THE FOSSILIFEROUS ROCKS. 37 



In process of time this alumina would accumulate to form a bed 

 of clay; aad as this clay has been directly derived from the 

 decomposition of the shells of animals, it would be fairly entitled 

 to be considered an organic deposit. Though not finally estab- 

 lished, the hypothesis of Sir Wyville Thomson on this subject is 

 of the greatest interest to the palaeontologist, as possibly serving 

 to explain the occurrence, especially in the older formations, of 

 great deposits of argillaceous matter which are entirely destitute 

 of traces of life. 



It only remains, in this connection, to shortly consider the 

 rock-deposits in which carbon is found to be present in greater 

 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 decom- 

 position 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 im- 

 probably 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, of veins or strings, or 



