Notes on the Microscopic Structure of some Shropshire Rocks

Abstract

The Ercal Hill, to the north of the Rhyoliytes of the Wrekin, so admirably described by Mr. S. Allport, is a mass of reddish granitoid rook. Microscopic exammation of a slide of this caused me to suspect that it was not a true granite, even before the investigation of Dr. Callaway's specimens and our joint visits to the locality. Besides the granitoid rock, there is seen, in the lower portion of a large pit, a compact greyish rock; both these have here been very fully examined with the following results:— (A) Compact type .—Five specimens in all have been examined, three from different parts of the greyish mass forming the lower portion of the pit, two from its upper left-hand corner, where the rock has a redder colour and a very fragmental aspect. As regards the first three, under the microscope they possess in common far more resemblance to the rhyolites described by Mr. S. Allport than one would suspect from their macroscopic aspect. All, seen by transmitted light, have a more or less glassy aspect, and with crossing Nicols exhibit the usual devitrified structure. For the details of this I content myself with referring to Mr. Allport's descriptions, and will merely call attention to the structural variations in each specimen. (1) A compact buff-grey rock from the base of the quarry (“fairly typical, very jointy”—C. C.). This exhibits a great number of somewhat faintly defined crystallites of a rather curved or wavy form, such as are figured by Zirkel in