430 DESCRIPTIONS OF ROCKS. 
Detritus (from the Latin for worn) is a generalééerm ap- 
plied to earth, sand, alluvium, silt, gravel, because the ma- 
terial is derived, to a great extent, from the wear of rocks 
through decomposing agencies, mutual attrition in running 
water, and other methods. . 
Soil is earthy material mixed with the results of vegeta- 
ble and animal decomposition, whence it gets its dark color 
and also a chief part of its fertility. 
12. Tripolyte (Infusorial Karth).— Resembles clay or chalk, 
but isa little harsh between the fingers, and scratches glass 
when rubbed on it. Consists chiefly of siliceous shells 
of Diatoms with often the spicules of sponges. Forms 
thick deposits, and is often found in old swamps beneath 
tne peat. 
This soft diatomaceous material is sold in the shops under the name 
of silex, electro-silicon, and polishing powder, and is obtained for com- 
merce in Maine, Massachusetts, Nevada, and California. A bed ex- 
ceeding fifty feet in thickness occurs near Monterey in California ; and 
other large beds in Nevada near Virginia City, and elsewhere. Itis 
used as a polishing powder; in the manufacture of ‘‘soluble glass; ” 
and also mixed with nitro-glycerine to make dynamite. Occurs some- 
times slaty, as at Bilin, Prussia; and also hard or indurated, from con- 
solidation through infiltrating waters. and thus graduates, at times, 
inte chzrt. Consists of silica in the opal or soluble state. 
II, Limestones or Calecareous Rocks. 
1. Not CRYSTALLINE. 
1. Massive Limestone.—Compact uncrystalline limestone 
usually of dull gray, bluish-gray, brownish, and black colors, 
sometimes yellowish-white, cream-colored, nearly white, 
and red of different shades ; in texture, varying from earthy 
to compact semi-crystalline. It consists essentially of cal- 
cite or calcium carbonate (p. 215), but often contains clay or 
sand, or other impurities. : 
VARIETIES.—The varieties depending on color are very numerous, 
and many of them, when pure and compact, are polished and used 
for marble. The gray and black colors are due commonly to carbo- 
naceous material, for they burn white ; but the yellow, red, and some 
other kinds to the presence of iron-oxide. There are also: a. Fossil- 
iferous or shell limestone. b. Coral or Madreporic limestone. c. Hn- 
clinital or Crinoida! limestone ; containing crinoidal remains in the 
form mostly of small disks. d. Nummutitic ; containing the disk- 
shaped fossils called Nummulites. e. Odlitic limestone ; a limestone 
having an odlitic texture. f. Bird’s - cye limestone; having small 
whitish crystalline points scattered through it, a rock of Western 
