• tion, which, as Professor Bonney has already shown, is con• sistent with derivation from the neighbouring series. The WLittle Cockup, Great Cockup, Burntod, and 22 ]. F. N. GREEN ON

Great Sca Fell. There is nothing to distinguish it from the ordinary granophyres of Carrick Fell. Perhaps the most suggestive component of the grits is in• tensely indurated mudstone or "spilosite." This has not been seen west of Great Cockup, and reaches its maximum on Great Sca Fell. The spilosite pebbles, which are well-rounded, unlike the black shale flakes, have the same fracture and buff colora• tion as the Thief Gill spilosites; but under the microscope they show deeper ,tint and more fine silt and clastic mica than the specimens of the Thief Gill rocks available, and they seem more altered. Some are rather spilositic silts, and often contain igneous grains. They are indeed what might be expected from alteration of the stony beds which usually occur about the top of the Upper Skiddaws in this region. The nearest point to the Carrick Fell intrusion at which the grits have been found is above Silver Gill (see Fig. 4). The edge of the outlier can be traced by means of the head under• lying the peat of the fell top, and on searching along it lumps of conglomerate were found. The flakes of Skiddaw Slate in this are often three or four centimetres long. The larger pebbles, half a centimetre or more in diameter, are mainly vein-quartz and felsite, but there is much spilosite and occasional granophyre. Two small pebbles of quartzite, of the type seen in meta• ~orphic regions, have been noticed at Watch Hill and Great Sca Fell respectively. A grain of delicate muscovite-schist or phyllite has also been observed. The provenance of these various materials is as follows :• (I) The rare metamorphics have probably travelled far; whence is unknown at present. (2) The flakes of black shales and silts from Skiddaw Slate, before cleavage. (3) The volcanics from the Borrowdale Series. Most of them resemble ra,ther the upper part of the Series than the rocks now exposed in the vicinity, but there is no reason to suppose that the Rhyolites did not once cover this region. The green matrix is like that produced elsewhere by the denudation of the Borrowdales. (4) Vein-quartz is common in the Ordovician rocks. (5) The quartz-felspar debris of a granitoid rock and the pebbles of granophyre come from the Carrick Fell intrusions. This is clear from their comparatively unworn condition and the decrease in complexity and coarseness on passing westward. (6) The spilosite is only found near the Thief Gill spilosite of the Carrick Fell aureole, from which it doubtless comes. The age of the Carrick Fell complex is thus fixed as earlier than the Watch Hill grits, which must now be correlated. They are folded up with the Skiddaw Slates, and are there- THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. Z3

fore anterior to the Devonian earth-movements j they are un• conformable to the Borrowdale Series. These conditions are fulfilled by the basal bed of the Coniston Limestone Series, described by the writer in the Duddon Estuary area. * This bed has now been traced right across the Lake District between the Coniston Limestone and the Borrowdales, and is everywhere composed, like the Watch Hill grits, of igneous detritus with great variations in thickness and texture. Fossils of recognisable species have not yet been noticed in situ in the Watch Hill Beds, but the specimen of Cybele ovata in the Keswick Museum, from a pebble of similar rock at Sandy Beck Wood, near Cockermouth,t must have come from them. It is preserved in black shale interbedded with green sandstone, both indistinguishable from the interbedded shales and sandstones of Watch Hill. No doubt, therefore, the out• liers will repav search for fossils. But if a line be drawn on the map from the western exposures near Watch Hill to the easternmost outcrop on Great Sca Fell, and continued for a mile and a half, it intersects the curious patch of Bala shales and igneous rocks at the Dry Gills of Carrick Fell. These exposures were discovered by Ward, who discussed them briefly in 1&76. t Noting that they seemed to dip under the surrounding rocks, he considered them an inlier of Skiddaw Slate. He included a detailed map in the Survey manuscript six-inch map dated 1885, showing two bands of lava and a faulted northern boundary. In 1887§ Messrs. Nicholson and Marr announced the dis• coveryof fossils in these beds, and compared them with beds :1t the base of F aleon Crag, . The one-inch map, 101 N.E., was published in 1890, with the boundaries of the shales and associated volcanics engraved on it; but in those copies with which the writer is acquainted the colour of the surrounding volcanics is continued over them.1i In 189zIJ Dr. Marr, on reconsideration, was disposed to refer the shales to the Coniston Limestone Series. In 1895** the fossils were studied by Miss Elles and Miss Wood, who showed that they must be equated with the Sleddale Group of the Coniston Limestone Series, the nearest exposure of which is 20 miles away. The position of the Drygill Shales thus seemed highly anomalous, and Dr. Marr suggested in 1900tt that they

* II Older Palreozoic Succession, Duddon," pp. 17·ZI, t Postlethwaite and Goodchild, Proc. Geol. Assoc., ix (1886), p. 465. ! Quarl. Journ. Geol. Soc., xuii, pp. 16 and 24• • Gtol. Mag., p. 339. 1/ The same accident has hefallen tbe Graystone House Limestone (" Older Palreozoic Succession, Duddon." p. 18), and also an interesting Skiddaw Slate inlier at Scarf Gap. 'lI Geol. Mag., p. lOS. ··Ibid.. p. '46. tt I'roc. G

* Qua1't. 10u1'11- Geol. Soc., lxii, p. !xxvi. t 5ee below. p. 25. : "Geol Northern Lake District," pp. 4~ and 47. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 25 above, pp. 9-10), which belong to the Mottled Tuffs. The resemblance is, however, purely superficial. Indeed, there could not be a much greater contrast than that between the homo• geneous, even-grained, rounded quartz-sandstone of Latter• , with its lapilli, and the variable angular polygenetic grits of the North. A likeness to the Ingleton grits has been noted bv Mr. Rastall,* and there can be no doubt as to the marked'similarity both in hand specimen and in microscopic section. As however will be shown in a separate note, there is reason to believe that the Ingleton rocks themselves are of Bala age. Finally, a minor similarity between the northern grits and the Coniston Limestone Series at Stockdale is the existence of acid igneous rock resting on each. The higher part of Watch Hill, which rises to over 700 ft. O.D., is composed of a columnar felsite, with spheroidal staining, as an outlier about half a mile long from east to west and probably not more than 200 yards broad, though the lack of evidence on its northern boundary makes that edge rather un• certain. The best exposure is in a little quarry in Hay Planta• tion, in which the lower junction has been cut, but is now unfortunately obscured. The rock is much altered, consisting chiefly of quartz and sericite with leucoxene and pyrites, but two generations of felspar can be made out, of which the earlier is orthoclase, so that it is presumably a rhyolite, originally with much glass. The absence of any sign of alteration in shales and sandstones originally not more than half a metre below the actual base of the felsite suggests that it is a lava rather than a sill. It may thus be placed with the well-known Stockdale Rhyolite which rests on the basal bed of the Coniston Lime• stone Series from Stockdale to Wasdale Head; and perhaps the rhyolite of the Dry Gills belongs to the same episode, though more evidence is necessary on this point. The ascription of the Watch Hill Beds to ,the base of the Coniston Limestone Series is, therefore, fully justified, and in consequence the Carrick Fell complex is certainly pre-Bala. Another result which emerges from this discussion is of interest, though not strictly germane to the subject of this paper. It is now clear that the greater part of the denudation of the enormous mass of volcanic and intrusive rocks of the Borrowdale episode was completed in Ordovician times, and that the erosion of the Devonian period was mainly limited to the removal of Silurian material. It follows that the distribu• tion of the lower palreozoic rocks was already asymmetrical when the great folding took place. There are reasons, which cannot be detailed here, for believing that the Borrowdale rocks under the south-eastern part of the Lake District are compara-

• Quart. journ. Ceol. Soc., lxvi (1910), p. 121. 26 ]. ~'. N. GREEN ON

tively thin, and, accordingly, the main resistant mass, after the volcanics had been cut down by the Bala sea, lay where it does now, in the Central Lake District. Under Devonian compression folding proceeded outwards from this great igneous block. Thus the Skiddaw anticlinal axis, which has hitherto been considered the main structural line, is really an axis only of the comparatively simple Ordovician folding, whereas the central axis of the far more intense folding of Devonian time crossed the district in the neighbourhood of and Scawfell. The importance of this line was detected as early as 1849 by Daniel Sharpe.* Another anti• clinal axis must run under the Carboniferous rocks north of the northern limit of exposed lower palreozoics, suggesting another mass of resistant rock there. The Skiddaw axis in Devonian times appears to have been synclinorial rather than anticlinal.

VI. GENETIC. It has now been proved that all the four masses intruded at the Skiddaw-Borrowdale junction are post-Lower Borrowdale and pre-Devonian; and that one at least, the Carrick Fell complex, is pre-Upper Bala. It has next to be seen whether there are grounds for assigning them to any particular episode within the limits set by structural considerations. The alkaline character of the rocks and the prevalence of perthite suggests some connection with the quartz-keraJtophyres of the Borrowdale Series, but such qualities are too common for the comparison to be worth much. There is however a minera• logical peculiarity very characteristic of the Borrowdale rocks and as yet undescribed elsewhere, namely, the sporadic develop• ment of almandine garnet. In a paper read before the Minera• logical Society in 1915,t the writer arrived at the conclusion that these garnets were formed by circulating solutions under high pressure during the solfataric stage of the Borrowdale volcanic episode. One of the best-known cases of gametiferous rock in the Lake District is that of the St. John's Vale granite-porphyry, especiany the eastern, or Threlkeld, part of it. The garnets are usually wen-formed dodecahedra, have the same tint as those in the volcanic rocks, and occur in the same capricious uneven manner. They are oHen associated with pyrites or pyrrhotite, and in a specimen from Bramcrag quarry the writer found a nest of almandine, pyrrhotite, and some opaque heavy black mineral.

~eol. • Qv.a"t. TouTn. SOC' l VI p. no. +.M'J". Mag., xvii, 81, pp. 207-217. THE .~GE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 27

Thus the garnets of the St. John's sill repeat in form, species, distribution, and association, the distinctive marks of those of the Borrowdale lavas, tuffs, and dykes. They should then be assigned to the same causation, and it may, therefore, be inferred that the sill was intruded hefore the solfataric stage of the Borrowdale episode. Macroscopic almandine garnets are also known in the Esk• dale Granite, though rare. But Messrs. Rastall and Wi1cockson* have found garnets plentifully as accessory, having recorded them in five of the six samples examined. The tint varied from colourless to brownish pink, the latter being commoner. It is some confirmation of the writer's view that the garnet is of determinative value that no trace of it was found in anv of nine samples of the Devonian Shap Granite. It is true th~t neither was it recorded in five samples of Buttermere Granophyre, but it is possible that further research might discover the mineral, as E. E. Wal ked found crystals in loose blocks believed to c;;ome from part of the Butt';rmere complex. The inference made in the case of the St. John's Micro• granite may, therefore, be extended to the Eskdale intrusion 7 and there are no grounds for separating the Buttermere and Carrick Fell complexes from their fellows. The whole of the available evidence points to the inclusion of all four in the Borrowdale suite.

VII. THE Q17ARTZ-DOLERITES.

Ophitic dolerites, commonly with rhombic pyroxene, bearing orthoclase and quartz in small proportion, are rather frequent in the Lake District, but do not appear to be found above the Bala unconformity. They show no differences from the similar dolerites in Ordovician rocks in other parts of Britain. In dealing with the eastern part of the Lake District the writer left the relations of the Haweswater quartz-dolerite, the largest of the isolated masses, uncertain, though pointing out that its outcrop resembled that of a folded sill.j Since the publication of that paper specimens have been noticed which show that parts of that intrusion were affected by cleavage. Considerable masses of similar quartz-dolerite occur as pro• ducts of differentiation among the Carrick Fell series of intru• sions and in the igneous complex of Burtness Combe, which is

• The Accessory Minerals of the Granitic Rocks of the English Lake District, Qua.t. Jou,,,, Gfot. Soc., lxxi (19,6), p. 592. t Qua.t.Jo"..... G'ot~ Soc., Ix (1904), p. 85. t P.oc. Geot. Assoc., xxvi (191S), p. 215. ]. F. N. GREEN ON

an offshoot of the Buttermere granophyre.* Now that these must both be referred to the Borrowdale episode, there can be no hesitation in including with them all the dolerites of similar character found in the Borrowdales and Skiddaws.

VIII. THE AGE OF THE BORROWDALES. The date of .the Borrowdale volcanic episode has usually been stated as Llandeilo, and it has been supposed to have con• tinued during the whole of the Llandeilo (sensu lato) and Lower Bala periods. The proof that there is a powerful and wide• spread unconformity at the base of the Coniston Limestone Series has given a changed aspect to the evidence. There is no intrinsic reason to suppose that the Borrowdale volcanic pile, thick though it is, took any long time in a geologi• cal sense to build up. The lavas may be almost neglected, as there is no evidence of any but short pauses between successive flows; the coarse Harrath Tuffs must have accumulated rapidly; and the only part of the series of chronological weight is the band of Middle Tuffs, averaging perhaps 700 ft. thick over the main area, but decidedly thinner near the Duddon Estuary, where there is less coarse material. This thickness has certainly been much increased by the compression which the band has undergone. This compression can be measured in the "birdseye slates," in which the"eyes," being now similar and similarly situated ellipsoids, must have originally been spheri• cal. t In an average specimen from Wrengill, at the head of Long Sleddale, the ellipsoids have semi-axes approximately I: 3 : 5, equivalent to an expansion along the cleavage dip (nearly vertical) of over 100 per cent., if the volume is supposed unchanged. This supposition is of course not correct, and something must be allowed for packing. It is probably sounder to assume that the intermediate semi-axis, being horizontal and perpendicular to the direction of lateral pressure, remained unaltered, in which case the volume was diminished to five• ninths the original, and the extension along the cleavage dip was 667'3 per cent. The deformation is much less in the coarser hands) but even so the 700 ft. of tuff could hardly have been more than 500 ft. originally, a thickness which, in the neighbourhood of volcanic cones, would be laid down on the \iea-floor with rapidity. t Thus the mere thickness of the Bor-

• Rastall.Quart.Jou..... Geol, Soc•• b:ii (1906), pp. 262. 263. f See MUltge. N. J. fur Min. (1893), BE. viii, pp. 6S3-tS8. t For the swift erosion of modern volcanic ",aterial. cf., Anderson and Flett, l'hil. Trans. Roy. Soc.• Ser. A, cc ('903), PP' 429-442 and .87. THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT. 29 rowdales, perhaps between 4,000 ft. and 5,000 ft. in the central region, does not necessitate a long period of deposition. They have now been shown to rest everywhere conformably on the Upper Skiddaw Slate. At some depth below this junction is a persistent graptolitic band, that of Didymograptus indentus, appertaining to the bifidus-zone of the Llanvirn. In the Lake District proper the only fossils yet recorded in the intermediate "Blue Shale" appear to be CaTyocaris and lingula __ but fossils have been found near the volcanic base in the Cross FeU area which were assigned by Goodchild to D. mUTchisoni, as part of the fauna of his" Milburn Beds." This name was instituted to include certain supposedly interbedded slates and tuffs, but it was ascertained during a visit by the writer in 1912 that the apparent succession was deceptive, the "Blue Shale" and Flagdaw type of the Mottled Tuffs succeeding one another exactly as in the Eastern Lake District,* but folded up so as to be repeated on traversing across the strike. The character of the folding is weli displayed in EIIergiIl. The" Milburn Beds" must therefore be abandoned, and, as Miss Elles has expressed the opiniont that the supposed Skiddaw Slate D. murcltisoni is in reality a large form of D. bifidus, there is reason to believe that the Borrowdales rest at all points on sediments belonging to the bifidus-wne. This reference is supported by the wide distribution of volcanic rocks at the junction of the bifidus and Murcltisoni-zones wherever they have been accurately discriminated. Their existence in North and Central Wales is well known. The volcanic mass on Ramsey Island, near St. David's, rests on biftdus-shales,! and Dr. A. H. Cox has lately described a series at Abereiddy, in Pembrokeshire, which repeats many of the lithological varieties of the Borrowdales, including tuffs (the D. murchisoni .liSa which" marks exactly the top of the D. bifidus-zone "§) closely resembling the "Mottled Shale" of the Duddon Estuary, quartz• keratophyres,/! and quartz-dolerites, often with rhombic pyroxene. ~ It seems probable that the tuffs of the Mount Partry beds of Mayo** are on the same horizon. As a result the Borrowdale Series is ascribed to the Middle Llanvirn and regarded as the eastern or south-eastern portion of the central mass of a widely spread volcanic area of that date. The Eskdale granite, Buttermere granophyre, St. John's granite-porphyry, and Carrick Fell complex all belong to the suite, being intruded before the solfataric stage, but at a late• period of the episode.

* Proc. Geol. Assoc., nvl ('9'5), p. 206. +Qua,t. .ToUffl. Geol. Soc., Iiv (,8g8), p. 467. 1 Proc. Geol. Assoc., xxII (,gIr), p. '38. f Quart.fou.... Geol. Soc.,lni ('9'5), p. 30'. I! Ibid., p. 3'9. ,. Ibid, pp. 322 to 330• •• Gardiner and Reynolds, Qua,/.Journ. Gtol. Soc., )xv ('909), pp. '07-'09.~ 30 THE AGE OF THE CHIEF INTRUSIONS OF THE LAKE DISTRICT.

Special thanks for kind assistance are due to Mr. J. W. Rhodes and to Mr. J. Postlethwaite, whose ready help is always available to anyone interested in Lake District geology. Much of the work has been directed by study of the six-inch maps, published and manuscript, of the Geological Survey, to the Library staff of which the writer is greatly indebted for constant kindness. Mr. G. Rarrow has kindly read through the manu• script and suggested some improvements.

ILLUSTRATIONS. Plate I.-Wilton to Borrowdale. Plate 2.-Cockermouth to Carrick Fell. Fig. I.-Index Map of part of the Lake District. Fig. 2.-Faults at Devoke Water. Fig. 3·-The 51. John's Vale intrusion. Fig. 4.-The N eighbourbood of Great Sea. Fell.