PEECAEBONIFEEOUS EOCKS OE CHAENWOOD EOEEST. 215 



browner dust remains opaque ; the more transparent granules appear 

 to be sometimes of rather vermiform aspect, showing clear bright 

 colours, pinkish, bluish, and yellowish, probably prehnite or some 

 allied mineral. The exterior rim of the crystal often seems to have 

 escaped better than the interior, or, as there is reason to think, 

 pseudomorphism is here more complete on the exterior*. In some 

 of the crystals a minute chloritoid mineral is also present. Separate 

 grains of quartz, such as are seen in granite, are rare, but a good 

 deal occurs intercrystallized with felspar in a very curious way. 

 This structure, which was noticed by Mr. Sorby in 1863 f, resembles 

 " a microscopic graphic granite or hebraic felspar." The felspar is 

 generally much decomposed, and occasionally stained green with 

 chlorite; it sometimes occupies irregular polygonal spaces among 

 the quartz, sometimes is more irregular and wavy in outline, pre- 

 senting, when green, a rough resemblance to an alga. The quartz 

 (with crossing Mcols) is uniform in colour over a considerable portion 

 of this structure, showing that it forms a kind of composite crystal. 

 Some examples rather favour the idea of this graphic structure being 

 of secondary formation ; but the evidence, on the whole, makes for 

 its being original. The hornblende appears generally to be rather 

 decomposed, is irregular in outline, often rather fibrous in structure, 

 especially at the end ; it is commonly partly replaced by viridite 

 and epidote ; some of it also looks like a pseudomorph after biotite ; 

 the iron is probably in the form of magnetite, but the mode of 

 decomposition of occasional grains suggests ilmenite. The apatite 

 occurs in clear, long, six-sided prisms, and is rather abundant in 

 the Groby rock, especially in the slide from the Pool Pit, where 

 some of the prisms are about '04-5 inch long. It appears to have 

 crystallized first in order. 



The Cliff-Hill rock is a little more decomposed than the others, 

 as we should expect from its appearance — fibrous zeolite, chlorite, 

 and epidote abounding in place of the felspar and some of the horn- 

 blende. The latter mineral, however, has commonly a very unusual 

 aspect, being fairly well preserved, with one well-defined set of 

 cleavage-planes often marked out by minute included microliths, 

 dichroism imperceptible when unaltered, and colour bright with 

 crossed prisms. In fact the mineral much more resembles a diallage, 

 and sometimes even augite, which in places is converted into uralite, 

 actinolite, or viridite (the anisotropic variety) ; one crystal is twinned 

 with the sections of the cleavage-planes at about 74° to the plane of 

 twinning. 



* In examining felspars in rather decomposed rocks, such as the Carboni- 

 ferous dolerites of Scotland, I have often observed that a felspar seems to pass 

 through the kaolinized stage to a clear pseudomorph, which shows only milky 

 white and blue-black colours with the Nicols, instead of tints common with 

 felspar.— T. G. B. 



t Geol. Mag. vol. ii. p. 448 (from Geol. & Polyt, Soc. of Yorkshire). The rock 

 here described is said to be from Mount Sorrel ; but Mr. Harrison has ascer- 

 tained (Geology of Leicestershire, p. 11) that the specimen came from Groby. 

 Mr. Sorby calls attention to the fluid cavities in the quartz, which indicate that 

 the rock crystallized under great pressure. 



