36 PRINCIPLES OF PALAEONTOLOGY. 



or less quantity. In the great majority of cases where rocks 

 are found to contain carbon or carbonaceous matter, it can be 

 stated with certainty that this substance is of organic origin, 

 though it is not necessarily derived from vegetables. Carbon 

 derived from the decomposition of animal bodies is not uncom- 

 mon ; 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 per- 

 centage 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 im- 

 pregnate fossiliferous rock-masses, is only what might be ex- 

 pected ; 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 con- 

 spicuous constituent of rock-masses in the form of graphite or 

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

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

 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 often the case in the Lau- 

 rentian formation, there can be little hesitation in regarding it 

 as of vegetable origin, and as an altered coal. 



* In the Huronian 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. 



