IV.—The Ashburton Limestone: Its Age and Relations

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410 A. Champernoicne—Age of the Ashburton Limestone. the surface of such enormous regions of the earth would in itself lead us to believe, that those creatures had really been long inhabit- ants of such countries, living and dying there for ages, while their final destruction may have resulted from aqueous debacles dependent on oscillations of the land, the ehvation of ridges, and the formation of much local detritus " (Russia and the Ural Mountains, vol. i. p. 495). Speaking of the herds of Mammoths that must have lived in the Northern Urals, the same writers say : " Such might we add, have been the position and condition of some of these creatures at the periods when, as we have imagined, the highest ridges of the Ural were thrown up, followed by the rupture of many lakes, and the consequent inundation of large tracts of the fiat country, previously frequented by these great herbivorous animals" {id. 498). This completes our survey of the Siberian evidence, and I take it that not only is it everywhere consistent with the conclusion of this paper, namely, that the Mammoth was finally extinguished by a sudden catastrophe, involving a great diluvial movement over all Northern Asia, and accompanied by an equally sudden and violent change of climate, but that it is consistent with no other conclusion. We are now in a position to follow up the problem as it presents itself in Europe, where, as we said, many subsidiary difficulties make the discussion of the main question a complicated one. It is clear that these difficulties must be frankly faced if our theory is to remain good, and I propose to examine them in another paper, where I hope to show that, in Europe as in Siberia, the evidence points overwhelmingly to the same conclusion.

IV.—THE ASHBURTON LIMESTONE : ITS AGE AND RELATIONS. By A. CHAMPEKNOWNE, M.A., F.G.S. T has been for some time a matter of surprise to me that I Dr. Holl, in his exhaustive memoir " On the Rocks of S. Devon and East Cornwall,"—after once raising the question " whether the limestones " (those of our present subject) " might or might not be the same as those of Ogwell, Ipplepen, and Dartington thrown over a broad anticlinal axis of the lower slates to the North-West,"'— should have ceased following out that line of thought to what appears to me the only possible issue, since he thoroughly recognized the existence of uniclinal structures in parts of the district. It may be as well to say at once that I do not believe these limestones to be on a lower horizon than those of Ogwell, Ipplepen, and Dartington, all appearances to the contrary notwithstanding; and I will proceed to give a summary of my reasons for holding so pronounced an opinion, to which, let me add, I have always more or less inclined. That the direct evidence is, as Dr. Holl says, against it, I grant; yet not " entirely " so, for, as he himself admits, " palasontological records would not altogether discountenance it."* This, as far as 1 Q.J.G.S. 1868, p. 442. 2 I.e. p. 443.

Downloaded from https://www.cambridge.org/core. INSEAD, on 07 Oct 2018 at 00:26:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800117558 A. Champemoicne—Age of the Ashburton Limestone. 411 it goes, is certainly direct evidence, but we shall see that there is abundant indirect evidence, tending to show that they are one and the same. Mr. Godwin-Austen did not, as far as I am aware, ever hint at these limestones having a uniclinal structure, but I am not familiar with his earlier papers, though quite so with that in the Trans. Geol. Soc. 2nd series, vol. vi. In the latter the Ashburton limestone is distinctly referred to as a " lower limestone." ' As regards the lithological varieties to be met with in this range of limestones, we need not dwell at any length. Suffice it to say that it is usually well-bedded and dark-coloured; occasionally dolomitized but not extensively so, like the Dainton mass and others which are crystalline and run into light colours; but that in other respects it may be paralleled with almost any of the limestones of South-East Devon or Plymouth. Moreover, it is quite possible that the highest beds, which at Dartington and elsewhere are the dolomitized portions, may be seldom seen, that is, of course, provided the uni-synclinal folding can be proved. But. be this as it may, dolomitization is always very impersistent, and the causes of its phenomena still very enigmatical. We now come to the fossils; the collector will be sadly dis- appointed in these limestones. I know of no spot where fossils can be extracted from the matrix, but the records can be fairly well read in the polished slabs of Ashburton marble, on the large sawn blocks in the quarry. Large Stromatoporm of the same kind as those which characterize the Ogwell, Dartington, etc., limestones, are very abundant, two may be cited—one of loosely reticulated and open structure (query, not named or var. of S. polymorpha, Goldf.)—another of remarkably fine structure, which, with my kind friend Dr. Carter, I regard as belonging to the same section, or identical with S. lypica, Rosen, the concentric layers " having the form of rhombs, triangles and pentagons,"3 thus simulating, though lacking the geometrical constancy of the Hexactinellidce. Interspersed with these we frequently observe fragmentary sections of Stringocephalus, shells which, whether from the large septum projecting from the concave side, or from the punctate shell- structure, are not readily mistaken. Other Brachiopod sections, with strias, betoken Uncites. Some portions of beds are full of Gasteropoda, among which the outlines of MurcMsonia can be de- tected. Others again are made up wholly of the so-called Caunopora ramosa, Phillips, which is also common in the Lemon Valley S.W. of Bickington, as well as in the Ogwell and Bishopsteignton beds. It abounds also in the Westphalian limestone, and at Paffrath. Am- plexus tortuosus, Phillips, simple Cyathophylla, apparently few as to species, Gystiphyllum vesiculosum, Goldf., Favosites (sp.), Aulopora repens, Goldf., can all be observed, and probably other forms might be added to this small but significant list, which links the Ashburton. limestone with that of Schwelm, Elberfeld, etc. 1 See also De la Beche, Geol. Report, p. 69. * Rosen, " Ueber die Natur der Stromatoporen," p. 17 and woodcut.

Downloaded from https://www.cambridge.org/core. INSEAD, on 07 Oct 2018 at 00:26:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800117558 412 A. Champernowne—Age of the Ashburton Limestone. The Chudleigh limestone has been classed as a lower limestone together with the Ashburton band by Mr. Godwin-Austen, and indeed looking at their strike on the map, no one would reasonably doubt their belonging to the same run of beds, both being more or less closely skirted along their North-West side by a downcast area of Culm-measures.1 The Chudleigh band is, to say the least, closely connected by its fossils with that of Ogwell. In addition to the rich Gasteropod fauna of Lower Uppercot and Kerswell quarries (of which a list was given in Mr. Keid's paper, GEOL. MAG. 1877, p. 455), Mr. Vicary, F.G.S., has shown me in his splendid collection two specimens of Uncites gryphus, Defr., and two or three perfect valves of, I believe, Megalodon carinatum, Schlottheim. But if any doubt lately existed as to whether the Chudleigh band is identical with the Ogwell beds, or lower, we may regard it as now set at rest. Prof. Eoemer (GEOL. MAG. 1880) assures us that the thin red limestone beds of Lower Dunscombe, near Chudleigh, abounding in Cephalopoda, are the Gon. intumescens stage, having the same relations as in Germany, namely, at the summit of the Eifler- kalk, which accordingly must be represented by the Chudleigh, no less than by the Ogwell beds.2 But the Chudleigh band is admitted to be the continuation of the Ashburton band, therefore we must logically concede the same identity between the Ogwell and Ash- burton beds. But why need we feel staggered at the uniclinal structure in- volved in this identification, merely because, in two or three instances, the angle of inversion is so great that the beds lie at 20° or even 15° to the horizon, as mentioned by Dr. Holl ? This folding is in fact but the natural continuation, with partial flatten- ing of the axes and widening of the area, of a great series of folds which pass North of the Plymouth limestone and round the granite by Ivybridge and Ugborough with some vertical dips, as clearly shown by Dr. Holl's section.3 In the large quarry at Dean the dip is 35° towards E. 5° N. Near Buckfastleigh some dips of 20° or under occur, and once more at Chewley, S. of Ashburton. Beyond this towards the North-East the dips again become high. Prom the town to Goodstone they are nowhere under 45°, along inverted margin. The line as laid down by Dela Beche needs but the most trifling alteration. Just a mile N.E. of the town, along the road that follows the boundary, two adjoining quarries show a steady dip of 50° S.E., and if from this point we cross the outcrop (one field to the turnpike road), on the other side we come first upon a disused quarry, beds dipping 30°, and then the great Ashburton marble quarry, where the dip is steady at 25° S.E., half the first angle. It seems the most reasonable inference that these planes meet at no great depth ; in other words that the limestone is doubled upon itself. From the road on the inverted margin, 156 1 Vide paper by the author, GEOL. MAG. 1880, last par. p. 361. 2 The Gon. intumescens stage is classed by the German geologists as the commence- ment of the Upper Devonian. 3 I.e. p. 138.

Downloaded from https://www.cambridge.org/core. INSEAD, on 07 Oct 2018 at 00:26:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800117558 A. Champernoicne—Age of the Ashburton Limestone. 413 paces takes us to the furthest limestone face, which distance at 50° gives a thickness of more than 300 feet (354, if yards were paced, and I cannot estimate it at much less). Between this and the turnpike road, along the level ground, we must presume the axis of syncline passes. The Eew Mill fault, dying out at this point as shown by De la Beche, probably exercises little or no effect on the limestone dips. There are other interesting facts connected with these limestones, but none more so than the apparent gaps in its continuity. For instance, after the first railway cutting South of Ashbnrton station there is no limestone until we reach the Pridhamsleigh fault: it is abruptly lost for f of a mile. This is caused by an Bast and West fault passing Chewley and Summer Hill, and meeting the Pridhams- leigh fault at an angle of about 40°. These two heave the wedge of country between them and " throw out" the limestone fold. The evidence is clear; for the mass of igneous rock constituting Pear Tree hill is shifted towards the East and laid open in the railway cutting (9th milepost from Totnes) dipping about 35° towards E. 25° S., the intervening ground towards the station being low and swampy where the fault passes.1 The rocks immediately S.E. of the town appear identical with those on the N.W., the limestone being troughed in them. A similar gap occurs immediately S.W. of Buckfastleigh, between that place and the Dean limestone ; but, interesting as the structure is, present space will not admit of an attempt to describe it, more than to say that the Dean band is evidently thrown down by a fault on the north, which is metalliferous, having been once worked as an iron mine. In the country to the East and South-East of the whole belt, taking Woodland as a centre between the Dart and the Lemon, the igneous rocks, ash-beds and lavas, rather than the "greenstones," will help to unravel whatever may be obscure in the structure ; my maps already begin to foreshadow the appearance, but we must remember that, igneous rocks excepted, we have to deal with an almost purely argillaceous series, and that there is little at first sight to dispel the illusion of an ascending series from the Ashburton to the Ipplepen, etc., limestones. It is necessary however to point out that, at a certain distance from the South-east of the limestone band, there runs a belt of coloured slates, purple and greenish, which have an important bearing on the structure. They appear about a mile East of Buckfastleigh, very narrow at first, but beyond the Pridhamsleigh fault, in the upcast country about Penrecca slate quarry, are more striking; they can be traced continuously towards the North-east, only affected by N.W. and S.E. faults which equally shift all the rocks, and they curve round with the strike near Knighton and by Knowle Hill, reappear- ing on the opposite side of the Teign, where they are exposed in the railway cutting near Combe cellars. Intrusive rocks, individually of small extent, break out along their 1 This shifted continuation of the Pear Tree rock is omitted on the map.

Downloaded from https://www.cambridge.org/core. INSEAD, on 07 Oct 2018 at 00:26:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800117558 414 A. Champernoicne—Age of the Ashburton Limestone. line, and for a short distance from the contact, the colour is gone from the slates. Instances of this can be seen conspicuously in a quarry at the Northern foot of Knowle hill near Newton Abbott, and again in the cutting near Combe cellars, where one or two small tongues a few feet thick appear isolated in the discoloured slates which surround them. They are probably only branches from the great mass that runs up to Eowdon Cross. Thus we see that these coloured slates which run S.E. of the Ashburton limestone pass North of the Ogwell mass, and South of that of Bishopsteignton, throwing them off on opposite sides. They form the crest of a denuded and overturned anticlinal line, and clearly correspond with the coloured slates North of Plymouth, at Mutley, etc., to which district we must now turn our attention. The apparent thickness of slaty rocks North of Plymouth is enormous. An excellent paper" On the Geology of Plymouth " was written by Mr. Worth, F.G.S., for the Trans. Plymouth Institution, 1875. In that paper (p. 9) he pointed out that three or four sub- parallel belts of coloured slates, separated by slates of the ordinary type, are doubtless only one group, repeated by folds, the same general dips to the south at high angles prevailing throughout, making the apparent thickness vastly greater than the actual. Having formerly stayed some months at Knackersknowle, I can testify to the truth of these statements. The folds, however, are certainly more numerous than in the corresponding South-East Devon area, and the purple slates, their axes less thrown over, rise in greater bulk. Dr. Holl has shown how, through a series of folds, the Liskeard and St. Cleer igneous rocks are brought down to Saltash,1 and Mr. Worth has pointed out how close the igneous bands come down to the outcrop of the limestone at Devonport ;2 so that one may now confidently predict that the so-called " Liskeard and Ashburton group" of Sedgwick and Murchison will resolve itself into a complicated series of folds of the " Plymouth and Torquay group," limestone becoming scarce North-west from Plymouth, or possibly not being "brought in" among the folds. But at any rate such fossils as occur in this direction, especially at Liskeard, have been shown by Dr. Holl to be of a Middle Devonian type;3 there is, in fact, no sign of a Lower Devonian or Coblentzian fauna or rock group in the neighbourhood of Liskeard, whatever the beds of the Looe river section, or those of Whitesand Bay, may be. In contrast to this let us compare, first, a small tract comprising strata known to be of Lower Devonian age,4 the Torquay Promontory, with, secondly, the slate rocks expanded West of the great limestones of Ipplepen, etc. In the former (saving some calcareous shales with lower Eifel Orthides, Phacops latifrons, Spirifera speciosa, etc., which peep out from under greatly faulted limestone in parallel stratification), we have a series of tough grey and brownish grau- wacke grits and schistose beds with fossils of the Coblentzian series, i I.e. p. 442. 2 I.e. p. 7. 3 I.e. table iii. p. 450. 4 Muxchison, Siluria, p. 398 ; Salter, Q.J.G.S. 1863, p. 483.

Downloaded from https://www.cambridge.org/core. INSEAD, on 07 Oct 2018 at 00:26:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800117558 A. Champernoicne—Age of the Ashburton Limestone. 415 passing up into a group of quartzose grits red and greyish, inter- stratified with purplish and variegated sandy shales—the Lincombe and Warberry grits1—with some fossils differing from the Meadfoot beds, notably spined Homalonoti. Moreover, we have a mass of rocks from which it may be safely asserted that not a decent roofing-slate could be obtained. In the latter, namely, the slaty district of Staverton, Woodland, etc., several slate quarries are worked, and there is apparently an utter absence of grit bands —I know of none. In fact the difference is, to my mind, so complete as to lead to the conclusion that, from the West of the Ogwell, Ipplepen, and Dartington limestones to the extreme North of the county, the Torquay grits nowhere reappear at the surface. This formerly presented a difficulty to my mind, at least whilst acquiescing in a downward sequence frem the Ogwell limestone to the Culm-measure fault; but after having convinced myself that the Ashburton is identical with the Ogwell limestone, and consequently that this downward sequence is illusory, and further, that the Lower Devonian is on all sides faulted through the Torquay limestones, the difficulty has disappeared. A suite of fossils, collected from several slaty localities West and South-west of Totnes, to which I hope some day to draw attention, will go far towards confirming the view that a large portion of the South Devon slates must be classed as Middle Devonian with the limestones, which they both laterally replace by interdigitation, and also partially underlie. In particular the slates of Englebourne Quarry near Harbertonford at once recall the Wissenbach slates, with which to some extent the organic remains also correspond. It is worthy of remark, as distinctly bearing upon this subject, that M. Dewalque, speaking of certain tints employed by H. von Dechen, especially for the slates of Wissenbach, observes parentheti- cally that there is a general consent now-a-days to regard them as the local equivalent of the limestone of Givet (Eifler Kalk).2 The writer then mentions the Lenne schists as having a special tint. If we draw a line due South from Elberfeld to Bensberg, across these schists, we find nothing to support the notion of a lower reef limestone ; on the contrary the Paffrath and Eefrath series, as prolific as any Middle Devonian succession, hold a position analogous to that of the Ashburton limestone, the older beds resting on the younger, a fact which greatly strengthens the views advocated in this paper.3 When we reflect that all the rocks of our peninsula are but a link in a system of plications that extend through South Ireland, under the South-Eastern Counties, through Belgium, the Rhine Province and the Hartz ; and on the one hand, at Killarney, we have the 1 These must not he confounded with the purple sandstones and slates of Cocking- ton, Ockham, Beacon Hill, Windmill Hill, Southdown Cliff, etc., etc., which are of Upper Devonian age, doubtless the equivalents of the Pickwell Down sandstones of North Devon. 2 Prodrome d'une description geologique de la Belgique, 1880, p. 114. 3 Murchison and Sedgwick, Trans. Geol. Soc. 2nd series, vol. vi. pp. 241—244.

Downloaded from https://www.cambridge.org/core. INSEAD, on 07 Oct 2018 at 00:26:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800117558 416 W. 0. Crosby—Absence of Joint-Structure at Great Depths. Carboniferous Limestone thrown under the Glengariff Grits, then the Coal-measures under the Carboniferous Limestone of the Mendips, and Eastward Dumont's well-proved Belgian inversions, etc., our present subject assumes but a very subordinate place in so grand a system. Such sound generalizations as a " contracting crust " with " lateral displacement" through " tangential thrust," now happily rank as part of the common stock of knowledge. No writing has tended to bring this about in a greater degree than Prof. Heim's magnificent work " On the Mechanism of the Formation of Mountains, and Monograph of the Todi-Windgalle group," embodying the grand discoveries of Escher von der Linth. Fortified by such established facts, we feel unfettered in identi- fying distant out-crops from the sum of their characters, palaeonto- logical, etc., a vera causa being recognized for their appearance in positions the most startling at first sight. I cherish the hope of being yet able to publish a geological map of this Calciferous district, including portions of Sheets 22 to 25 of the One-inch Survey, having spared no pains to lay down the lines as correctly as the imperfect topography and half-worn-out engraving of the old maps will admit of. Note.—This seems a fitting opportunity, since several igneous rocks have been referred to, for saying that Mr. Rutley, of the Geological Survey, has kindly examined and given me a written opinion on many slices of South Devon rocks, which have been cut for me by Mr. Cuttell. They are chiefly from the important lavas of Upper South Devon (Upper Devonian) age that run East and West through Totnes and the adjoining parishes of Harberton, Ashprington, Cornworthy, etc. Whilst hoping to do justice to them in their proper place, I will only now observe (being quite a tyro at the polariscope myself) that there is the strongest concurrence between the verdicts given and the aspect of the rocks in the field.

V.—ON THE ABSENCE OF JOINT-STRUCTURE AT GREAT DEPTHS, AND ITS RELATIONS TO THE FORMS OF COARSELY CRYSTALLINE ERUP- TIVE MASSES. By W. 0. CROSBY, of Boston, Mass., U.S.A. "TT is often said by vein-miners who have worked at great JL depths, that the jointed structure of rocks fades away and disappears in the deeper parts of the mines. This, however, is probably a case of the rule ' de non apparentibus et non existentibus eadem est ratio.' All the joints near the surface are more or less acted on by the weather. The deeper-seated rocks may be just as much traversed by joints, but they are merely mathematical planes of division, the faces of the blocks adhering as closely as if they did not exist till the weather makes them apparent, or some force tears the blocks asunder." This extract from Jukes and Geikie's Manual of Geology' is 1 Third edition, p. 18i.