CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 2*7 



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 un- 

 commonly " anthracitic," arid 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 Hack-lead. In this 

 form it occurs in the shape of detached scales, or of veins or 

 strings, or sometimes of regular layers j 1 and there can be 

 little doubt that in many 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 Lau- 

 rentian formation, there can be little hesitation in regarding 

 it as of vegetable origin, and as an altered coal. 



1 In the Huroriian formation at Steel River, on the north shore of Lake 

 Superior, there exists a bed of carbonaceous matter which is regularly in- 

 terstratified with the surrounding rocks, and has a thickness of from 30 to 

 40 feet. This bed is shown by chemical analysis to contain about 50 per 

 cent of carbon, partly in the form of graphite, partly in the form of anthra- 

 cite ; and there can be little doubt but that it is really a stratum of "meta- 

 morphic " coal. 



