Prof. Bonney and Miss Raisin — Rocks from Kimheyley. 497 



(3) Mica. The flakes are somewhat irregular in outline, of 

 a pale but rather warm brown colour, well cleaved. The 

 pleochroism is from light dull buff to an ochre-brown. Cases of 

 twinning occur; one of the twins sometimes greatly dominating 

 over the other. The mica at its exterior is changed into a mass 

 of a minute, very pale green mineral, which sometimes extends up 

 cleavage cracks. This apparently is the material designated vaalite 

 by Professor Maskelyne. 



(4) Garnet : (rare in this specimen) wine-red in colour. In one 

 case this mineral is surrounded by a zone about 'OOG" thick (except 

 on one side, where it becomes a mere film). This zone exhibits 

 a radial fibrous ci-ystallization and a banded structure. It varies 

 in colour from a rather light to a very dark umber-brown, and the 

 constituents seem to have a straight extinction. Beyond this, in 

 three directions, is a much broader border, variable in thickness, 

 barely translucent, and a deep umber-brown in colour ; but perhaps 

 only a more felted mass of the mineral already mentioned. Tlie 

 more diaphanous part does not appear to be sensibly dichroic, and 

 can hardly be biotite or an altered mica. It might, however, be 

 a mixture of iron oxide with fibrous quartz, as in the so-called 

 crocidolite. In another slice biotite does occur as a decomposition 

 product of a garnet, but here the former mineral eats into the latter 

 in a somewhat ii'regular manner. 



(5) Chromite. Feebly translucent brown grains, occasionally 

 octahedral in form, are doubtless chromite, but other iron oxides are 

 present in the slide. 



(6) Pseudobrookite (?). Other brown grains, occurring in a rudely 

 square form or as an association of granules, are anisotropic, and 

 sometimes exhibit, with crossed nicols, specks of rather bright colour, 

 indicative, probably, of aggregate structure. Small crystals, however, 

 of a mineral to be mentioned below (perofskite) are sometimes 

 clustered about these grains and may mask the colour effects. We 

 refer these brown grains to pseudobrookite, though the tint is less 

 rich than usual. This, however, may be due to decomposition. 



(7) Rutile. A few small crystals occur, among them a geniculate 

 twin, and in serpentine, as observed by Prof. Carvill Lewis. We 

 doubt, however, whether it is an alteration product. 



(8) Perofskite. The grains, usually from -0002" to -0008", are 

 sometimes hexagonal in outline, often showing a central spot and 

 radial lines, a very faint yellowish tinge with transmitted light, 

 and a whitish or yellowish-grey with reflected. These grains liave 

 a habit of clustering, and they sometimes wreathe around fragments 

 of crystals and rocks. 



Leaving for a moment the rock fragments, we proceed with the 

 matrix, to which, indeed, the last-named mineral might be referred. 

 Its hardness is barely 3, though among its microscopic constituents 

 is one which scratches glass. When powdered, it effervesces briskly 

 with HCl. Apparently it is composed of the following constituents : 

 (.t) a clear colourless isotropic serpentine; (6) brown mica, in small 

 films, sometimes irregular in outline, having a general resemblance 



DECADE IV. VOL. II NO. XI. 32 



