320 Mr. J. Stevenson on the Chemical and 



little doubt, however, but that in the drills or boring trials 

 which have already been made in all parts of the world, to 

 all degrees of depth and for all kinds of purposes, the average 

 amount of organic or carbonaceous matter has been at least 

 y T Q of a per cent, by weight. This is equal to a layer of 

 coal 5 feet thick in each half-mile thickness of the earth's 

 crust. It should be noted here that coal has only about half 

 the specific gravity of other ordinary rocks, and therefore a 

 layer of coal must be 5'28 feet thick in order to be equal in 

 weight to a layer of ordinary rock of similar extent but only 

 2*64 feet in thickness — 2*64 feet being ^ per cent, of half 

 a mile. 



If we assume, then, that the total mass of existing sedi- 

 mentary rocks on the earth is equal to a stratum of rock half 

 a mile thick over the entire continental and transitional areas, 

 and that it contains on the average ^ per cent, of carbon- 

 aceous matter, then the total amount of such matter would 

 be equal to a layer of coal 5 feet thick over these areas ; and 

 this, as we have already seen, would be practically equivalent 

 to the total free oxygen of the atmosphere. 



Also if, as is quite probable, the total mass of sedimentary 

 rocks should be greater than the amount just supposed, then 

 obviously the total amount of carbonaceous matter in the 

 world would be even greater than the 500,000,000,000,000 

 (500 million million) tons which are required to be equivalent 

 to our free oxygen. And so far as the percentage of carbon- 

 aceous matter is concerned, it is just as likely to be over as to 

 be under jL. Not only have we the eoal itself, which alone 

 might possibly reach up to that percentage, but there is the 

 carbonaceous matter diffused through all kinds of shales, 

 clays, slates, limestones, ironstones, &c. The proportion of 

 organic or carbonaceous matter in these rocks is often ^ per 

 cent., and sometimes much more. 



Of course if the total quantity of carbonaceous matter on 

 the earth should be greater than 500,000,000,000,000 (500 

 million million) tons, it would exceed the upper limit laid 

 down for it by Lord Kelvin, in the free oxygen of the atmo- 

 sphere. But after all this consideration introduces no serious 

 difficulty into the problem ; at least it can be readily sur- 

 mounted by the hypothesis that the primitive atmosphere of 

 the earth not only contained no free oxygen, but that it con- 

 tained a considerable quantity of free hydrogen or hydro- 

 carbon gases. It is easy to see how the presence of these 

 gases would affect the production of free oxygen by vegetation. 

 If we assume that the primitive atmosphere of the earth 

 contained also carbonic-acid gas, and that the primeval 



