430 DESCRIPTIONS OF ROCKS. 



Detritus (from the Latin for worn) is a general term 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 Earth)"!— Resembles clay or chalk, 

 but is a 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 

 the peat. 



This soft diatomaceous material is sold in the shops under the name 

 of silcx, 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 Xevada near Virginia City, and elsewhere. It is 

 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, 

 into c/urt. Consists of silica in the opal or soluble state. 



II. Limestones or Calcareous F.ocks. 



1. Xot Crystalline. 



1. Limestone. Calcyte. — 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. En- 

 diaital or Crinoidal limestone; containing crinoidal remains in the 

 form mostly of small disks, d. JYummulitic ; containing the disk- 

 shaped fossils called Xummulites. e. Oolitic limestone ; a limestone 

 having an oolitic texture, f. Bird's - eye limestone ; having small 

 whitish crystalline points scattered through it, a rock of Western 



