442 Mr. J. Stevenson on the Chemical and 



cent, of the total depth, and the weight of the coal about 

 2*25 per cent, of that of the total rocks present; and of 

 course there might be a large quantity of thinly-disseminated 

 carbonaceous matter in addition to the coal itself. 



There is, therefore, nothing very fanciful or far-fetched in 

 this theory about the production of metallic iron by volcanic 

 action, but of course the question as to whether the reaction 

 has taken place on a scale sufficiently great to affect the com- 

 position of the whole of the upper crust of the earth to a great 

 extent is a very wide and somewhat difficult one, involving as 

 it does a number of important considerations. In particular 

 the occurrence of the reaction to such an extent would require 

 a truly enormous quantity of coal or carbonaceous matter to 

 effect the reduction of the oxide of iron. Let us suppose that 

 the acidic or upper and altered part of the crust of the earth 

 is on the average 10 miles thick, and that it is composed of 

 rocks having an average specific gravity of 2*6. Its weight 

 in that case would be about 20 X 10 18 tons, and if 6 per cent, 

 oxide of iron has been removed from it by reduction with 

 carbonaceous matter, the quantity of oxide removed would 

 be 1*2 x 10 18 tons ; and this would require for its reduction 

 about 0*1 X 10 18 tons carbon, or fully 200 times as much as 

 would be required to combine with and use up the total free 

 oxygen of our present atmosphere, It is obvious that such 

 a quantity of carbonaceous matter would take an extremely 

 long time to produce by the growth of vegetation and the 

 deposition of coal or bituminous matter in the ground at 

 anything like the rate at which these natural operations are 

 carried on at present. If the average percentage of carbon- 

 aceous matter in the crust of the earth should be only j 1 ^- of 

 1 per cent., and should have been only -jL of 1 per cent, 

 during the whole of geological history, it would take a series 

 of sedimentary strata 50 miles in thickness all over the earth 

 to give the amount required. Then if we assume (as is 

 most probable) that the ordinary geological operations of 

 denudation and deposition have been confined to the rocks 

 that lie within a comparatively short distance from the 

 surface of the earth, say a layer or outer crust not 

 exceeding 10 miles in thickness, then the whole of the 

 materials composing this crust would require to be denuded 

 away, redeposited, and heated to fusing-point or thereby no 

 less than five times in the course of geological history. 

 This supposition is hardly credible, but still it is quite 

 possible, not only that the proportion of carbonaceous matter 

 is greater than ^ per cent, in the present crust of the earth, 

 but also that it was very much greater in very ancient times. 



