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III.—Notes on the North-Western Region of Charnwood Forest2

Published online by Cambridge University Press:  01 May 2009

T. G. Bonney
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Charnwood Forest, since 1891, the date of the last paper by Canon E. Hill and myself, has been investigated by the Geological Survey. Though part of their map and the accompanying memoir have not yet been published, the general results of their work have been announced by Professor W. W. Watts, by whom most of it was executed. As we stated at the time, we were far from being satisfied with some important points in our own conclusions; so that since my return to Cambridge I have studied my specimens and slices from the north-western region, which had presented to us the more serious difficulties. In 1891 I had been led to regard the characteristic rocks of Peldar Tor and High Sharpley as lava-flows, but considered the dominant rocks of Bardon Hill to be mainly pyroclastic. Professor Watts, however, maintained the intrusive character of the first and second, while taking the same view as myself about the third. The lava-flow hypothesis had appeared to me the more probable, because I doubted whether a mass so large as the Peldar-Bardon porphyroid, if intrusive, could have maintained throughout a texture so uniformly fine-grained, and I had found in the Bardon quarries fragments of it embedded in rock which I then supposed to be a somewhat altered tuff, closely related to the High Sharpley lava.

Type
Original Articles
Information
Geological Magazine , Volume 2 , Issue 12 , December 1915 , pp. 545 - 554
Copyright
Copyright © Cambridge University Press 1915

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References

page 545 note 1 Walcott, C. D., Smiths, Misc. Coll., vol. liii, 1908Google Scholar.

page 545 note 3 Quart. Journ. Geol. Soc., vol. xlvii, p. 78, 1891Google Scholar.

page 546 note 1 In our Charnwood papers I accepted all responsibility for the microscopic work.

page 546 note 2 I gladly tender my thanks to the owners or managers of the quarries—to Mr. B. N. Everard, of Bardon Hall, for giving us the help (as he was himself unable to meet us) of his foreman manager, Mr. R. B. Grant, who personally conducted us over Messrs. Ellis & Everard's quarries and called our attention to particular sections which other geologists had found interesting. The knowledge thus acquired has led him to put aside specimens which strike him as remarkable, and I am indebted to him for two of much importance, which are mentioned in this paper. I have also to thank Mr. J. H. Robinson, of the Peldar Tor quarry, and Mr. J. T. Briers, of the Forest Rock quarry, for much kindly assistance.

page 547 note 1 Quart. Journ. Geol. Soc., vol. xxxvi, pp. 342–5, 1880Google Scholar.

page 547 note 2 Now the fourth, counting upwards from the lowest. The quarry in 1878 was divided into two stages. In 1880 one was opened a little farther down the hill, so the middle quarry of 1891 (Q.J.G.S., vol. xlvii, p. 86) is the lower of our earlier papers. There are now six quarries one above the other, the fourth, counting upwards, corresponding with our ‘middle’, and two quarries (besides trial holes) on the north-western side, one of them less than 100 feet below the summit (912 feet).

page 547 note 3 Quart. Journ. Geol. Soc., vol. xlvii, p. 82, 1891Google Scholar.

page 548 note 1 Signs of fluxion, as well as of subsequent pressure, may sometimes be noticed, and the former seem to be more common in the neighbourhood of a junction. Many of the Pre-Cambrian and Ordovician felstones, in which some amount of fluxion is perceptible, show, in a single slice, appreciable, variations in their microscopic structures.

page 548 note 2 Viridite is an old term which I think might well be retained. Of course it is vague, but so is the thing thus denoted. It often is obviously made up of tiny flakes, but not seldom has hardly any effect on polarized light (perhaps because they are so minute). Some of the larger flakes appear to be a chlorite, while others have more resemblance to a greenish mica ; others may, perhaps, be serpentine. In fact, I believe it to be often more or less of a mixture, varying in composition with the mineral which has been replaced; the above-named colour change from purple to green meaning the formation of a hydrous iron silicate.

page 549 note 1 Quart. Journ. Geol. Soc., vol. xxxiii, pp. 785–6, 1877. The mistake is noticed, id., vol. xlvii, p. 91, 1891Google Scholar.

page 549 note 2 Locally of a reddish colour.

page 549 note 3 We find sometimes, within 2 or 3 inches of a junction, solitary grains of quartz and half-destroyed felspars, which can only have been derived from the Peldar porphyroid.

page 550 note 1 In that case it may be a zeolite, but Mr. Rastall suggests to me that some grains more resemble dolomite.

page 550 note 2 A pale red with dull green spots.

page 551 note 1 Geology in the Field, p. 770 ; also ch. ii in Mem. Geol. Surv., Expl. Sheet 141 (C. Fox-Strangways), p. 5Google Scholar.

page 551 note 2 Loc. cit., p. 776.

page 551 note 3 Supposing this to be pyroclastic, I interpreted the other as a fragmant ejected in advance of a lava-flow (represented by the Peldar porphyroid) ; but in any case such a relation makes it in the highest degree improbable for the latter to be the intruder.

page 551 note 4 For the preservation of this, which (with the second mentioned) is now in the Sedgwick Museum, I have to thank Mr. Grant. The only difference between it and the specimen collected by myself is that the ‘Bardon’ rock in the one case is greenish, in the other purplish in colour, which, as said above, may be disregarded. Under the microscope they are identical.

page 551 note 5 I have never detected any hardening of the Bardon rock at the junction with the Peldar porphyroid, and doubt whether the redness mentioned can be interpreted as a contact phenomenon.

page 551 note 6 A notice of it is given : Q.J.G.S., vol. xxxiii, p. 777, 1877Google Scholar.

page 552 note 1 Well exposed in a little ridge running roughly north and south.

page 552 note 2 Q.J.G.S., vol. xlvii, p. 95, 1891Google Scholar.

page 552 note 3 Q.J.G.S., vol. xlvii, p. 85, 1891Google Scholar.

page 552 note 4 Geology in the Field, p. 774.Google Scholar Possibly also, they think, the underlying “felsitic agglomerate” may have been similarly treated.

page 552 note 5 We noticed also that the slate fragments exhibited a rough parallelism.

page 553 note 1 I write this after spending a long time over rock slices from other districts in my collection, and comparing those from Charnwood with others, like them very ancient, in which the structure was less ‘blurred’ by the formation of secondary minerals, or in which any subsequent change was improbable. There must, no doubt, be some micromineralogical secondary change in the ground-mass, but the main question is, how far that ‘spotty’ structure of the Peldar rock, which disappears with crossed nicols, is a consequence of this. This disappearance favours an affirmative answer, but I find a generally similar structure exhibited by slices in a (purchased) collection of Hungary rocks, where it must, I think, be primary, and it occurs in a slightly more pronounced state at the Lower Quarry, Enderby (near junctions with a sedimentary rock considered by Mr. A. J. Lowe to be Stockingford Shales), and in the two quarries between Narborough and Croft. But in a slice labelled Thonstein porphyr, Mohorn, near Freiberg, I find a ‘spotted’ structure which mostly disappears with crossed nicols, and in rock from the Wrekin district, which must have been devitrified, I find indication of a very clear separation of the more felspathic from the more quartzose part, which may be secondary. For a discussion of devitrification, see Quart. Journ. Geol. Soc., vol. lix, p. 440, 1903. So at present I prefer to regard the question as still unsettled.