Geological History of the Atmosphere. 401 



of ascertaining the probable amount of the world's supply of 

 coal, or at least of ascertaining whether a high estimate of the 

 total carbonaceous matter of the earth — high enough to make 

 it equivalent to our free oxygen — is at all credible. We think 

 that the general results indicate that this high estimate is 

 quite credible. But even supposing that the evidence 

 pointed distinctly in the opposite direction — supposing that 

 there was good reason to believe that there was not enough 

 fuel of a carbonaceous nature to use up all our free oxygen, 

 it would not follow that there must have been at least some 

 free oxygen in the earth's primitive atmosphere. It is quite 

 possible that a large quantity of organic matter may have 

 been deposited in certain rocks, and may afterwards have dis- 

 appeared while the free oxygen corresponding to it still 

 remains in the free condition. For it is well-known that 

 there is found distributed through many rocks a considerable 

 amount of sulphide of iron, which is an eminently oxidizable 

 substance. There is also little doubt but that it has, in the 

 case of strati ried rocks, been produced from oxide (or car- 

 bonate) of iron and sulphate of lime by the reducing action 

 of carbonaceous matter derived from animal and A r e2;etable 

 remains. It may therefore, like coal, be regarded as a fuel of 

 vegetable origin, and the amount of oxygen that would be 

 required for its complete oxidation may be regarded as so much 

 oxygen that has been separated from carbonic acid by the 

 action of vegetation. We should therefore be justified, when 

 estimating the free oxygen produced by vegetation on the 

 earth, to include not only the oxygen corresponding to all 

 the carbonaceous matter derived from animal and vegetable 

 remains, but also the amount of oxygen corresponding to the 

 sulphide of iron found in sedimentary rocks. We cannot give 

 even an approximate estimate of the percentage of sulphide of 

 iron in such rocks, but we know that it is fairly high — quite 

 comparable with that of the organic or carbonaceous matter 

 itself, and therefore of great importance in the question of 

 the history of free oxygen. 



A large amount of sulphide of iron is also found in metal- 

 liferous lodes or veins. Other metallic sulphides are also 

 found in these veins, and, as is well-known, constitute the 

 principal ores of lead, zinc, mercury, antimony, copper, and 

 other metals. The circumstance that these ores consist 

 principally of sulphides, and contain but little oxygen except 

 in the upper part of the veins, where they have in all proba- 

 bility been oxidized by the action of air and water, indicates 

 that there is, most probably, a deficiency of oxygen in the 

 regions of the earth from which the material filling the lodes 

 is derived. It is of course just possible that the ores or 



