Naming Rocks. 153 
Obsidian—A black, brown, red or green volcanic 
glass, generally or most always of some dark color, 
looking and breaking like dark bottle glass. 
Pumice-stone—A light, spongy, grayish rock, which 
floats on water. It is a volcanic foam, so to speak. 
Syenyte—A metamorphic and eruptive rock close- 
ly related to granite. It consists of orthoclase, horn- 
blende, oligoclase, mica, nepheline, augite, and some- 
times zircon. It is generally a grayish and flesh-colored 
rock, from the colors induced in it by its chief typical 
minerals, orthoclase and hornblende. Zircon syenyte 
is rare and occurs chiefly in intrusive dykes. 
G.=2.7—2.9. Silica 58—63 p. c. - 
Porphyryte—A volcanic rock, close-grained and 
breaking with an even fracture. It is formed almost 
entirely of feldspar with some magnetite. It is known 
by various names, according to the development of cer- 
tain minerals. Mica-porphyryte has much mica; horn- 
blende-porphyryte, much hornblende, etc. Porphyryte 
was largely outpoured during the carboniferous age, and 
would appear to be an imperfectly crystallized lava. 
Trachyte—A rock of various colors, hard, but brit- 
tle, with rough fracture. It is the volcanic representa- 
tive of syenyte. It was originally poured out as a thick, 
viscous lava-stream. Sanidin is always well represent- 
ed, which forms glassy crystals in the base. The min- 
erals hornblende, augite, apatite, magnetite and titan- 
ite are often present. It is an orthoclase rock, with also 
oligoclase and crysolite. G@. = 2.6—2.7. Silica 60—64 
p- ¢. 
Minette—A volcanic rock consisting of biotite, or- 
