KINDS OF ROCKS. 43% 
dules in limestone are whitish, owing to the limestone 
material they contain. Chert sometimes contains cavities 
which are lined with chalcedony or agate, or with quartz 
crystals. 
5. Jasper rock.—A flinty siliceous rock, of dull red, yel- 
low, or green color, or some other dark shade, breaking with 
a smooth surface like flint. It consists of quartz, with more 
or less clay and iron oxide. The red contains the oxide in an 
anhydrous state, the yellow in a hydrous; on heating the 
latter it turns red. 
6. Buhrstone.—A cellular siliceous rock, flinty in texture. 
Found mostly in connection with ‘Tertiary rocks, and 
formed apparently from the action of siliceous solutions on 
preéxisting fossiliferous beds, the solutions removing the 
fossils and leaving cavities. 
Buhrstone is the material preferred for millstones. The buhrstone 
of the vicinity of Paris, France, has long been largely exported for 
this purpose. Good buhrstone is obtained also from the Tertiary in 
Greenville District, South Carolina, 100 miles up the Savannah River. 
7. Fioryte. (Siliceous Sinter, Pearl Sinter, Geyserite.)— 
Opal-silica, in compact, porous, or concretionary forms, 
often pearly in lustre; made by deposition from hot sili- 
ceous waters, as about geysers (Geyserite), or through 
the decomposition of siliceous minerals, especially about 
the fumaroles of volcanic regions. 
Geyserite is abundant in Yellowstone Park, and about the Iceland 
geysers ; after long exposure it crumbles down and becomes changed 
to ordinary silica, or quartz. 
2. MICA AND POTASH-FELDSPAR SERIES. 
1. Granite.—Consists of quartz, orthoclase, and mica, and 
has no appearance of layers in the arrangement of the mica 
or other ingredients. G.=2°5-2°8. The quartz is usually 
grayish- -white or smoky, glassy, and without any appearance 
of cleavage. The feldspar is commonly whitish or flesh- 
colored, and may be distinguished from the quartz by its 
cleavage surfaces, which reflect light brilliantly when the 
specimen is held in the sunlight. The mica is usually in 
small bright scales, either silvery, brownish-black, or black 
in color, ‘and the point of a knife carefully used will easily 
split them into thinner scales ; the silvery mica is muscovite, 
but sometimes of the allied hydrous kinds, margarodite or 
