Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/552

466 At Shap Wells, close to the medicinal spring, occurs a singular band of Coniston Limestone, to which we shall have occasion to refer again.

The band in question is a calcareous breccia composed of innumerable fragments of various rocks, mostly under a quarter or half an inch in diameter, imbedded in a calcareous matrix. Thin sections of this rock are very difficult to prepare, owing to the large number of angular fragments of quartz which they contain; in addition to which there are numerous fragments of limestone, felspathic ash, and perhaps traps, the cementing matter of the whole being a granular limestone apparently devoid of fossils. That this breccia is more or less altered by the near vicinity of the Shap granite cannot be doubted, the more so as the graptolitic mudstones are seen in an unequivocal form about 200 yards to the south of the breccia, with numerous graptolites, but so highly indurated as to have become almost flinty in their fracture. In connexion with this we may briefly refer to the microscopic characters of the more highly metamorphosed Coniston Limestone, about a quarter of a mile from the mineral spring, near the top of the Blea Beck. Here the Coniston Limestone is penetrated and apparently overlain by a mass of felstone, doubtless emanating from the granite of Wastdale Crag. In the immediate vicinity of this intrusive mass, the limestone is converted into an olive-green splintery rock, which shows, in microscopic sections, particles of iron pyrites and disseminated specks of a dark green mineral (hornblende?), together with a few traces of minute fossils. At a distance of a few yards from the felstone this limestone presents itself as a dark grey crystallized rock, which is shown by thin sections to be completely granular, with hardly any indications of fossils.

About two miles west from Shap Wells, at a short distance to the south-west of the farm-house of Wastdale Head, the Coniston Limestone exhibits itself in a very highly metamorphosed condition. Here, in the course of a small stream flowing from the north, a small patch of the limestone occurs, having a white colour and a crystalline structure. A few feet to the north of this patch a small exposure of rock is seen, having a gneissic character, the particles of which are very crystalline. This gneiss is very nearly in contact with the south-west portion of the Wastdale-Crag granite. It is probably either metamorphosed shales of the Coniston Limestone, or of the representatives of the Dufton Shales.

The white crystalline limestone has peculiar features. It has a fine scaly structure, with a pearly lustre, and resembles fine-grained Schiefer spar. It contains within it Idocrase; and in the upper portion, where it has been subjected to the action of the water of the stream, the idocrase almost remains alone, the carbonate of lime having been removed.