426 GEOLOGY. 



of the deep sea are the great examples. Sometimes layers of in- 

 fusorial earth, tripolite, arise from the shells of diatoms and other 

 aquatic organisms secreting silica. The waters in which such earths ac- 

 cumulate are rather shallow, and either fresh or salt. The most 

 familiar examples of indurated rocks formed in this general method 

 are the flints and cherts (impure flints) that occur in Hmestone and 

 chalk, chiefly as nodules, but sometimes in distinct beds. 



Organic rocks. — While most limestones, chalks, flints, cherts, and 

 the silicious and calcareous oozes are formed through the agency of 

 organisms, they are not themselves strictly organic. There is, how- 

 ever, a small but important group of rocks formed directly from organic 

 matter. In favorable situations the woody parts of plants, faUing into 

 water, are so far preserved from decay that they accumulate in beds, 

 and by slow changes pass into peat, lignite, bituminous coal, anthracite, 

 and graphite. The first of these is composed essentially of carbohy- 

 drates and hydrocarbons much as plants are, while the last two are 

 mainly carbon, and the intermediate members represent stages of 

 passage from the first to the last. They are all derived from the strictly 

 organic part of the plants, and spring essentially from the atmosphere 

 and hydrosphere. They are only indirectly associated with the evolu- 

 tions of the inorganic series. 



INTERNAL ALTERATIONS OF ROCKS. 



Besides the extreme alterations of rocks at the surface of the eart^" 

 by which they pass into solution and into residual mantle-rock, and 

 at length by tranportation and re-sedimentation become stratified 

 rocks, as just described, those rocks which are not at the surface are 

 subject to changes that give rise to several varieties of altered rocks. 

 These changes are taking place constantly under ordinary conditions, 

 though usually very slowly. Under great pressure and heat the changes 

 are relatively rapid and intense, and lead to results not reached under 

 other conditions. These more profound changes are termed metamor- 

 phism, and will be considered later. It is, however, important to recog- 

 nize the great fact that the outer part of the earth, for perhaps 20,000 or 

 30,000 feet, is more or less fractured and permeated by water contain- 

 ing in solution various substances dissolved from the atmosphere, the 

 soil, and the rocks through which it has already passed, and that this 



