On the Physical History of the English Lake District. with Notes

110 Rev. J. Clifton Ward—Geology of the Lake District. also wanting, unless we regard the presence of large boulders at high levels, as the diallage blocks of Crousa Down for instance, as the unremovable debris of an old glacier system, and ascribe the presence of large boulders, at some distance from their parent rocks, in river gravels, to the relics of moraine, carried down to successively ^ lower levels in the excavation or deepening of the present lines of drainage. However, if, as I agree with Mr. Godwin-Austen in thinking (op. cit), the land stood at a much greater elevation during the Glacial epoch, a great and constant snowfall may have given rise to local glacier systems; and as the present area of the county would offer little more than the generative sources of the (imaginary) glaciers, all traces of pre-existent deposits and of moraine matter, except very large boulders, would be swept down by the flood waters of the succeeding period of subsidence to levels now submerged. But as all such glacial theories are purely hypothetical, it behoves one to fall back on the probability that Cornwall, during the Glacial epoch, stood at a much greater elevation, and that its highlands were crowned with constant snows, the melting of which during the succeeding amelioration, accom- panied by subsidence, caused the liberation of great quantities of surface water with torrential power carrying off the pre-existing detritus to lower lands, now submerged. (To be continued in our next Number.) I beg leave to correct the following errata in Part I. of my paper. On p. 33, line 40, for "10to20" read " 10 to 12;" on p. 34, line 20, for " B.c. 400" read "B.C. 440," and for "e/ctras" read " iovaat," also for " yriaat" read " " and on p. 79, line I, for " on " read " or."—W. A. E. U.

III.—ON THE PHYSICAL HISTOET OP THE ENGLISH LAKE DISTEICT. WITH NOTES ON THE POSSIBLE SUBDIVISIONS OF THE SKIDDAW SLATES.1 By the Key. J. CLIFTON "WARD, P.G.S., F.E.M.S. Post- Carboniferous Periods. As the Carboniferous rocks form a framework around the Silurian mountain country, so do the Permian rocks for considerable distances lie as an outer flat and bevelled edge to the slightly raised Carboniferous frame, the edge indeed overlapping the frame itself along the west coast-line between St. Bee's Head and the Duddon Estuary. It is not purposed to give a detailed description of any Post- Carboniferous formations lying outside the mountain district. The two thick Permian sandstones (the Penrith and St. Bees), separated by a set of shales and marls, are believed by some to indicate the conditions of an inland sea, and the formation in some parts of its English development to suggest cold or glacial conditions. How- ever this may be, it seems more than probable that during the whole of the Permian Period the present Lake District was dry land, and undergoing that subaerial waste which, long continued, was to give 1 Concluded from the February Number, p. 61.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 Rev. J. Cliflon Ward—Geology of the Lake District. Ill rise to the present beautiful forms of mountain and valley. If the Permian were partly a cold period, then an early set of glaciers may have found their home among the early Cumbrian mountains, and any amount of ice-smoothing and polishing of the rocky surface then produced must, to a certain degree, have hindered the work of denudation for a long period afterwards. Indeed, it is more than likely that the very large amount of denudation produced by glacial means during a glacial period is more than counterbalanced by the retarded action of subaerial forces working upon a generally smoothed and rounded surface. In post-Permian times, when extensive north and south movements and dislocations occurred (as along the foot of Cross Fell range, and the Pennine Anticlinal), sym- pathetic movements may have taken place in the mountain nucleus, and very likely the majority of the N. and S. faults ranging through the Silurians, and frequently shifting the previously formed E. and W. ones, were produced at this period. As we pass in review the periods succeeding the Permian, nothing more can be said with regard to the history of the Lake District than that in all probability the Cumbrian centre remained an area of dry land, more or less lofty, though slowly being eaten down and into by the forces of denudation. From the mountains as they then existed one could have looked northwards and southwards into areas of water beneath which the New Bed deposits were being formed. Subsequently the Liassic sea with its marine monsters probably skirted the mountain district at no great distance. Onwards in time through the Jurassic Period, with its Australian- like climate, during which the Cumbrian mountains may have experienced many a storm of semi-tropic violence, through the long Cretaceous Epoch and Tertiary times must the District have been suffering wear and tear, at times doubtless nearly or quite united to the neighbouring high ground of the Pennine Range and of Wales. No wonder so much has been carried away by apparently weak subaerial powers of denudation, rather the wonder is that the mountains should have been preserved at all, instead of all being levelled to the sea. This will again be commented on in following pages, when the question of the numerical equivalents of periods will be dis- cussed. That the District experienced many vicissitudes of climate throughout this enormous length of time seems certain—now tempe- rate, now semi-tropic, and now again probably glacial, and the last marked geological change which it underwent was during the so- called Glacial Period proper. Glacial Period. This period having been treated of in full, as regards the Lake District, in my former Papers, and in much detail in the Survey Memoir, chap, xiii., nothing but a mere necessary outline will be now given. The glacial phenomena of the district are as follows:—Till, • mainly the product of a confluent glacier-sheet. Drift gravel, and Stratified sand and gravel, often occurring in the form of eskers, due

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 112 Rev. J. Clifton Ward—Geology of the Lake District. principally to marine action. Boulders carried far from the parent- rock by glacier-ice at one time, and floating-ice at another. Glacial scratches partly produced by a confluent glacier-sheet, partly by small separate glaciers; also occasionally made by floating-ice during the period of submergence. Moraines left by an early set of glaciers, much modified by subsequent aqueous action; and more perfect moraines left by the most recent local glaciers. Many years study of these phenomena has led me to draw the following genera] conclusions, which I take in part from my Survey Memoir, p. 97 : " At the commencement of the cold period small glaciers occupied the heads of the various valleys, and, as the cold continued and increased, they became larger and larger, until, in many cases, they united, overlapping the lower ridges parting valley from valley, and forming one great confluent ice-sheet, the movement of which was determined to the north and to the south, or east and west—as the case might be, in different parts of the district—by the main water- parting lines. A great quantity of rocky debris was moved onwards and left scattered over the country, partly by the first formed moraines being pushed forward, and partly by the ice overriding the same and dragging on and triturating the fragments beneath it. In this way the Till was formed, sometimes left in rock-sheltered upland hollows, but most largely deposited on the lower and less inclined ground. " Whether this first land-glaciation was interrupted by one or more mild periods, the deposits in this district do not prove. As the final close of this epoch of intense glaciation drew on, moraines were left by the retreating glaciers plentifully scattered in every valley, but the glacial streams and rivers must have made much havoc amongst them, cutting them up and bearing away their material to lower levels. "Then, when the cold had disappeared, began a submergence of the district to a very considerable extent As the land sank, the old moraine material was sifted, sorted, and partly rounded. At the ends of some of the fiords or straits, sand-bars were formed; but, as there was no floating-ice during the earlier stages of submergence, these sand and gravel deposits enclosed no large boulders. The district became gradually converted into an archipelago, and currents circulated among the islands. When depression had gone on to the amount of 1,000 feet or less, the cold returned and ice-rafts bore blocks from one part to another.1 In many cases the direction in which currents swept the floating-ice was the same as that of the old glaciers, and thus boulders were transported along the same course at different periods and by different means. Sometimes, however, or at certain parts, marine currents bore floating ice, with its boulders, in directions opposed to, or much at variance with, the old glacier courses. Thus, when the land stood about 1,200 feet lower than at present, a current, sweeping the north-western out- 1 For maps showing probable form of land at different stages of submergence, see papers by the author on " Lake District Glaciation," Quart. Journ. Geol. Soc, yol. xxix. p. 422, and vol. xxxi. p. 152.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 Rev. J. Clifton Ward—Geology of the Lake District. 113 skirts of the district, carried boulders from Sale Fell southwards on to Broom Fell. Not until the submergence reached over 1,500 feet was there any direct communication between the northern and southern halves of the Lake District, except by the straits of Dunmail Raise. Under such conditions a current very probably ran through those straits from south to north, turning mainly to the east on reaching Keswick Vale, though probably sending a branch off to the west; while other currents may have set through the straits between Skiddaw and Blencathra. The case mentioned of an ash boulder at the upper sources of the Caldew would seem to point to a current having at one time passed from south to north, up the Glenderaterra Valley and down through that of the Caldew. The block could not have reached its present situation from any of the volcanic deposits lying north of Carrock and Comb Height, and is scarcely likely to have come from the very limited ash exposures of -Eycott Hill, to the south-east of the Caldew at Mosedale. It is not likely that this boulder could have been transported by glacier-ice or any form of ice-sheet, because it is in the very midst of lofty mountains which would have produced sufficient ice to have filled the valleys between them and kept out any ice-sheet foreign to this group. Hence, I am inclined to consider this case as a proof of submergence to the height of at least 1,300 feet, and of the existence of marine currents passing through the Skiddaw mountain group. " The submergence continued until the land must have sunk more than 2,000 feet below its present level, as the position of boulders in many parts of the district seem to show, and notably those on Starling Dodd (see page 93, also Quart. Journ. Geol. Soc. vol. xxix. p. 437, and vol. xxx. p. 96). Then the whole district was represented merely by scattered islands clad in snow and ice, each a little nursery of icebergs. " As the land was re-elevated, the glaciers crept down to the level of the sea, sometimes forming moraines just at the sea-margin, as was the case beneath Wolf Crag, Matterdale Common, when the land stood 1,400 feet below its present level. During all this time numerous boulders were let fall upon the early-formed mounds of sand and gravel, and as the sea shallowed, more of such mounds were deposited, now, however, frequently containing ice-borne blocks. " Finally, when the land had regained its former height above the sea, glaciers still lingered in the recesses of the mountains, but this second set of glaciers at no time equalled the first in size. Some, in Borrowdale, were sufficiently large to creep down probably as far as Eosthwaite, and the more or less perfect moraines in every upland valley remain as the last traces of the Glacial Period. "I think too that the immediate cause of the numerous lake- hollows, many of which are now completely filled with stream- borne detritus, was the onward movement of the glacier-ice when at its thickest, as suggested by Professor Ramsay's theory; for not only are these lake-basins extremely shallow when compared with the heights of the mountains and the thickness of the former ice- sheet, but in most cases the agreement is remarkable between the DECADE II.—VOL. VI.—NO. III. 8

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 114 Rev. J. Clifton Ward—Geology of the Lake District. spots at which the greatest depth of water occurs and those points' where, from the confluence of several ice-streams or from a narrow- ing of the valley, the onward pressure of the ice must have been greatest.1 "I have sometimes been asked whether the glaciation described in mv papers on this district was not wholly belonging to the close of the Glacial Period, and whether previously the whole mountain district had not been overridden by a great ice-sheet from the north ? To this I answer that, so far as I am able to speak, there is no evidence in the district of such a mountain- and valley-ignoring sheet, and not one boulder of foreign northern rocks to be found among the mountains. Therefore the burden of proof lies with those who advocate this theory." Post-Glacial Period. "With times subsequent to the Glacial Period we have here but little to do, though a few remarks may not be out of place. I have often been struck with the scarcity of delicately poised perched boulders, and cannot but think that this may be partly due to shocks of earthquake in post-Glacial times, of sufficient severity to dislodge any such. The country is so thoroughly glaciated, that their absence is all the more conspicuous. But I fancy there are also other indications of earth-movements. There are several cases of fissuring upon mountain summits, and even of fissures crossing mountain ridges, which would appear to be the work of earth-movements more general than the slips so often occurring upon mountain slopes. Thus, on the summit of the Screes Mountain, and Kirk Fell, Wastdale; upon and distinctly crossing the narrow ridge of High Stile, between Buttermere and Ennerdale: at Eosthwaite Cam, Borrowdale, and Helm Crag, Grasmere, are instances of fissures, many of them deep, long, and more or less parallel to one another. In some of these cases there is no appearance of slipping upon the mountain flanks, and I cannot but think that at least some of them are the result of the passage of earthquake waves. Since the close of the Glacial Period, the atmospheric denuding agents have been much occupied over many' areas in the work of clearing away glacial debris, such as once again opening out valleys partly choked by glacial drift—example, the valley between. Threl- keld and Keswick. In other parts the denuding agents have had even harder work in roughening the ice-smoothed surface, and one may confidently say that the general contour of the country as we now see it can be but very little different from that which prevailed immediately anterior to the Glacial period. " Stone implements of two kinds have been found in the mountain district; some are of the smooth and polished neolithic type similar to the stone axes from Ireland, figured in Sir J. Lubbock's Pre- Historic Times, at page 88 (2nd edit.); others are generally longer, more narrow in proportion to their length, and have a roughly 1 See papers by the author.

PART II.—CHRONOLOGY OF THE LAKE DISTRICT. I wish now to make an experiment, namely, to try whether, by using the most reliable estimates yet made, (1) upon the rate of deposition, (2) upon the rate of denudation, it be possible to draw up a Chronology of the Lake District to place beside the ascertained facts of History. I am sensible how uncertain are many of the data, yet it is well for the geologist occasionally to take stock of time, so far as he is able—that element in which he deals oftentimes so freely, and at times even recklessly. For my purpose I shall avail myself of my friend Mr. Lloyd Morgan's article upon " Geological Time " (GEOLOGICAL MAGAZINE, 1878, p. 199), in which he has conveniently brought together the estimates of various eminent geologists as to rates of deposition and denudation. Skiddaw Slate Period.—The deposits of this period, if not alto- gether deltaic, are not far removed in character from such. We have seen how large a proportion of them must have been laid down in shallow water. If then we take TV of an inch per annum as the average rate of deposition for deltaic sediments, and allow that 1 I may here mention that this museum has been started with the hope of illus- trating, as completely as possible, the Natural History of the district. Any help from naturalists visiting the country would be most gladly received, as the working resources are somewhat limited. 8 I have placed in the Keswick Museum of Local Natural History a map of the District on the 1-inch scale, upon which are marked in different colours and patterns (1) all pre-historic remains, (2) Roman remains, (3) camps, etc., of uncertain age; and a detailed account of these will be found in a paper entitled " Notes on Archaeological Remains in the Lake District," published in the Transactions of flie Cumberland and Westmoreland Antiquarian and Archaeological Society, J878, p. 241. : * Geology of the Northern Parts of the English Lake District, by the Author.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 116 Rev. J. Clifton Ward—Geology of the Lake District. distance from the actual area of a delta means a decreased rate of deposition, we may assume perhaps that the 10,000 feet of Skiddaw Slates were laid down at the rate of ^V of an inch per annum, and this would give a period of six million years for the formation of the whole series. If, however, we take the rate at tV instead of -fa, the period is reduced to less than 1^ million. Volcanic Period.—No reliable estimate can be made of the rate of deposition of volfcanic material. The length of the periods of repose between successive eruptions appears to be very variable. In this particular case, however, there is nothing to lead one to suppose that the eruptions were not tolerably continuous. There seem to be no great breaks in the series, and, for aught we know to the contrary, the whole thickness of some 12,000 feet may represent continuous and violent action. Hence I am inclined to put the rate of accumu- lation at not less than -fe of an inch per annum, which gives a period of \\ million years. If we take i as the rate—as Mr. Morgan has assumed in the case of Etna—the length of the whole period is reduced to 720,000 years. The gap between this and the next period might be put down at ^ a million of years, perhaps some 500 feet having been removed at the rate of 1 foot in every thousand years, the denudation acting upon a sinking area. Upper Silurians of Lake District.—The same remarks may be made on this series of sedimentary deposits as on those of the Skiddaw Slates. Although in great part they must have been formed in tolerably shallow water, yet might the deposition have gone on more or less outside the limits of a delta proper. Taking therefore the rate of deposition at gV of an inch per annum, a thickness of some 14,000 feet of beds may represent a period of about &| million years. Old Bed Period.—Here we have not facts of deposition to calculate from in this district, but those of denudation. What length of time must have elapsed between the close of the Upper Silurian and the commencement of the Carboniferous, to allow of the removal of probably more than 20,000 feet of rock, so that the Mell Fell con- glomerate could be deposited transgressively upon both Volcanic Kocks and Skiddaw Slates ? The length of time must have been great indeed, for the extensive denudation necessitates also a great amount of. continued elevation to allow of the deeply buried strata coming within the range of the denuding powers. A planing action is required to cut off the rising land along the main axis of upheaval; and the sea alone can supply us with this. The work, in fact, would be most efficiently done if we could suppose the elevation to be going on but very little faster than the sea could gnaw and eat the land down. Now are we warranted in assuming that the rate of denudation under such circumstances would be greater than in the case of a tract of country lowered mainly by subaerial agencies? I think we are. Prof. Geikie has calculated that the area of country drained by the Mississippi is being lowered at the rate of one foot in 6000 years, while that drained by the Po is being lowered at one foot in 729 years; the Ganges, Hoang Ho, Ehine, Danube, and Nith

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 118 Rev. J. Clifton Ward—Geology of the Lake District. within the last million of years sank deeply beneath the sea, and was re-elevated without, apparently, any long periods of rest, and therefore without having left upon it many well-marked lines of marine action. Moreover, supposing Mr. Croll's theory to be the right one as to the cause of the Glacial Period, and taking the time indicated by that theory, this submergence and re-elevation must have taken place in some 200,000 years. Now, from what we know of such movements at the present day, the rate of two feet per century would appear high, and this rate would allow of no halt. There might be plenty of time for the District first to become shrouded in confluent glacier-ice, and then to be depressed and re- elevated under changing climatal conditions ; but I must acknowledge that the submergence to such an amount is somewhat embarrassing. Post-Glacial Period.—According to Mr. Croll, about 13 feet may have been removed from the general surface of the country since the close of the Glacial Period (i.e. in 80,000 years). I doubt very much whether, taken all round, more than six feet has been thus removed in the same period over the area of the Lake District mountains, and this would be at the rate of one foot in every 13,000 years. The ice-action is yet so evident over large tracts, that I question whether even this may not be too high an average. The Skiddaw Slate mountains with their greater softness may, however, more than make up for the unyielding nature of the hard ice- sinoothed volcanic rocks. General rate of subaerial denudation in the Lake District.—The low rate of denudation just now assumed for the period since the close of the Glacial may be partly put down as resulting from the polished state of the country, but that the general rate over so small a mountain district must have been low is, I think, very probable. If an accurate model of the Lake District be examined, it will be seen that more material has been removed in the formation of the valleys than has been left to form the mountains. The mountain district might, indeed, as well be described as a valley district, so close to one another are the several valleys, and so narrow the mountain ridges separating them. The question then arises, at what average rate has denudation been carried on since Carboniferous times ? Is the amount of denudation effected ridiculously small for the amount of time supposed to have been spent upon it ? or the reverse ? At the close of the Carboniferous Period we must, I think, believe the area under description to have been elevated as a more or less lofty table-land, constituting the rough-hewn block out of which such a valley-system as that we now see has been carved. The height of that table-land we cannot indeed accurately estimate, but it may well have been higher than the height of the highest mountains as they now stand. This is not, however, a necessity, for much denudation may have been effected, ere, in times long after the close of the Carboniferous, the base of the country, so to speak, had been raised to its highest level, or, in other words, ere the movement of elevation had finally ceased. Still, I think we may assume,

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 Rev. J. Clifton Ward—Geology of the Lake District. 119 •without much affecting the result, that at the close of the Carbon- iferous Period, the Cumbrian nucleus was elevated to its greatest height as a table-land with slight ridges and furrows, the early promise of mountain and valley. Uniting the summits of the chief mountains by means of a plane, we restore some such plateau; but as it is certain that the denuding agents would lower the country generally as well as along certain lines of valley, we must place the original plateau at a higher level than any plane now joining the mountain tops. Now the highest mountain, Scafell Pikes, is a little above 3,200 feet in height, and there are several others over 3,000 feet. Might we not suppose that the original plateau stood at 300 to 500 feet higher than the present highest point? If so, and we can form an idea of the present mean level, we get the thickness of rock removed over the area of the district in the work of scenic elaboration. If we take the Keswick one-inch sheet, we find that of a list of 47 mountain summits in this sheet, there are 34 above 2000 feet in height, and 18 above 2,500 feet. Probably the mean level is not less than 1,500 feet, and not much greater than 1,800 feet. Hence, if we assume the height of the original plateau to have been 3,500 feet (which may very likely be under the mark), and the present mean level to be 1,500 feet, we have to account for the removal over the area in question of some 2,000 feet of rock by purely subaerial agents. It should be borne in mind that none of the rocks thus removed are calcareous to any degree, and therefore they are not readily soluble in water; still the wonder remains that, if the original plateau did not much exceed 300 feet above the present highest point, more has not been effected by denudation during that long period from the close of the Carbon- iferous to the present, for the actual amount denuded seems insig- nificant indeed in comparison with that removed from the same district during the Old Red Period. Still, the changed conditions may make all the difference; in the latter case—that of the Old Red—conditions the most favourable for denudation may have prevailed—land rising scarcely faster than the rate at which the sea could plane it down; while in the case of Post-Carboniferous ages, the conditions are those of a small tract of only moderately high ground containing no soluble rock material, and eaten down and into by subaerial agencies. If the basin of the Mississippi be lowered by such agencies one foot in every 6,000 years, are we likely to err much if we assume that this small mountain area was denuded at the rate of one foot in every 10,000 years ? Calculating on this supposition, the 2,000 feet removed, in the formation of mountain and valley out of the rough-hewn block provided at the close of the Carboniferous epoch, would represent a period of some 20 million years. In our former calculation of the length of time represented by the formation of of 4C,000 feet of Post-Carboniferous strata, we arrived at a period of 24 millions of years. Summary. — The foregoing time-estimates may now be placed together in one column, and in parallel columns the rates of deposition or denudation which have been assumed.

CHRONOLOGICAL SUMMARY.

Million of years. Sate of Deposition. Rate of Denudation.

Skiddaw Slate Period THJ in. per annum. Volcanic Period tV in- per annum. Possible Gap lft. in 1,000 years. tipper Silurian of Lake District in. per annum. Old Eed Period 15 1 ft. in 750 years. Carboniferous Period ( 12,000ft. @ -Ar in. perann. 61 \ 8,500ft. @ ^yin. „ Post - Carboniferous Periods 24 fg in. per annum.

62 The above result may, after all, in itself be of little value; but if it helps to make one realize the respective magnitude of the various operations which the Lake District has undergone in the course of its long history, one main purpose will have been served. Whether the Old Ked Period was one of such duration as here indicated or not, its length, in respect to the other formations, is probably shown •with some degree of truth. Whether 60 millions of years have elapsed since the beginning of the Skiddaw Slate Period or not, it is more than likely that the time which elapsed between the first laying of the foundations in the early Skiddaw Slate sea and that time when th*e roughly-hewn block of country was turned out at- the close of the Carboniferous epoch, for the tracing of scenic details upon it, was greater than the time consumed in the actual formation of our present mountain and valley system by subaerial denudation.

PAET III.—NOTES ON THE POSSIBLE SUBDIVISIONS OF THE SKIDDAW SLATES. Introduction.—In making some observations upon the correlation of the Skiddaw Slates with the Silurian rocks of Wales, I wish to bear in mind the following axioms. (1). Physical evidence, such as character and thickness of deposits, can be taken as no absolute guide in the correlation of distinct groups of strata separated from one another by considerable distances. (2). A general sequence of similar deposits in two distinct but not widely separated areas may be held to indicate a succession of similar conditions prevailing at or near the same period of time. (3). The evidence of fossils as to the age of a deposit is generally trustworthy when it is grounded upon assemblages of life, and when corroborating evidence is to be gathered from the formations above and below. (4). In determining the age of a formation due weight should be given to both physical and palseontological evidence, but neither should be relied on exclusively. Applying these axioms to the case of the Skiddaw Slates, I wish to see whether the palseontological dictum as to their Lower Llan- deilo or Arenig age can be unhesitatingly supported.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 Rev. J. Clifton Ward—Geology of the Lake District. 121 First, I would remark that this name—indicating the supposed age and the equivalent in Wales—has been given to the whole thickness of some 10,000 feet. Second, that this opinion is founded almost entirely upon the presence of certain groups of Graptolites. Third, that Graptolites are notably capricious in their distribution— witness in the Lower Silurians of Scotland the immense thickness of non-graptolite bearing and nnfossiliferous strata associated with the one or two black shale bands bearing Graptolites. Fourth, that in the Skiddaw Slate Series as a whole there are several alternations of deposits varying much in character—an upper black slate series, a marked grit or gritty series, again a black and irony series of slates, passing down into a great thickness of sandy and gritty beds. Fifth, that the majority of the Graptolites have been found in beds above the upper grit, but that they also occur in those below the grit, where the sediment seems to have been favourable for their existence; while there are a few cases apparently of their occurrence even among some of the flaggy beds of the lower sandy and gritty series. With these preliminary remarks, I will pass, first, to the consider- ation of physical evidence as to subdivisions and their possible equivalency to Welsh Lower Silurians. Points of Physical Evidence.—I had been working for some years upqn the Lake District rocks, when Prof. Eamsay called me into Wales to trace a base to the Arenig series marked by a thin grit. In carrying out this work, I was often struck by the general resemblance in sequence of the deposits both above and below the grit to that of a like series in Cumbria, and on returning to the Lake country examined more closely the characteristics of the various . parts of the Skiddaw Slates. The resemblance in physical character between the Arenig grit of Wales and the upper grit of the Skiddaw Series is perfect. With one or two local exceptions, the Cumbrian upper grit is thicker than the Welsh Arenig, but both are made up of similar materials, both vary much in coarseness, though almost always presenting some coarse gritty or conglomeratic portion, and both are much traversed by quartz strings. The constancy of the Arenig grit in N. Wales, though often not exceeding 12 or 20 feet, is surprising, and the fact is at any rate worth recording that among strata of somewhat parallel age in Cumbria there occurs a like grit. Moreover, in Wales the Arenig slates above the grit soon become mixed with volcanic deposits, submarine volcanic ejecta, and in Cumberland we have in the northern or Skiddaw district a considerable thickness of black Graptolite-bearing slates succeeded by a Volcanic Series, and in the south-western part, at Lank Eigg and Latterbarrow, the same Volcanic Series lying directly upon the grit. (See also previous remarks on this subject, page 61.) Turning to the beds below the grit, we have in Wales the iron- stained Tremadoo Slates underlaid by a great thickness of flaggy, sandy, and gritty beds, the Lingula Flags. In Cumbria there is a like physical sequence; beneath the grit come black iron-stained slates, often quite indistinguishable from the Tremadocs of North •Wales, underlaid by a great but unknown thickness of flaggy, sandy,

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 122 Rev. J. Clifton Ward—Geology of the Lake District. and gritty beds. If reference will be made to the Horizontal Section through the Skiddaw Slate Series, No. 2, page 54, the course of which is marked in the accompanying Sketch-map, the sequence and lie of these subdivisions will be clearly seen. The central range of high mountains along this section shows the lower sandy series in their fullest development, the physical equivalents—as it would appear from above—of the Lingula Flags; and the black slates beneath the grit at either end of the section (shaded more darkly)— physical equivalents of Tremadocs—come in as a faulted synclinal in the valley between Whiteside and Grasmoor, the latter mountain presenting a very exceptional example of a summit formed by an anticline.1 Eeferring to Horizontal Section No. 1, page 54, it will be seen that the grit of Great Cockup dips southwards under the mass of Skiddaw (see also Map), and only reappears in an anticlinal fold on the south side of that mountain. Thus, the area of Skiddaw and surrounding mountains is mostly made up of the slates above the grit, the physical equivalents—and in this case the palasontological equivalents also—of the Welsh Arenigs or Lower Llandeilo. In a set of rocks so highly contorted as are those of the Skiddaw Slates, taken as a whole, it is often impossible to detect and trace the faults; but if my reading of the district be at all correct, there must, I think, be some large N. and S. fault or series of faults separating the tract of Skiddaw, etc., from that W. and S.W. of Derwentwater and Bassenthwaite Lake. This I have indicated in the Sketch-Map. That there are many such N. and S. faults of great size is proved by the frequent shiftings of the grit west of Great Cockup. In Hor. Sect. No. 1, page 54, a large strike-fault throws down the Vol- canic Series against beds far below the grit, and while this is very generally the case for many miles west of Carrock Fell, yet is there clear evidence at several points of the passage upwards into the Volcanic Series of the higher beds of the slates above the grit To sum up the physical evidence : while fully acknowledging the frequent inconstancy of physical characters, there does seem to be in this case of Cumbria and Cambria, some 60 to 80 miles distant from one another, a striking sequence of similar deposits, such, indeed, as to lead me to infer, so far as physical evidence can do so, that in Cumbria we have the following groups represented which are more or less equivalent to those of Wales. Arenig Slates—True Skiddaw Slates. Arenig Grit. Tremadoc Slates, Lingula Mags. It is impossible everywhere to define the limits of these groups, so contorted and faulted is the District; but that they exist as generally definable subdivisions I have myself no doubt. It would seem— as already called attention to in the Survey Memoir—that the lowest 1 This may be well seen looking up on to Grasmoor End from Crummock Water. In Horizontal Section No. 2, the upper black slate should have been represented as conformable to the underlying grit of Elva Hill,

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 08:08:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800170086 Rev. J. Clifton Ward—Geology of the Lake District. 123 sandy series of beds so largely developed in Grasmoor and Whiteside, either thin eastwards or have their place taken by less sandy and more slaty deposits in this direction ; this, combined with excessive faulting, and the difficulty in tracing the faults, owing to contortion and cleavage, makes it exceedingly hard to draw the actual boundaries of these groups in many places. Points of Palceontological Evidence.—In assigning the whole thickness of the Skiddaw Slates to the Lower Llandeilo, and that mainly on the strength of the Graptolite fauna, Axiom (3) is distinctly transgressed in both its clauses, for the Graptolite family cannot be taken as a characteristic assemblage of life in general, and the formation above (the Volcanic Series) is unfossiliferous, while nothing is known of that below. Still, the group of Graptolites being such as is known to be characteristic of rocks of Lower Llandeilo age elsewhere, is evidence of considerable value, though not to be too exclusively relied on; but when the palaeontologist says further that the whole of this Skiddaw Slate Series must be L. Llandeilo because Graptolites do occur throughout, or nearly so, I for one am inclined to demur. Could a L. Llandeilo assemblage of fossils be shown to prevail throughout the series, the case would be different. But it so happens that among the Trilobites there are several Tremadoc forms —see notes by Mr. Etheridge, F.E.S., at the end of the Survey Memoir—and with them a species of Cybele hitherto only represented by species not occurring below the Caradoc rocks. Certainly the scanty evidence to be derived from the group of Trilobites is con- flicting, and besides Trilobites and Graptolites, we have but a few doubtful plant-remains, carapaces of Phyllopod Crustacea, a IAngula (brevis), and doubtful Discina, and worm-tracks. Under these circumstances no wonder that the Graptolites are referred to as the most important. Hence we have the evidence of this group in favour of the L. Llandeilo age of the Skiddaw Slates, and the evidence of at least part of the Trilobite fauna in favour of the Tremadoc age of portions of the series. The question arises, are we justified in saying that such and such beds below a certain horizon cannot be of Tremadoc age because beds believed to be of that age in Wales do not contain Graptolites ? A thin grit in Wales separates black slates above, containing Graptolites, from black slates below not containing them; the upper are called Arenig, the Lower Tremadoc: are we to assert from this that, such being the case in Wales, so far as our experience goes, no black slates, whatever their physical position, which contain Graptolites, can be of Tremadoc age ? Surely not. In Cumbria we have a grit, similar to the Arenig grit of Wales, with black slates above containing Arenig Graptolites, but in this case the black slates below also contain them; therefore the palaeontologist says these cannot be Tremadocs, and yet there may be but 50 feet or less of grit separating the two series. For my own part, while ready enough to accept the evidence of paleontology when it is presented in the form of life assemblages, I do decline to pin my faith as to the succession and equivalence of rock-groups to the evidence afforded by the finding of a few

EXPLANATION OP SKETCH-MAP, PLATE II. (GEOL. MAO. Feb. 1879, p. 49.) FIG. 1. Diagrammatic Section, from N. to S., to show general structure of District. FIG. 2. Diagram to show plateau of marine denudation, at the close of the Old Red period. The pattern used to denote the Upper Silurians is not meant to indicate that the strata are vertical, though they are generally highly inclined. Sketch-map of the Geology of the Lake District. The lines along which the Horizontal Sections on p. 54 are drawn are indicated in the Sketch-map. APPENDIX. LIST OP PAPEKS, BY THE AUTHOB, ON LAKE DISTRICT GEOLOGY, TO BB REFERRED TO IN CONNEXION WITH THIS PAPER. Notes on the Microscopic Structure of some Ancient and Modern Volcanic Rocks. Quart. Journ. Geol. Soc. vol. xxxi. p. 388. On the Granitic, Granitoid, and associated Metamorphic Bocks of the Lake District. Ibid. voL ixxi. p. 568, and vol. xxxii. p. 1. PART I.—On the Liquid Cavities in the Quartz-bearing Bocks of the Lake District. PART II.—On the Eskdale and Shap Granites, with their associated Metamor- phic Bocks. PART III.—On the Skiddaw Granite, and its associated Metamorphic Rocks. PART IV.—On the Quartz-Felsite, Syenitic, and associated Metamorphie Rocks. PART V.—General Summary. The Geology of the Northern Part of the English Lake District. Memoirs of the Geological Survey. Longmans & Co., and E. Stanford. Notes on the Occurrence of Chlorite among the Lower Silurian Volcanic Bocks of the Lake District. Mineralogical Mag. No. 4.. On the Lower Silurian Lavas of Eycott Hill, Cumberland. Monthly Microscop. Journ. 1877, p. 239.

Glaciation of the Northern Part of the English Lake District. Quart. Journ. Geol. Soc. vol. xxix. p. 422. On the Origin of some of the Lake Basins of Cumberland. Ibid. vol. xxx. p. 96. The Glaciation of the Southern Part of the Lake District, and the Glacial Origin of the Lakes of Cumberland and Westmoreland. Second Paper. Ibid. vol. xxxi. p. 152. Notes on the Archaeological Remains of the Lake District. Trans, of the Cumb. and West. Ant. and Arch. Soc. part ii. vol. iii. p. 241.

IV. — THE DEVONIAN QUESTION. By A. CHAMPERNOWNE, M.A., F.G.S. N last month's GEOLOGICAL MAGAZINE there is much (by three writers) that bears upon the Devonian Question, and I would Inow add a few words on the same subject, which is of the greatest interest to me. In the first place, notwithstanding that two of the writers are my personal friends, I think that the united criticisms upon Prof. Hull's proposed classification exceed what the nature of the case calls for, as that scheme contains the most essential elements. of truth. I have myself been in North Devon during the last autumn with Mr. Ussher, and we hope to give a further account of our work and the conclusions arrived at. My chief object in going there was to test certain opinions I had before expressed with great confi- dence (in these pages and elsewhere), and, as a result, I have proved to my satisfaction that those opinions were wrong. Neither by fault, nor inverted junction along the line indicated by Jukes, is there any wholesale reduplication of the North Devon, Series. The fault I had already taken to be disproved by Mr." Etheridge's paper (Q. J. G. S. 1867). In the last paragraph of Mr. Kinahan's paper he observes, that " Jukes' fault has not been disproved, and that the Irish geologists who have been in Devon believe in its existence." This, however, from Prof. Hull's classification, is evidently not his opinion, as he places the Ilfracombe and Morte slates beneath the Pickwell or (Upper) Old Eed Sandstone, but he suggests an unconformity at the base of the latter, of which there is no evidence. Eegarding the Pickwell as what Mr. Kinahan aptly calls the Carboniferous Old Ked, I will venture to say that in nine out of ten Western European areas a break1 might be expected at a corre- sponding horizon, but the North Devon marine sequence is almost unique in its completeness. My former belief in an inverted passage assumed perfect conformity at that junction, and of this conformity there are indisputable proofs. 1 Of all areas in which rocks belonging to the Siluro-Carboniferous interval are developed, that of the Welsh border counties, although geographically the nearest, is perhaps the least comparable of any to the North Devon area, and the greatest stumbling-block to a would-be "Devonian" disciple. In vain maybe the search there for a break of any magnitude to be filled in from the latter area. Lacustrine conditions may have prevailed more or less, from the final retreat of the Ludlow types, until the waters of the Lower Limestone Shale united the " Welsh Lake " (see Prof. Geikie's Lectures oh the Old Red Sandstone) with the ocean. In South Ireland sod Devonshire, on the other hand, there are plain indications of an open sea to the South and East, extending into the Rhine country.

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