CARBONIFEROUS PERIOD. 359 



General Observations. 



1. Origin of Coal. — (1.) Coal derived from Vegetation. — As the coal 

 beds and accompanying strata abound in the impressions of leaves 

 and stems, and the coal also consists of vegetable fibres (p. 328), the 

 vegetable origin of coal is beyond all reasonable doubt. 



(2.) Plants of the Coal. — The plants that have contributed most to 

 the formation of the great beds of vegetable debris which were 

 afterwards converted into coal, are the Sigillarids, the Catamites, 

 and the Conifers, with the Lepidodendra for that of the Lower Coal 

 measures. The Conifers and Lepidodendra probably spread also over 

 the dry land covering the plains and hills, while the Sigillarids and 

 Calamites were mainly plants of the great marshes. Along with 

 these were numerous herbaceous ferns, but rarely tree-ferns : even 

 the stems of the small ferns are not common in the coal itself, 

 though abundant in the accompanying shales. 



(3.) The plants either land or fresh-water species. — That the plants 

 were not such as frequent salt marshes, but, on the contrary, those 

 of the land or fresh-water marshes, is obvious from (1) the nature 

 of the plants themselves ; (2) the absence of sea-weeds from among 

 the species of the coal beds ; (3) the presence of the remains of 

 insects. It is not possible that some of the beds have originated 

 from the vegetation of salt marshes and others from that of the 

 land or fresh-water marshes, because there is a great uniformity in 

 the plants of the several beds, — showing that all are of one mode 

 of growth and origin. 



(4.) Coal a result of the decomposition of plants. — Mineral coal is 

 simply the element carbon along with some kinds of bituminous 

 substances that consist of carbon and hydrogen, and also admixtures, 

 small or large, of earthy impurities. 



Dry vegetable matter consists of about 49 per cent, of carbon, 

 6.3 of hydrogen, and 44.G of oxygen. The first, the essential ele- 

 ment of the coal, is solid at the ordinary temperature ; the other 

 two elements are gases. In the decomposition of wood, the gaseous 

 part escapes, carrying off part of the carbon in combination with it, 

 and leaves the rest of the carbon as coal, with more or less bitumen, 

 derived from a union of some carbon and hydrogen. In this decom- 

 position, the oxygen may combine (1) with part of the hydrogen to 

 form water ; (2) with part of the carbon to form carbonic acid or 

 carbonic oxyd ; also the hydrogen may combine with the oxygen of 

 the atmosphere to form water ; or some of it with some of the carbon 

 to produce carburetted hydrogen gas, or (with or without oxygen) 

 the bituminous substances. By these means, half or more of the 



