712 HISTORICAL GEOLOGY. 



The presence of this large percentage of carbonic acid and moisture would 

 have given the atmosphere a correspondingly greater power of absorbing 

 non-luminous heat, or that radiated by the warmed earth, and it therefore 

 accounts for the uniformity in the earth's climate. 



With conditions in the climate and atmosphere so favorable, the plants 

 would have been- rapid in growth, in covering emerged lands with jungles 

 and forests, and in supplying vegetable debris for the thickening peat-beds. 

 Although the era was one of more clouds than sunshine, growth must have 

 been, if possible, more exuberant than it is now in tropical America. 



The conditions were also favorable for decay. Old stumps of Lepidoden- 

 drids and Sigillarids, poor in wood, decayed within as they stood in the 

 swamps, while the debris of the growing vegetation, or, in some cases, the 

 detritus borne by the waters, accumulated around them ; so that their hollow 

 interiors received sands, or leaves, or bones, or became the haunts of reptiles, 

 as was their chance. 



FORMATION OF COAL FROM THE BEDS OF VEGETABLE DEBRIS. 



The formation of coal out of the beds of vegetable debris probably only 

 made a beginning while these beds lay as open beds of peat. The process is 

 carried forward imperfectly in the modern peat-bed, and the best result is a 

 poor coal, as it contains 25 per cent or more of oxygen. The deposits of 

 clay or sand over the peat accumulations of the Carbonic era prevented the 

 atmospheric oxygen from participating in the change, and to this is due the 

 better product. The making of coal from wood has been explained on page 

 124, under Chemical Geology. The resulting mineral coal consists (1) chiefly 

 of carbon; but (2) anthracite contains usually 2 to 5 per cent of oxygen and 

 hydrogen, and the bituminous coals often 12 per cent in weight of oxygen 

 and 4 to 6 of hydrogen ; while brown coal, the bituminous coal of later 

 formations (which ordinarily gives a brownish-black powder), contains 20 

 per cent or more of oxygen with 5 or 6 of hydrogen. 



Mineral coal, therefore, is not carbon, but a compound, or a mixture of 

 two or more compounds, of carbon, hydrogen and oxygen, associated proba- 

 bly with some free carbon in anthracite, and possibly in some or all bitumi- 

 nous coal. In this view, coals are mainly oxidized hydrocarbons, or mixtures 

 of them. They are feebly acted on by ether or benzine, if at all, and hence 

 contain little or no mineral oil, or only a trace of any soluble hydrocarbon ; 

 but, at a high temperature, hydrocarbons (compounds of hydrogen and car- 

 bon) are giv^en out, and often very abundantly, in the form of either mineral 

 oil, tar, or gas. 



The process of the conversion of woody material into coal is briefly 

 described on the page referred to. The vegetable material from which coal 

 is made may be (a) woody fiber ; (6) cellular tissue ; (c) bark ; (d) spores of 

 Lycopods (Lepidodendrids, etc.) ; (e) resins and associated substances. The 

 following is the composition of (1) dried wood in the mass ; (2) cork (the 



