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II.— Petrological Notes on some Lake District Rocks

Published online by Cambridge University Press:  01 May 2009

W. Maynard Hutchings
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Extract

During the last three years I have at various times collected and studied rocks from the Lake District. Some of them are, I think, of sufficient interest to render them worth describing. It is my opinion that a great deal of very interesting petrological work still remains to be done among these rocks, and that when they are more studied in detail they will be found to present more diversity of type than is usually supposed.

I will first note the occurrence of a variety of rock not previously recorded in this district. It is a quartz-andesite, or dacite. It is exposed on the ridge between Greenburn and Wytheburn, not very far from Dunmail Raise, and near to a new wire-fence which bounds, I believe, the property of the Manchester Waterworks. It is a dark-coloured rock, on a newly-fractured surface of which are seen numerous light spots of porphyritic felspars, and some of calcite, with many grains of quartz, some of good size. It was the grains of quartz which instantly attracted my attention to this rock when a bit was chipped off it in passing the exposed crag.

Type
Original Articles
Information
Geological Magazine , Volume 8 , Issue 12 , December 1891 , pp. 536 - 544
Copyright
Copyright © Cambridge University Press 1891

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References

page 539 note 1 Mr. Ward refers to rocks of a grey-blue colour, very compact, which are outwardly not unlike those above described. The closest resemblance is in a lava from near Lodore Hotel, which is singled out by him on account of its nonporphyritic nature and the smallness of its felspar-needles. He gives the average length of these as th of an inch. They are, therefore, very much larger than in the rocks referred to by me, and from his description the Lodore lava was evidently very much less vitreous than these. It resembles, as Mr. Teall points out, the ground-mass of the average andesites of the district; a ground-mass in which the felspars are above the average sizes of typical andesites elsewhere.

page 541 note 1 Mr. Teall uses the term “andesitic dolerite” on several occasions, pointing out that some authors would simply call such rocks augite-andesites; and he adds the very sensible reminder that “it is a matter of indifference what we call them, provided we recognize their true characters and relations” (British Petrography, pp. 194, 195).

page 544 note 1 Prof. Cole points out (“Aids in Practical Geology”) that the older trachytes (or quartzless porphyries, orthophyres, etc.) may often be difficult to mark off from the rhyolites, and suggests that the marked absence of porphyrite quartz in some of the “altered rhyolites” may give rise to a suspicion that they were trachytes. There is a “rhyolite” exposed in the plantation in front of Shap Wells Hotel, which differs notably from the other rhyolites near to it. Macroscopically it shows crystals of felspar more prominently than any of the others do. Microscopically it shows an abundance of felspar-laths and microlites, all orthoclase, lying in a rather plentiful cryptocrystalline base, “speckly” in polarized light, apparently devitrified glass. Such laths and microlites are absent from the other local rhyolites. There is none of the spherulitic structure so abundantly developed in these rhyolites, and there is no sign of free quartz or of “granophyre.” The silica-percentage of a specimen collected by me is 61·15, as contrasted with over 75 per cent, found in rhyolites of Wasdale Head and Stockdale by Mr. Garwood. The microscope shows nothing whatever to lead to the supposition that the silica-percentage has been lowered by decomposition or infiltration, the rock being very free from chlorite, etc. This was apparently never a rhyolite, but was most probably a trachyte with a large amount of glassy base.