402 Mr. J. Stevenson on the Chemical and 



metallic compounds were at first well oxidized, and were 

 afterwards reduced to sulphides by the action of hydrocarbon 

 gases or similar compounds derived from carbonaceous matter 

 in the adjacent rocks. If this were the case, a considerable 

 quantity of carbonaceous matter would be required to account 

 for the total amount of sulphides in metalliferous veins. It 

 would, however, be rather difficult to form an estimate, and 

 it is possible that the amount required might not be so great 

 as to be at all comparable with the whole of our free oxygen. 

 But it is mucn more likely that the sulphides and similar 

 compounds (selenides, arsenides, <&c.) were present as such from 

 the very beginning of the history of the lodes; that is, from 

 the time when the metallic compounds and the accompanying 

 materials were injected or otherwise conveyed into the veins ; 

 and as these materials have probably been derived from more 

 deeply lying regions of the earth's crust, their composition 

 points to a general deficiency of oxygen on the earth as a 

 whole. 



Reference may be made at this stage to the large quantities 

 of petroleum and natural gas which are found in many places 

 in the earth's crust. These may not be so great as to be 

 comparable in amount with our total supply of coal, but still 

 they probably constitute a by no means negligible quantity of 

 oxidizable matter. If we bold the theory that they are 

 entirely derived from animal and vegetable remains by 

 destructive distillation, or by the molecular transmutation of 

 fatty matters, the quantity is at least sufficient to add an 

 appreciable amount of support to a high estimate of the 

 world's supply of fuel derived from organic remains. And 

 if we adopt Mendeleeff's theory that they have been formed 

 by the action of steam or water on carbide of iron and other 

 metallic carbides in hot interior regions of the earth, we must 

 infer that there is a deficiency of oxygen on the earth 

 relatively to the amount of oxidizable matter present. 



The observations made by Prof. Tilden on the nature of the 

 gases enclosed in granite and basaltic rocks, and quoted by 

 Lord Kelvin himself, have a similar corollary. That is to 

 say, if granite and basalt have been produced out of ancient 

 sedimentary rocks, then there must have been an appreciable 

 quantity of organic matter or "reducing" matter of some 

 kind present in these rocks ; while if they are derived from 

 more primitive materials than sedimentary rocks, they go to 

 indicate a deficiency of oxygen on the earth as a whole. 



This question about the excess or deficiency of oxygen on 

 the earth relatively to the amount of oxidizable matter present, 

 is of course not identical with our main question regarding 



