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MATERIALS FOR A CENSUS OF DEVONIAN
GRANITES AND FELSITES.
BY R. N. WORTH, F.G.S.
(Read at Plymouth, July, 1892.)
I do not think that this paper requires any justification, but should it be deemed otherwise, such may possibly be found in the remarks of Sir Archibald Geikie in his recent Presidential Address to the Geological Society of London, on the igneous activities of Devonshire, in which he comments on the great quantity of work there remains to be done by way of investigation in this county. One cannot help feeling, however, that had a tithe of the official attention been given to the West of England that Scotland has received, the balance of interest and information might have been more evenly held. At the same time, while it is quite natural that the Scottish Directors of the Geological Survey should see that Scotland is not forgotten, it is equally natural that Devonian geologists should be inspired by a similar spirit.
The chief object of this contribution is to give some idea of the extent of variation in the granites and felsites of the county, a point to which very little attention seems to have been paid, either officially or independently. Years of geological work on Dartmoor and its borders have made me fairly familiar with the range of that variation; and the bulk of the present paper consists of a descriptive list of specimens of granites and felsites which I have, with
few exceptions, collected, and all of which I have seen. I do not think it possible to suggest any condition or gradation of granitic or felsitic material in composition, structure, colour, or metamorphism, that is not represented, or that is not linked by intermediate phases to all the rest. What we attempt to distinguish as separate rocks in this connection are, in fact, nothing more than passing stages. Their condition and appearance will vary with their history, but at bottom they are essentially one and the same, and nowhere in the whole series can any hard and fast line be drawn.
The best method of forming such a collection as is here described is to hunt the beds of the Dartmoor streams, the alluvia of their valleys, or the piles of material left by the ancient tin-workers, which are simply detrital matter in another form. For all practical purposes, in this connection, the river beds on Dartmoor may be regarded as out-door museums of the rocks of the valleys through which they run; and thence one may obtain specimens of felsitic dykes which would probably escape even careful search; for while the courses of the rivers form to a large extent rough sections of the country-where they traverse rocky channels—there are many points at which they pass over alluvial deposits by which the rocks are masked. But it may fairly be assumed that at some time or other the loose material of the river beds has been derived from some points of the watershed higher up their courses ; and that, in such an estimate of Dartmoor petrology as the present, is practically all we wish to know. The difficulty of tracing dykes in situ over a wide extent of moorland country, when they are not merely a few feet but often a few inches only in breadth, is all but insuperable-how difficult I have practically learnt from my own utter inability, after repeated efforts, to trace the block of Trowlesworthite which I found several years since on Trowlesworthy Tor, to its origin, though within fairly narrow limits it is perfectly certain where that origin must be.
The description given by De la Beche of the Dartmoor granite is, in a general view, as good as could be framed :1
“The granite of Dartmoor is, as a whole, a coarse-grained mixture of quartz, felspar, and mica, the latter sometimes white, at others black, the two micas occasionally occurring in the same mass. It is very frequently porphyritic from the presence of large crystals of felspar, and here and there schorlaceous, but the latter character is chiefly confined to the outskirts, where the Dartmoor granite adjoins the slates. The schorl not unfrequently occurs in radiating nests, of variable size and abundance. A complete passage may generally be traced between the compound of schorl and quartz, usually termed schorl rock, and the ordinary granite. The mica usually disappears as the schorl begins to be abundant, but sometimes, though not very commonly beyond limited areas, the granite rock is a mixture of mica, schorl, felspar,
1 Report on the Geology of Cornwall, Devon, and W. Somerset, 157, 158.
and quartz, in nearly equal proportions. After the absence of mica, the next mineral which usually disappears is the felspar, leaving the compound a mixture of schorl and quartz. . . . Even the compound of quartz, felspar, and mica would seem more variable towards the outskirts of the mass than in the interior, and schorlaceous veins and veins of fine granite appear more frequent under such circumstances.”
Generally speaking, De la Beche also notices the very variable character of the Devon and Cornwall granites; and incidentally describes many varieties of elvans or felsites; recording some of the changes that take place in short distances in elvan dykes themselves, alike in colour and in structure: as for example when he says of some of the West Cornwall elvans that they sometimes acquire a more granitic structure in the central parts of the dyke. He remarks : 2
“They may be considered as mere granite dykes, the chemical composition of which bears great analogy to that of the chief granitic masses in the vicinity of which they occur, the mineral structure considerably depending on the conditions for cooling to which they have been exposed.”
It was hardly possible when De la Beche wrote to have more clearly indicated the general conditions of the problem.
Many years ago the late Mr. Godwin-Austen suggested that the granites and granitic rocks of Devon were of three periods—the ordinary porphyritic granite of Dartmoor, the schorlaceous granite, and the elvans; and for a long period that opinion was generally held. At length it was recognised that the schorlaceous varieties were the product of alteration in the mineral structure of the original rock, and that while subsequent in form they were not really later in origin. The periods were thus reduced to two_that of the formation of the granite, and that of the formation of the elvans or felsites, which are clearly inore recent than the mass of the granite, since they intersect it in common with the adjacent rocks. And this view has generally been accepted up to the present day.
As the result of my own investigations I was led in 1888 to suggest that in Dartmoor we had the base of a volcanic region, which at some point or points reached the surface definitely as a volcano, but the superstructure of which had been denuded. I suggested also that in the elvans or felsites, we had a series of rocks intermediate between the plutonic granites, and the sub-aerial lavas which formed part of that superstructure.
Op. cit. 184, 185. VOL. XXIV.
The question was then put, and naturally: If Dartmoor was a volcano, where are its volcanic rocks? It is, I think, a sufficient answer to point out that the amount of denudation to which Dartmoor has been exposed would naturally remove this class of rocks, in laying bare the basal granite. As a matter of fact, however, felsitic fragments do occur here and there on Dartmoor, which show flow structure and approximate very nearly to eruptive rocks; and alike in South and North Devon, (as in detritus at Cattedown and among the porphyritic pebbles of Rockham Bayh) example of andesites and volcanic grits are found, associated with ordinary granites and felsites, which cannot be traced in Devon in situ, but which such a volcano as that suggested would naturally yield.
Still further light was thrown upon the subject by an inquiry into the origin of the igneous fragments in the breccias at the base of the Trias of South Devon, and specially along the coast from Teignmouth to Exminster. It was found that these consisted :—of granites precisely resembling those of Dartmoor; schorlaceous rocks of various kinds, to all appearance of similar origin; a wide variety of felsites, mostly indistinguishable from Dartmoor elvans, while others had a strongly marked rhyolitic or andesitic character; and a number of more or less felspathic traps, ranging from andesites to basalts, most of them closely resembling in situ traps of the Exeter, Crediton, and Tiverton areas.5
Thus sundry gaps between the plutonic granites and their volcanic representatives were
Morever it was noteworthy that the granites were much less frequently represented than the felsites and andesites and their allies, which is precisely what we should expect to find at such a comparatively early stage of denudation as that which supplied the Dartmoor materials to these breccias.
A further step was taken when a boss of the Cawsand felspathic trap at Withnoe was found to pass in the course of a few feet from "a rhyolitic pitchstone to a trachytic quartz felsite 6" that is, from a rock which would unhesitatingly be pronounced volcanic, to one which would unhesitatingly be accepted as a felsite or elvan—a rock that is known to pass absolutely and certainly into granite.
3 Trans. Devon Assoc. xxi. 77-80. 4 Ibid. xxiii. 400-406. 5 See “The Igneous Constituents of the Triassic Breccias and Conglomerates of South Devon.” Quar. Jour. Geo. Soc. xlvi. 69-83.
6 See “Additional Notes on the Cornish Trias.” Trans. Roy. Corn. Geo. Soc. 1891, 338-346.
Since then Mr. Ussher has observed in the vicinity of the Trias of Thurlestone a dyke of quartz-porphyry (Horswell Quarry, North Milton) or elvan, which with equal certainty passes into an andesite. In fact we seem to be getting this sort of evidence on all hands.
The list given will, I think, show in its wide variety and close and intimate gradation that our granites, felsites, and andesites, are not distinct species, but changing conditionsrather pathological than physical--of a common original.
What I have to suggest therefore is this : that instead of our granites and felsites being of three periods as Mr. GodwinAusten held; or of two periods as we have been accustomed to hold of late; that they with the felspathic traps of Central and East Devon-are really of one. I do not of course mean that they are absolutely contemporaneous, but that they are part of one and the same great series of igneous activities. That the elvans and felsites are later than the granites is clear, because they traverse them; but this may have happened at any period subsequent to consolidation, and need not infer any great range in time. What is equally to the purpose is the fact, hitherto too much overlooked, that porphyritic felsites traverse some of the felspathic traps, and must so far therefore be subsequent to them. I have found fragments in the red-rock breccias, showing junctions between porphyritic felsite and andesite; and examples occur in situ.?
There is no doubt that the granite of Dartmoor is later than the surrounding Carboniferous rocks, because it sends veins into them. It is evident also that the granites, the felsites, and the felspathic traps are earlier than the red-rock breccias, because their fragments are found therein.
In my paper on the “ Igneous Constituents of the Triassic Breccias,"'' I concluded
“ That the elevation of Dartmoor and the associated igneous phenomena, which have been commonly regarded as post-Carboniferous and pre-Triassic, may in all probability be assigned to narrower limits, and be regarded as not earlier than Permian times, and possibly as occupying the Permio-Triassic interval, continuing into the earlier stages of the Trias. Certainly if the eruptions of the 'felspathic traps' of Devon and their associated elvans are related to the great Dartmoor movement (and, as we have seen, these traps are in part of Triassic date), we cannot well give the origin of that movement a higher than Permian antiquity."
7 There are dykes of quartz-porphyry in a cliff quarry of andesite near Withnoe, noted and described by Mr. Bernard Hobson, F.G.S.—Quar. Jour. Geo. Soc. xlviii. 500. 8 Op. cit. Quar. Jour. Geo. Soc.