Geological History of the Atmosphere. 4:4t7 



the rocks composing the crust of the earth, it is quite possible 

 that it may have been much greater than the 6 per cent, pos- 

 tulated above. If the average composition of the earth as a 

 whole is at all similar to that of the meteorites which fall 

 upon it, and there is some reason to suppose that it is, 

 the percentage of iron present should be very large, and 

 therefore we should not be surprised if the rock -mass forming 

 the earth's crust originally contained very much more than 

 8 or 10 per cent, oxide of iron. To postulate a considerable 

 addition to the quantity of oxide of iron originally present 

 would no doubt intensify some of the difficulties discussed in 

 the preceding paragraphs, but it would not necessarily make 

 them insurmountable. However, our information with 

 regard to the various data required, viz., the composition 

 of the primordial basic rocks or rock-mass of the earth, and 

 the average composition and total quantity of the acidic and 

 other derived rocks, is too indefinite to admit of them being 

 used as the basis of a decisive verdict either one way or the 

 other. At the same time, it is worth noting that the total 

 quantities of water and carbonic acid (or carbonates) on the 

 earth, so far as they can be estimated roughly, seem to accord 

 fairly well with the requirements of our theory, at least in so 

 far as the question of whether there is enough carbon and 

 hydrogen on the earth to form the carbonaceous matter 

 necessary for reducing the oxide of iron is concerned. The 

 quantity of carbonic acid on the earth as inferred from 

 Dr. Sterry Hunt's estimate of the carbonates present, viz. 

 enough to form an atmosphere 200 times greater in extent 

 than our present one, may be taken roughly at 1'4 X 10 18 tons, 

 which corresponds to about 0'38 x 10 18 tons carbon. As we 

 have already estimated that the quantity of carbonaceous 

 matter which would be required to remove 6 per cent . 

 of oxide of iron from a shell of the earth's crust ten miles in 

 thickness would be 0*1 x 10 18 tons, it is obvious ihat there is 

 more than enough carbon on the earth, if it formerly existed 

 in the unoxidized condition, to effect the reduction in 

 question, and indeed that there is enough to remove fully 

 20 per cent, of oxide of iron from a shell of the earth's crust 

 ten miles thick even if each particle of carbon took part only 

 once in the reaction. And besides the carbonic acid there is 

 a very large quantity of water on the earth, even larger 

 than the quantity of carbonic acid as estimated above and 

 containing a larger proportion of oxygen. If a large pro- 

 portion of the hydrogen of the water originally existed in the 

 unoxidized condition, either quite free or in combination 

 with carbon or other elements, there would obviously be 



