Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 ON THE DEVONIAN ROCKS OF SOUTH DEVON. 4S7 29. The D~.voNIA~ ROCKS of SO~TH D~.v0N. By W. A. E. USSUE~, Esq., F.G.S., of the Geological Survey of England and Wales. (Read April 30, 1890.) By permission of the Director-General of the Geological Survey of the United Kingdom. CONTENTS. wI. INTRODUCTION. wII. GENERALDESCRIPTION'. w RELATIONSOF THE Ct'L~ AND DFVO~IA.~. wIV. Lower DEVONIAN. 1. Torquay Area. 2. Paignton Area. wV. MII)DI.EDEVONIAN. 1. Eifelian. 2. Middle Devonian. 3. Middle- and Upper-Devonian Limestones. wVI. THE ASIIPR[I~'OTONVOLCANIC SERIES. wVII. UveEa DEVOSIAX. 1. Massive Limestones. 2. Goniatite-beds. 3. Cypridinen-Schiefer (Entomis-slates). wVIII. CONCr.~SIONS. w I. INTRODUCTION. " I~r South Devonshire the rocks are greatly disturbed, broken by faults, standing at various angles, folded, and distorted ; eruptive rocks frequently cut through them, and beds of volcanic ash are interstratified with them, Hence has arisen great dispute and uncertainty as to the true order of succession of deposits, although their fossils were referred to the Devonian age nearly fifty years ago. It will be seen without comment that the South Devonshire sections, from which most of the Middle- and Lower-Devonian fossils have been obtained, are valueless for determining the order of sequence of the faunas. There are many places (I saw such near Newton-Abbot) where limestones, appearing very similar in colour and structure, and within a stone's throw of each other, hold distinct faunas. In fact, the interpretation of the order of the beds is a matter of the greatest difficulty, even when occasional fossils appear." l~ere is the geology of South Devon as epitomized by Professor H. S. Williams, of Ithaca, N.Y., in a paper read before the American Association for the Advancement of Science as lately as Aug. 30, 1889. Explorations .in the rocks of Devonshire gave rise to the Devonian System; explorations in France, Be]glum, Germany, Russia, and :North America have made ds acquainted with the succession of 2M2 Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 48S MR. W, A. E. USSHER ON THE DEVONIAN the Devonian faunas, and the extraordinary persistence of homo- taxcous horizons, such as those characterized by Stri~,.Iocephabts, Rhy~chonella euboides, and Goniatites, at the same time presenting us with the varied evidences of stratigraphical succession. The disturbances to which Prof. Williams refers, the distortion of fossils in consequence, and the paucity of localities rich in charac- teristic fossils, renders South Devon a most unprofitable field to the casual investigator, whilst depriving the stratigraphist of the only means of piecing the scattered details into a connected whole. The late Mr. J. E. Lee's ~ discoveries of the Biidesheim and Adorfer- Kalk faunas at Saltern Cove and Chudleigh proved that the Gouia- tite fauna had extended to Devonshire; but the faulted occurrence of the beds containing it precluded the observation of their t.rue "position in the formation. The South-Devon limestones were, with two or three exceptions, regarded as Middle Devonian, and the presence of Lower Devonian was only proved in the Torquay Promontory. The old 1-inch Ordnance Map on which De la Beche, Godwin- Austen, and subsequently Dr. Harvey Hell and Champernowne, were compelled to record their observations on the stratigraphy of South Devon is much too small to note minutim which have the most important bearing on the question. Dr. Hell t, in 1868, placed both the Plymouth and Paignton sections in the wrong sequence, through the acceptation of inverted dips as natural evidences of superposition; yet his sections account tbr the position of many of the types which he correlated by inverted plication. The succession he advocated in descending order is :--Staddon and Cocldngton grits, slates, limestones. He likewise endorsed Godwin-Austen's view that the limestones of Ashburton were a lower series than those of Newton and Ipplepen. During a re-survey of portions of the country around Newton- Abbot and Tor, luay in 1874-75, my colleague, H. B. Woodward +, arrived at the conclusion that the Devonian rocks occurred in descending order as follows :-- Limestones. Slates. Red Sandstone (of Cockington). When this general succession was published in 1S76, it was adopted by Mr. Champernowne as affording "the clue to unravel the country." Mr. Champernowne w in "Notes on tile Devonian and Old Red Sandstone of North and South Devon," gave the true succession in sections of the Mudst~)ne-Bay coast. "This," he says, "brings the Red Sandstone into direct and natural relation with those of Cockington, the Warberry, Lincombe, &c. at Torquay, whicl, are Geol. Mag. 1877, p. 100, and 1880, p. 145. Quart. Journ. Geol. Soc. vol. xxiv. p. 490. ,+ Geol. Mag. 1876, p. 576. w Ibid. 1878, p. 193. Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 :ROCKS OF SOUTH DEVO:~. 489 beneath the limestones." Unfortunately, subsequent investigations near Cockington, Marldon, and Paignton induced Mr. Champer- nowne to abandon his previous views and to endorse, on strati- graphical grounds, the erroneous succession given by 1)r. Holh The independent observation of the Plymouth section led me to adopt the same views. We were both under the impression that the Coekington beds were unfossiliferous, and in ignorance of the dis- covery, by the :Rev. G. F. Whidborne, of Lower-Devonian fossils in them, no detailed account of which had been published beyond incidental reference to it in Dr. Davidson's Supplement to Devonian Brachiopoda, Pal. Soc. Mon., note by G. F. W. at p. 4. Thus, through a mere accident, the geologist whose painstaking devotion has furnished the first detailed map of the Devonian rocks of South Devon, and vastly increased our knowledge of that forma- tion, was deprived of reaping the results of his exhaustive labours. For the publication of the erroneous succession given in Proc. Geologists' Assoc. vol. viii. pp. 442 &c., I am alone responsible, Mr. Champernowne's views having, at my request, been promptly reduced by him to their simplest form in spite of the uncertainty he entertained respecting them. The existence of contemporaneous volcanic action,--the definition of the Ashprington series and of sporadic evidences of local vulca- nicity outside its borders,--the correlation of the Ashburton lime- stone with that of Newton and Ipplepen,--and palmontologieal contributions, adding to our knowledge of the Middle and Lower Devonian, stand prominently tbrth amongst the labours of my deceased friend. Had he lived, the task I briefly and inadequately undertake would have fallen into better hands. During our friendship I became so thoroughly conversant with the special difficulties he encountered, and with the doubts he entertained, that, when it devolved upon me to carry on his work on the six-inch Map, I started already equipped with the results of his experience, and was stimulated to researches in quest of fossils in beds before regarded as unfossiliferous in character and anomalous in position. So the present communication must be taken as the outcome of my friend's life-work in Devonian geology, and will, I trust, form a not unfitting tribute to his memory. After the London Meeting of the Geological Congress in 1888 1 had the advantage of conducting MM. Gosselet, Kayser, Tschernyschew, :Holst, and Frech over the North-Devon section and over those parts of South Devon near Newton-Abbot, Chudleigb, and Torquay, which would in a short time best show the different varieties of rock and evidence of their fossil contents. Dr. Kayser embodied the results in a pamphlet entitled "Ueber das Devon in Devonshire und im :Boulonnais," Neues Jahrbuch fiir :Mineral. &c. Bd. i. (1889). The identification of the Cypridinen- Schiefer, by the Lewell-Path road and at Whiteway Farm, was of immense service to me. The identification of the Hope's Nose thin limestone with the Calceolen-Kalk, and the correction of t~h. cuboides to zvrocuboides, were also important. Downloaded from http://jgslegacy.lyellcollection.org/ at New York University on January 17, 2016 490 MR. W. A. E. USSHER ON THE DEVONIA~ M. Frech kindly furnished me with the names of fossils found in the different localities visited. I have great pleasure in acknowledging my indebtedness to MM. Gosselet and Barrels, and subsequently to Dr. Kayser, for their kind and prompt assistance in identifying fossils sent to them from time to time in 1889. I am indebted to M. Tschernyschew for the assistance afforded me, by the presentation of his admirable contributions to the Devonian geology of Russia ; and to M]K. Gosselet, Kayser, and Barrois for similar favours. My friend Prof. Gosselet devoted a week last September to con- ducting me over the typical sections of the ,~rdennes rendered classic by his investigations. His kindn~s supplied me with more information and encouragement than I can hope to repay. w li. GE.'~ERALDESCRIPTION. The area to which this paper more particularly refers lies north of the River Dart and East of Dartmoor, and comprises the most complex and lithologically varied tract in South Devon. We naturally turn to the limestone districts to obtain evidence of structure. Owing, however, to crushing, it is impossible to obtain persistent and reliable dips beyond the actual exposures; and the acceptance of apparent superposition of limestone on slate, or vice versd, in any one place must lead to endless confusion in its general application, as inversion is rather the rule than the exception. Evidences of stratigraphieal relations are often so dubious as to suggest alternative explanations, such as anticlinal or synclinal inversion or consecutive sequence.