156 Naming Rocks. 
Doleryte—A very hard, crystalline eruptive rock. 
Its color is always dark, from grayish and bluish to 
greenish-black and brownish-black. It is a crystalline 
variety of basalt, and consists of labradorite, augite, oliv- 
ine, and oftens contains the minerals magnetite and apa- 
tite. G.=2.75—3.1. Silica 50—55 p. c. Granitoid to 
aphantic in texture. 
Basalt—A compact, minutely crystalline mixture 
of labradorite and augite, with olivine, magnetite and 
titanite. The olivine occurs in the base like green bottle 
glass. Basalt is very hard and of various shades and 
colors, from gray to black. It is a very common eruptive 
rock, filling volcanic vents, fissures, and occurring in 
vast sheets covering large areas of country. Basalt is 
often vesicular and assumes many columnar forms. 
Diabase—An ancient doleryte or crystalline basalt. 
It is composed of the minerals labradorite, augite, oliv- 
ine and chlorite. It is a dark, compact rock, resembling 
doleryte and basalt, from which it can generally be dis- 
tinguished by the presence of light-green patches of 
chlorite, arising from the decomposition of the olivine 
constituent. No glass in base. 
Silica 53 p. c. Alumina, about 20 p. ¢. 
Breccia—A rock formed out of the angular frag- 
ments ejected from volcanoes. It is of frequent occur- 
rence in lava-flows. Some are of sedimentary origin 
(Dana). Tufa—A similar rock and of like origin, but 
with the fragments smaller. It is a fine sand conglo- 
merate. 
Mica-schist—A foliated arrangement of quartz and 
mica; probably a schistose greisen-granite, or dioryte. 
The mica probably derived from the decomposition of 
