ROCKS 45 
(Plate IV. 4) or an acid felspar or both; green augite also is 
often present, and not infrequently biotite or hornblende; less 
common are rhombic pyroxenes. Ptchstone-porphyry is the 
name given to this rock, when the phenocrysts are numerous 
and prominent. Obsidian (see Plate XIII.) is grey to dark 
grey and black, seldom red or brown. The lustre is vitreous, 
and the fracture conchoidal. Phenocrysts are not common—- 
quartz rarely or never appearing. Sometimes this glass is 
crowded with crystallites, spherulites, microlites, etc.; in other 
cases the rock is almost devoid of such bodies. 
The structures characteristic of glassy rocks have already been 
described (p. 34). Perlite is a glass characterised by the prevalence of 
perlitic structure, just as Spherulite-rock is so named from the abundant 
development of spherulitic structure. Pumzce is a frothy, foam-like, 
stringy, cellular, spongiform acid glass: it does not form individual rock- 
masses, but occurs as a crust on acid lavas, cr as loose blocks, scoriz, 
cinders, etc. When a glassy rock becomes crowded with crystallites, 
spherulites, and microlites, it acquires a stony aspect, and is said to be 
devitrified. 
Obsidian is usually associated with effusive rhyolites—into which 
indeed it frequently passes. It occurs in Hungary, the Lipari Islands, 
the Canary Islands, Iceland, the Western United States, Mexico, 
Ecuador, New Zealand, etc. Pitchstone is also somewhat widely 
distributed, occurring in various parts of Germany, Tyrol, N. Italy, 
Scotland, etc. ; it appears commonly in the form of dykes and intrusive 
sheets or sills. 
The rocks described in the foregoing pages are all acid 
rocks, having a similar chemical composition, and their 
different and often strongly contrasted petrographical aspect 
would appear, therefore, to be due to the varying conditions 
under which they cooled and solidified. Rapid cooling of 
molten matter, as we have seen, results in the production of a 
vitreous rock, while protracted cooling gives rise to a hemi- 
crystalline or even a holocrystalline type. Geologists, there- 
fore, look upon granite as the deep-seated equivalent of our 
acid lavas, or rhyolites and rhyolite-glasses or obsidian. But 
between the deep-seated plutonic granites and the volcanic 
rhyolites, occur rocks which are to some extent intermediate 
in character—that is to say, they are not quite so crystalline 
as granite, and not usually so vitreous as the rhyolites and 
obsidians; these are the quartz-porphyries. Thus the same 

