tarkI common rocks 39 



mated that it requires fifteen to thirty feet of peat to produce 

 one foot of coal and that the process takes three hundred years. 

 Another estimate, probably nearer correct, is that it requires 

 eight or nine thousand years for the formation of one foot of 

 coal. About three hundred feet of coal (the total thickness of 

 all the beds) is known in the Appalachian region, hence the time 

 required to deposit it varies from 90,000 to 2,700,000 years, if 

 we base our estimate on the above rates of accumulation. Some 

 single anthracite coal seams are sixty feet thick, so we can get 

 some idea as to the great length of time it required to make this 

 one seam. After accumulation has gone on for a long time 

 sands and muds may be washed into the swamp and the ac- 

 cumulated material buried. Various gases are driven off and 

 the carbon of the original wood with some of the hydrocarbons 

 and the ash are left behind to form the coal as we find it today. 

 The two common kinds of coal are soft or bituminous coal 

 and hard or anthracite coal. The bituminous coal fields are 

 found rather widely distributed in the United States, but the 

 productive area of the anthracite is confined to 480 square miles 

 in eastern Pennsylvania. The United States produces more 

 coal than any other country in the world. Bituminous coal 

 breaks into cubical blocks, is soft, brittle, appears banded and 

 burns with a long smoky flame. Anthracite has a shell-like frac- 

 ture or break, is hard and shiny, very brittle, does not ignite 

 easily and burns with a short flame. 



Summarizing then, the sediments can be arranged according 

 to origin as follows : 



Mechanical deposits of residual material. 



Conglomerate. 



Sandstone. 



Shale. 

 Chemical deposits of dissolved material. 



Salt. 



Gypsum. 



Some limestone and dolomite. 

 Deposits made through the agency of life. 



Coal. 



Limestone and dolomite. 



Siliceous rocks, as diatomaceous earth. 



The last division of rocks is the metamorphic class. This 



group includes rocks which have originated by the changing of 



the other two classes, hence the name metamorphic which means 



''change of form." By means of great heat and pressure and 



