Geological History of tlte Atmosphere. 317 



is that afforded by forming our estimate of the total coal- 

 supply of the world, so far as this can be done from actual 

 observation and measurement. Unfortunately the world has 

 been so very imperfectly explored in this respect that the 

 question as to the amount of the world's supply of coal is 

 nearly (if not quite) as problematic as the question of the 

 history of free oxygen. But still a good deal of information 

 has been gained, and it is interesting to see in what direction 

 it points. 



The best-explored region of the world is probably the 

 United Kingdom itself, and the best-known estimate of its 

 coal-supply is that given by the Report of the Royal Com- 

 mission on Coal, which sat from 1866 to 1871. In this 

 report the " available coal/'' consisting of seams over one foot 

 in thickness, and within 4000 feet of the surface, was esti- 

 mated at 146,000,000,000 (146 thousand million) tons, while 

 the coal at greater depths than 4000 feet was estimated at 

 48,000,000,000 (48 thousand million) tons. Since then 

 Prof. Hull has given a much lower estimate of our coal- 

 supply. He allows only 83,000,000,000 (83 thousand million) 

 tons or so ; but his reduction of the estimate seems to have 

 been obtained principally by raising the limit of the work- 

 able thickness of coal-mines at considerable depths from one 

 foot to two feet, as he thinks that seams less than two feet in 

 thickness will not pay to work, even at depths considerably 

 less than 4000 feet. But this question about the thickness of 

 the seams does not at all affect our present inquiry, Even 

 the seams under one foot in thickness, and carbonaceous or 

 bituminous matters thinly distributed through shales, clays, 

 limestones, &c, are of quite as great importance for our 

 present purpose as the thickest and best deposits of workable 

 coal, So far as I am aware no estimate of this thinly dis- 

 tributed carbonaceous matter has been published as yet, but 

 it may possibly be much greater in amount than the total 

 coal-supply itself, especially if we understand by the term 

 coal only the seams that are at least one foot thick, and 

 contain at least about 80 per cent, of combustible matter, 

 that is to say, not more than 20 per cent, of ash. Let us 

 suppose, however, that the total amount of this thinly dis- 

 tributed carbonaceous matter is equal to only half of the total 

 quantity of coal over one foot in thickness. Let us also 

 suppose that the Royal Commission's estimate of the total 

 amount of coal at all depths in seams of one foot and up- 

 wards in thickness is fairly correct, and let us call it in round 

 numbers 200,000, 000,000 (200 thousand million) tons, as it 

 comes pretty near that figure. This would make the coal in 

 Phil. Mag. S. 5. Vol. 50. No. 304. Sept. 1900. Z 



