
' Parr IL. § vi] CRYSTALLINE ROCKS—MASSIVE. 141 
bottle glass, having a perfect conchoidal fracture, and breaking into 
q sharp splinters, semi-transparent or translucent at the edges. Its 
~ eolours are black, brown, or greyish-green, rarely yellow, blue, or red, 
but not infrequently streaked or banded with paler and darker hues. A 
thin slice of obsidian prepared for the microscope 1s found to be very 
pale yellow, brown, grey, or nearly colourless, and on being magnified 
shows that the usual dark colours are almost always produced by the 
_ presence of minute opaque crystallites. Less frequently obsidian 
appears as a perfect glass without any foreign admixture. Its 
erystallites present themselves as black opaque trichites sometimes 
beautifully arranged in eddy-like lines showing the original fluid 
movement of the rock (Fig. 12); also as rod-like transparent 
microliths. They occasionally so increase in abundance as to make 
the rock lose the aspect of a glass and assume that of a dull flint-like 
or enamel-like stone. This devitrification can only be properly 
studied with the microscope. Again spherulites of a dull grey 
enamel appear in some parts of the rock so abundantly as to convert 
it into pearlstone. These spherulitic enclosures may be observed in 
Lipari in great abundance drawn out into layers so as to give the 
rock a fissile structure, while steam or gas cavities likewise occur 
sometimes so large and abundant as to impart a cellular aspect. 
Now and then the vapour vesicles are found in enormous numbers 
of extremely minute size, as in an obsidian from Iceland, a plane 
of which, about one square millimetre in size, has been estimated to 
include 800,000 pores. The average chemical composition of the 
rock is—silica, 71:0; alumina, 13°8; potash, 4:0; soda, 5:2; lime, 
1:1; magnesia, 0°6; oxides of iron and manganese, 3°7 ; loss, 0°6; 
total, 100°0,—mean specific gravity, 2°40. Obsidian occurs as a 
product of the volcanoes of late geological periods. In Europe it is 
found in Lipari, Iceland, and Teneriffe; in North America it has 
been erupted from many points among the Western territories; it is 
met with also in New Zealand.’ 
Perlite (Pearlstone), another vitreous condition of sanidine 
lava, consists, as its name indicates, of enamel-like or vitreous 
globules, occasionally assuming polygonal forms by mutual pressure. 
These globules sometimes constitute the entire rock, their outer 
portions shading off into each other so as to form a compact mass; in 
other cases they are separated by and cemented in a compact glass 
or enamel. ‘They consist of successive very thin shells, which, in a 
transverse section, are seen as concentric rings, usually full of the 
same kind of hair-like crystallites and crystals as in obsidian (Fig. 
12). As these bodies both singly and in fluxion-streams traverse the 
globules, the latter may be conjectured to be a structure developed 
in the rock during its consolidation analogous to the concentric 
spheroidal structure seen in weathered basalt. Occasionally among 
these concentrically laminated globules are found true spherulites 
where the internal structure is radiating fibrous. A predominance 
of these bodies forms spherulitic perlite or spherulite rock. 
1 On obsidian, see Zirkel, Micro. Petrog. 
