Geological History of the Atmosphere. 321 



vegetation grew in practically the same way as more recent 

 vegetation, viz. through the decomposition of carbonic acid 

 by sunlight — a process which involves the liberation of free 

 oxygen ; the oxygen at first liberated would not remain long 

 in the free condition, because it would probably, as may be 

 inferred from Dr. Phipson's experiment, soon combine with 

 the hydrogen and hydrocarbon gases of the atmosphere to 

 form water and carbonic acid. There would therefore be no 

 permanent free oxygen so long as free hydrogen or hydro 

 carbon gases still remained in the atmosphere, and in the 

 meantime a large amount of coal might have been deposited. 



We admit that only indefinite results can be obtained as 

 yet by the foregoing method of inquiry, but still they are 

 definite enough to make a very high estimate of the total 

 carbonaceous matter in the world quite credible and tenable. 

 This view is also fairly well confirmed by studying the question 

 from the standpoints provided by such general considerations 

 as the length of geological time, the rate of deposition of 

 sedimentary rocks, and the rate of the growth of vegetation. 



We have now good reason to suppose that the formation of 

 coal has been going on for the whole period of geological 

 history which is recorded for us in sedimentary rocks. 

 Though ordinary coal has not been found, so far as I am 

 aware, in Cambrian and Laurentian rocks, graphite has been 

 found in considerable quantity; and this points with a high 

 degree of probability to the previous existence of ordinary 

 bituminous coal. We also know that the deposition of coal 

 was still going on in Tertiary times, and even now there is 

 reason to believe that the process is going on to a not in- 

 considerable extent. If we therefore divide the total amount 

 of coal by the number of years of geological time, we shall 

 have the average rate of its annual production. No doubt 

 estimates of the length of geological time vary very much. 

 Lord Kelvin's estimate is that about 20,000,000 years have 

 elapsed since the solidification of the earth's crust from 

 the liquid condition, and also since the possible and 

 probable beginnings of vegetable life. If, then, we divide 

 500,000,00(1,000,000 by the above figure, we obtain 25,000,000 

 tons as the average amount of coal that would have to be 

 deposited annually daring the whole of geological time, to give 

 us an amount equivalent to the free oxygen of the atmosphere. 

 And if we require, with certain biologists and geologists, a 

 period of at least 080,000,000 years, the rate of production of coal 

 becomes very much less, only about 740,000 tons per annum. 

 These two estimates are no doubt widely divergent, but still 

 even the larger one is quite a modest and credible figure, and 



