322 Chemical and Geological History of the Atmosphere. 



would still be so though it were multiplied several times in 

 order to make allowance for coal that may have been destroyed 

 by oxidation after its first deposition and consolidation. 

 This latter is a point of some importance ; for though coal is not 

 readily oxidized by the air at the ordinary temperatures, yet 

 when organic matter is deposited in contact with oxide of iron 

 and sulphate of lime, it is very liable to be oxidized, sulphide 

 of iron being formed at the same time. Probably also, when 

 rocks containing coal are highly heated by the irruption of 

 igneous rocks, a quantity of the coal may be oxidized at the 

 expense of the oxides with which it comes into contact. 

 However, we need not take much account of such considera- 

 tions at present, and we can say confidently enough that the 

 presence of a quantity of coal in the earth equivalent to the 

 whole of our free oxygen is not at all incredible when we 

 have regard to the length of time during which the deposition 

 has most probably been going on. 



A similar verdict is obtained if we compare the rate of 

 deposition required by the above calculations with the rate 

 of deposition of the other rocks derived from the denudation 

 of the earth as a whole. Estimates of the rate of denuda- 

 tion no doubt vary just about as much as estimates of the 

 age of the earth ; but even if we take Darwin's estimate of 

 1 inch per century, which seems to be regarded as rather a 

 low estimate, this would give us an annual denudation from 

 the present land-surface of the earth of 85,000,000,000 (85 

 thousand million) tons of rock. This is equal to 3400 times 

 the 25,000,000 tons of coal that would have to be deposited 

 annually in order to give us 500,000,000,000,000 tons in 

 20,000,000 years ; or the average proportion of carbonaceous 

 matter that would have to be present is only 0'03 per cent. 

 This seems a modest enough figure when we remember the 

 great amount of peat that is annually produced in many 

 parts of the earth, and may be regarded as a geological 

 deposit, and also the vast quantity of blue mud that is 

 accumulating on the sea-bottom within the first few hundred 

 miles from land. Analyses of this mud would be very useful 

 if they were available, but at any rate it must contain, or 

 must have at first contained, sufficient organic matter to change 

 a considerable amount of oxide of iron and sulphate of lime 

 (the latter being derived from sea-water) into sulphide of iron 

 and carbonate of lime. 



There is no doubt a difficulty in this line of reasoning, due 

 to the obvious circumstance that as new strata are being 

 deposited, old ones are worn down and washed away; but 

 probably this difficulty is not so great as it looks at first. 

 Coal and other bituminous matters are only very slowly, and 



