23. on the ASHPRINOTON VOLCANIC S~Rn~S of SOUTH DEVON
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Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016 ON THE 2kSHPRINGTON VOLCANIC SERIES OF SOUTH DEVON. 369 23. On the ASHPRINOTON VOLCANIC S~Rn~S of SOUTH DEVON. By the late ARTHUR Crrx~te~a~OW~E, Esq., M.A., F.G.S.* (Read May 8, 1889.) (Communicated by Prof. k. GEIKIE, LL.D., F.R.S., F.G.S.) INTRODUCTIOn'. TaERE exists in South Devon an extensive series of igneous and quasi-igneous rocks occupying a considerable area, mainly east of the River Avon, which have as yet received scarcely more than passing notice, and the study of which is attended with much perplexity Sir Henry De la Bechet noticed the u trappean rocks south-west of Paignton, which are included in our subject, and spoke of them as interposed between the Yalberton limestone and that of Watton, which forms the western termination of the Berry Head mass. The relations of the igneous rocks to the Devonian limestones will be considered de hove as we proceed. Dr. Hell + mentions the rocks of this series in the neighbourhood of Totnes, Ashprington, &c., as "thick slates, in which much vo!- canic matter is disseminated." He observes that "volcanic rocks are frequent," and that " beds which are light-coloured often yield a red soil." This was written with reference to rocks which occupy the ground between Harberton and the Dart. Though we might reasonably take exception to the term " slates " as not strictly ap- plicable to any portion of these rocks with which we are acquainted, yet as it is quite certain that Dr. Hell well appreciated the im- portance of the group, and as no spot could be pointed out where they attain a greater development than [at] Ashprington and its neighbourhood, the provisional term " Ashprington volcanic series" has been here retained. Many patches and lines of " greenstone" were laid down by De la Beche within the tract of country to be described; but these represent only a very small fraction of the actual spread of the rocks, which occupy on the east of the Avon a large part of Dipt- tbrd, Harberton, Totnes, Ashprington, Cornworthy, and Dittisham parishes, and on the east of the Dart, Berry Pomeroy, Stoke Gabriel, and Churston Ferrers. On the west of the Avon they range through North Hewish and Ugborough towards the Yealmpton and Plymouth district, but are * [The MS. of this paper was found by Mrs. Champernowne among her husband's papers after his death, and was handed over to me. I believe it to have been intended as an instalment of a general description of the Devonian rocks of the Totnes district, which, at my request, he had agreed to prepare. Though incomplete and evidently the first rough draft, it possesses much interest as an expression of some of the latest views of one of tbe most careful geologists who ever studied the rocks of Devonshire. The map to which reference is made is the 1-inch Ordnance Survey Sheet, no. 22, which was coloured after an original geological survey by the author. This work was presented by him to the Geological Survey, and it will be embodied in the new edition of the Survey Map (no. 22) now in preparation.--A. GEIKZ~.] 72. r Rep. Geol. CornwaU, Devon, and West Somerset, 1839, p. Quart. Journ. Geol. See. 1868, voh xxiv. p. 434. Q.J.G.S. No. 179, 2E Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016 370 THE LATE MR. A. CHAMPERNOWNE ON THE less expanded than on the east of that river, partaking of the narrow folds into which the country is thrown immediately south of the granite. To the north of the parallel of Paignton and Brent they nowhere occupy a large continuous area ; nevertheless various tufts seen in the Dartington, Kingskerswell, and Ogwell districts ought probably to be placed in the same category from their relations to Devonian limestones, although not attaining the development that the series exhibits on the banks of the Dart. Lavas, frequently amygdaloidal and vesicular, or even scoriaceous, but at other times very compact or aphanitic, constitute a great part of this series. They are either altered porphyrites, or basalts, or both. Tuff-beds are largely intermingled with them, all these rocks being highly basic in character. It is possible that some beds of purely detrital origin may be here and there interbedded (if not doubled in), but, so long as we have only the imperfect one-inch maps, they are too insignificant to be shown on paper. Some reddish schists, for ex- ample, are met with in a road descending from Weston within a mile east of Totnes to the head of the valley leading down to Fleet Mill. The lavas, where freshest, are usually of a blackish-green colour, often porphyritic in structure, from the presence of crystals of felspar, which sometimes in hand specimens appear as dark as the ground- mass. These harder rocks appear to run in lines, and have even been represented as dykes on the map, as, for instance, at Sharpham on the Dart, but they dip with the rocks among which they occur, and even pass into them. It is true they form projecting bosses by the river-bank, but it would be impossible to trace them away from the foreshore. I believe they are not dykes, but are simply inter- calated, and I know of no single instance of a line of hard rock cutting across the strike. Sometimes the lavas become flaggy, breaking into irregular, long, wedge-like flags, weathering brown or purplish near the surface, and even splitting up into a shaly substance, from which, neverthe- less, a perfect passage can be traced into the compact, dark rock used for road-metal; and these facts can be observed in one and the same quarry, as, for instance, in a quarry by the roadside on Totnes Down Hill. A mass of this rock at " Red Hill" or Pheasant's I-Jill quarry, Totnes, where it rests on limestone, as further to be described, is weathered brown on one side, but abruptly changes its colour to a deep red on the other, where I noticed a knob of limestone, imme- diately under the "trap," coated with a thin film of hmmatite. In a plantation above Sharpham Lodge, near Totnes, the rock is amygdaloidal, as often happens, the cavities being filled with a yellow powder, which appears to be pure limonite, and is doubtless a product of decomposition. At Broomborough quarry, also near Totnes, there occurs a singular rock, evidently included in this series, of a dull purplish and brownish colour, in which patches and strings of a greenish, felstone- like substance resembling porcellanite are included, with amygda- Downloaded from http://jgslegacy.lyellcollection.org/ at Nanyang Technological University on June 15, 2016 ASHPRINGTON VOLCANIC SERIES OF SOUTH DEVON. 371 loidal kernels interspersed. At a small quarry above Allabeer on the right bank of the Dart and elsewhere some purplish and yellowish flecked shales are seen, the paler patches having a steatitic aspect. Again, the lavas are often highly calcareous, probably from sub- sequent infiltration, and as they are both aphanitic and sometimes flaggy, as above mentioned, they appear (without always assigning definite names, such as " porphyrite " or " basalt," to rocks which are so highly altered) to correspond to descriptions of "slaty calc- aphanites," even leading towards "' schalsteins." Of the fact of such alteration, the microscope leaves no room for doubt. The felspars are blurred, as if changing to saussurite, like the felspars in the Lizard gabbros ; or they exhibit a veined appearance. Some unaltered augite is usually present, as also a plentiful sprinkhng of magnetite or ilmenite ; but owing to the extent of alteration the thinnest sections let very little light pass. Nowhere do true grits appear to form any part of this series ; it is somewhat perplexing that the purple grits of Cockington, Beacon Hill, and Windmill Hill, which support the Triassic rocks of Paignton, do not apparently extend across the Dart between Totnes and Sharpham, as the same beds do south of Greenway and Ditti- sham. However, with the exception of a narrow strip which appears to be thrown down by a fault, and some beds at Langcombe Farm, they certainly form no feature south-west of a line extendi~g from Langcombe Cross to Stoke Gabriel. There are some signs of a N.W. and S.E. fracture bearing south-easterly for many miles, and its existence between the south-west of Ash and Stoke Gabriel was mentioned by Dr. Hell. If these strips really belong to the slaty beds at the base of the Cockington grits, then they overlie the volcanic series, and are thrown down by faults sensibly at right angles to the one just named, but they disappear before reaching the Dart. Were it not that the greenish aphanitic rocks can be actually observed passing to a deep red, the outlines of the felspars being still traceable, one might be inclined to regard all the soft, raddled, earthy-looking rocks as tufts; but for the reason just stated we could not safely do so. The quantity of magnetite or ilmenite which appears in every section I have had cut would furnish a ready source for any degree of peroxidation ; some sections, when seen by refiected light, show these specks turned brown, and when this destructive process is carried far enough, all distinctive cha- racters of the rock are lost. These decomposed rocks generally yield freely to the knife, differing in this respect altogether from grits of Devonian age.