THE CARBONIFEROUS AGE. 117 



be vegetable matter which has been buried in the 

 strata of the earth's crust, and there subjected to 

 certain cl:emical and mechanical changes. The proof 

 of its vegetable origin will grow upon us as we pro- 

 ceed. The chemical changes which it has under- 

 gone are not very material. Wood or bark, taken as 

 an example of ordinary vegetable matter, consists of 

 carbon or charcoal, with the gases hydrogen and 

 oxygen. Coal has merely parted with a portion of 

 these ingredients in the course of a slow and imper- 

 fect putrefaction, so that it comes to have much less 

 oxygen and considerably less hydrogen than wood, 

 and it has been blackened by the disengagement of 

 a quantity of free carbon. The more bituminous 

 flaming coals have a larger amount of residual hydro- 

 gen. In the anthracite coals the process of carbonis- 

 ation has proceeded further, and little remains but 

 charcoal in a dense and compact form. In cannel 

 coals, and in certain bituminous shales, on the con- 

 trary, the process seems to have taken place entirely 

 under water, by which putrefaction has been modified, 

 so that a larger proportion than usual of hydrogen 

 has been retained. The mechanical change which 

 the coal has experienced consists in the flattening 

 and hardening efiect of the immense pressure of 

 thousands of feet of superincumbent ro6k, which has 

 f^rushed together the cell-walls of the vegetable 

 matter, and reduced what was originally a pulpy 

 mass of cellular tissue to the condition of a hard 

 laminated rock. '['o understand this, perhaps the 



