218 GEOLOGY. 



gradually new combinations, as they do quickly in com- 

 mon combustion. Heat is more or less an agent in ef- 

 fecting eremacausis, as it is in effecting the combustion 

 which is sudden and attended with palpable phenomena. 



314. Condensation of Coal by Pressure. While the 

 coal is forming it is subjected more or less to pressure 

 by the masses of matter which accumulate above it, and 

 form the rocky strata. It is therefore condensed, in- 

 stead of being light and porous, like charcoal, or the 

 coke which is obtained in the making of gas. In peat 

 we have an example of the looseness of coal formation 

 when subjected to but little pressure. 



315. Bituminous and Non-bituminous Coal. Anthra- 

 cite, or non-bituminous coal, is almost pure carbon, but 

 bituminous coal contains considerable hydrogen and ox- 

 ygen. It is the hydrogen, in connection with the car- 

 bon, that produces its flame when it is burned. The 

 flame is really the burning of the carbureted hydrogen, 

 which in our gas-works is evolved from bituminous coal 

 by a low degree of combustion, and is then passed about 

 by pipes to be burned in our houses. The flame which 

 plays over an anthracite fire when it is not wholly kin- 

 dled is the burning of a different gas, which I have no- 

 ticed in Part II., 67. It is supposed that anthracite 

 coal was once bituminous, and that it was freed from its 

 bituminous qualities by a process essentially the same 

 with that by which we make gas in our gas-works, the 

 difference being simply that we have left in the one case 

 a solid non-bituminous coal, and in the other a porous 

 one common coke. 



316. How the Coal was Deposited. You are now pre- 

 pared to see how the coal was deposited in beds or 

 strata between strata of rocks. In the first place there 

 was a growth of plants of various kinds, varying in size 

 from small mosses to enormous trees, in the dirt-bed of 

 which I have spoken in 310. This was, for the most 

 part, a swampy growth, and that, too, as you will soon 



