CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 35 



not necessarily derived from vegetables. Carbon derived from the 

 decomposition of animal bodies is not uncommon ; though it never 

 occurs in such quantity from this source as it may do when it is 

 derived from plants. Thus, many limestones are more or less highly 

 bituminous ; the celebrated siliceous flags or so-called " bituminous 

 schists " of Caithness are impregnated with oily matter apparently 

 derived from the decomposition of the numerous fishes embedded in 

 them ; Silurian shales containing Graptolites, but destitute of plants, 

 are not uncommonly " anthracitic," and contain a small percentage 

 of carbon derived from the decay of these zoophytes ; whilst the 

 petroleum so largely worked in North America has not improbably 

 an animal origin. That the fatty compounds present in animal 

 bodies should more or less extensively impregnate fossiliferous rock- 

 masses, is only what might be expected ; but the great bulk of the 

 carbon which exists stored up in the earth's crust is derived from 

 plants ; and the form in which it principally presents itself is that of 

 coal. We shall have to speak again, and at greater length, of coal, 

 and it is sufficient to say here that all the true coals, anthracites, and 

 lignites, are of organic origin, and consist principally of the remains 

 of plants in a more or less altered condition. The bituminous shales 

 which are found so commonly associated with beds of coal, also derive 

 their carbon primarily from plants ; and the same is certainly, or 

 probably, the case with similar shales which are known to occur in 

 formations younger than the Carboniferous. Lastly, carbon may 

 occur as a conspicuous constituent of rock-masses in the form of 

 graphite or black-lead. In this form it occurs in the shape of detached 

 scales, or of veins or strings, or sometimes of regular layers ; and 

 there can be little doubt that in some instances it has an organic 

 origin, though this is not capable of direct proof. When present, 

 at any rate, in quantity, and in the form of layers associated with 

 stratified rocks, as is sometimes the case in the Laurentian formation, 

 there seems to be considerable probability in the hypothesis which 

 would regard it as primarily of organic origin and as of the nature of 

 an altered coal. 



