
ROCKS 85 
(a) Highly Vitreous Felspathic Rocks are easily determined by their 
lustre. A glassy rock showing a uniform texture, either black or some 
dark colour, which breaks usually with a conchoidal fracture, and is 
translucent on thin edges, is most probably ods¢dian. Pttchstone may 
generally be distinguished from obsidian by its pitchy or resinous lustre, 
by its fracture, which is more frequently splintery than conchoidal, and 
by its feebler translucency. Its colour is variable—dark or light shades 
of green, brown, red, yellow, etc. Obsidian and pitchstone are acid 
glasses, and are, therefore, often found passing or graduating into acid 
hemicrystalline rocks. The dase glasses are associated with such rocks 
as basalt, into which they pass. This is often seen, for example, along 
the marginal surfaces of dykes and sills where, owing to the chilling 
influence of the surrounding rocks, the molten mass has cooled too 
rapidly to permit the development of crystallisation. 
(4) Compact and Fine-grained Felspathic Rocks are usually very 
difficult to determine in the field; indeed, it is often quite impossible to 
tell one from another in hand-specimens. The mode of their occurrence, 
the general character of the rocks with which they are associated, their 
structural features, and their mode of weathering, will often aid one in 
forming an opinion as to their nature. But only a microscopic examina- 
tion will suffice to determine precisely what the rocks really are. The 
following notes on some of the more commonly occurring compact 
felspathic rocks may, however, be of use to the beginner. 
A fine-grained or smoothly compact rock, which on fresh surfaces 
can barely be scratched with a knife or not at all, which is not affected 
by acid, and is fusible in thin splinters, is probably a fé/szfe. Should it 
contain more or less numerous phenocrysts of felspar and quartz, and 
possibly other minerals, such as biotite and hornblende, it is most likely 
a guartz-porphyry. Rocks of this type usually weather with a thin white 
or light-coloured crust. Their colour is very variable, but generally 
not dark—white, grey, yellow, brown, or red, being the prevailing tints. 
Quartz-porphyry might sometimes be confounded with rhyolite—a hemi- 
crystalline rock. Rhyolite, however, is not nearly so common or widely 
distributed a rock as the former. It often exhibits a finely cavernous 
character—the cavities being lined with quartz or some other form of 
silica—a structure which is not characteristic of quartz-porphyry. While 
the groundmass of a rhyolite not infrequently has the smoothly compact 
or dull horny-like aspect of that seen in felsites and quartz-porphyries, it 
is more commonly glassy, enamel-like, or porcellaneous, and very often 
highly perlitic ; but spherulitic and fluxion structures are more especially 
characteristic. Scattered through this matrix are conspicuous crystals 
of glassy felspar, granules of quartz, and small crystals of some dark 
ferromagnesian mineral (usually biotite or augite). The rock has often a 
peculiar, harsh, rough feel. All the Aighly acidic felspathic rocks have, 
on fresh, unweathered surfaces, a hardness of 6-5 or thereabout ; they are 
usually light-coloured, and generally weather with white or light-coloured 
crusts, which are relatively thin, and more or less well defined, but not 
readily detached from the rock. 
