436 Mr. J. Stevenson on the Chemical and 



would be equivalent to the whole o£ our present free oxygen ; 

 and as the continental and transitional areas occupy only 

 -about one-half of the surface of the earth, it is obvious that 

 there would be as much carbonaceous matter in each 5-mile 

 thickness of the whole of the earth's crust (or each shell or layer 

 of the crust \ mile thick) as would be equivalent to the whole 

 of our present free oxygen, if the rocks underlying the great 

 oceans should also contain ^ per cent, of carbonaceous 

 matter. We know practically nothing on this last point, but 

 as regards the continental and transitional areas it is highly 

 probable that the average percentage of carbonaceous matter 

 is greater than -^ , and that the rocks containing carbonaceous 

 matter have a much greater thickness than ^ mile. Further, 

 there is a considerable quantity of sulphide of iron present in 

 many rocks, and this has probably been formed by the 

 reducing action of carbonaceous matter on sulphates. All 

 these circumstances point (as was explained in the paper) to 

 the probable existence of an atmosphere in very ancient 

 times containing much hydrogen or hydrocarbon gases, and 

 if we knew the exact quantity of carbonaceous matter derived 

 from organic remains, and also of sulphide of iron produced 

 by the reducing action of organic remains, we might hope to 

 be able to make some estimate of the ancient atmosphere or 

 of the quantities of hydrogen or hydrocarbon gases present in 

 it. But the above data, even if they were ascertained with a 

 fair degree of completeness, would still be insufficient for 

 our purpose, as the question is complicated by other con- 

 siderations. We must on the one hand allow for free oxygen 

 that has been removed from the atmosphere and absorbed or 

 fixed in the ground through the oxidation or peroxidation of 

 various substances, especially the protoxide and the sulphide 

 of iron ; and on the other hand we ought to allow for 

 carbonaceous matter which may have been destroyed in the 

 course of geological history, but in such a way that its 

 former existence would have an important bearing on our 

 inquiry. 



As regards the quantity of oxygen used up in oxidizing 

 iron compounds, it was calculated by Ebelmen, about 1845, 

 that if all the sesquioxide of iron on the earth was originally 

 protoxide, there must have been at least as much oxygen 

 abstracted from the atmosphere to effect the peroxidation as 

 there still is of free oxygen in our atmosphere. This estimate 

 is confirmed, and more than confirmed, by Messrs. Clarke 

 and Hillebrand's series of rock-analyses published in Bulletin 

 148 of the United States Geological Survey (Washington, 

 1897), from which Mr. Clarke calculates that the average 



