Geological History of the Atmosphere. 443 



It is generally supposed that there was comparatively little 

 coal deposited in pre-carboniferous epochs, but this is a 

 mistaken idea ; at least there is a great abundance o£ carbon- 

 aceous or bituminous matter in the rocks o£ these epochs, 

 though it may not be available for use as coal. In both 

 Devonian and Silurian rocks in certain parts of America 

 there are thick strata of bituminous shale and of limestone 

 rich in bituminous matter and organic remains ; and the 

 Laurentian rocks of Canada contain so much graphite that 

 (according to Sir W. Dawson) their average percentage of 

 carbonaceous matter must be at least as high as that of the 

 rocks of the carboniferous epoch. We have therefore no 

 evidence to lead us to suppose that there was less vegetation 

 growing or less coal deposited on the earth in the Laurentian 

 epoch than in subsequent epochs, and we know from the 

 presence of much ferric oxide in the rocks of that period that 

 there must have been free oxygen in the earth's atmosphere. 

 The conditions therefore as regards the presence of free 

 oxygen on the earth, and (inferentially) the decay or erema- 

 causis of vegetable remains, must have been similar to those 

 of the present day, and probably only a small proportion of 

 the vegetation of that epoch would be preserved in the form 

 of coal or graphite. But, as is evident from Dr. Phipson's 

 experiment on the growth of plants in various gases, vegeta- 

 tion might quite well have grown on the earth in the periods 

 anterior to the existence of free oxygen. 



In the absence of free oxygen the remains of the vegetation 

 would not be subject to eremacausis, and would therefore 

 be preserved (for a considerable time at any rate) in the 

 form of coal or graphite or mineral carbonaceous matter of 

 some description. Obviously then it would require a very 

 much shorter time under these conditions to produce 0*1 x 10 L8 

 tons of coal than it would under modern conditions when 

 probably less than 1 per cent, of vegetable remains is preserved 

 as coal or carbonaceous matter. In fact the time required to 

 produce our Ol x 10 18 tons of coal would come within quite 

 reasonable or credible limits, for it is surely not unreasonable 

 to suppose that the ordinary geological operations of de- 

 nudation and deposition have been going on for a period at 

 least two or three times as long as the time that has elapsed 

 since the earlier part of the Laurentian epoch, and a period 

 of this extent is probably long enough under the conditions 

 above specified to produce 0*1 x 10 18 tons of coal or other 

 carbonaceous matter as the result of vegetable growth. This 

 quantity is equivalent to about 05 per cent, of the weight of the 

 whole crust of the earth for the first ten miles of its thickness. 



2G2 



