ON COAL. 143 



bution of lieat and moisture on the surface of the earth durins: the 

 coal period — is evidenced by the remarkable uniformity of the flora. 

 The general character of the coal flora was very much the same in every 

 portion of the earth's surface, and in many cases even the same species 

 are found in the most distant countries. Thus many identical species 

 have been found in Europe, United States, New Holland and Mell- 

 ville island, countries the existing flora of which differ entirely. Now, 

 although I cannot accede to the doctrine that diversity of climate is 

 the physical cause of diversity of fauna and flora, yet, whether we con- 

 sider the physical or the flnal cause, the result would evidently be the 

 same, viz : the perfect harmony between the climate and the fauna 

 and flora, the perfect adaptation of the one to the other. 



That the atmosphere was highly charged with carbonic acid is ren- 

 dered probable by the astonishing luxuriance of the vegetation of the 

 period. Some experiments recently made by Mr. Gladstone seem to show 

 that up to a certain limit the growth of ferns is rendered more rapid by 

 the addition of carbonic acid to the atmosphere in which they grow. 

 This probably becomes a certainty, when we reflect upon the enormous 

 amount of carbon contained in the coal deposits, all of which must 

 have been extracted from the atmosphere. It has been estimated that 

 " all the forests of the United States gathered into one heap would 

 fail to furnish materials of a single coal seam equal to that of Pitts- 

 burg." Again, that " that there is laid up in the earth, in the form 

 of coal, six times as much carbon as now exists in the atmosphere. If 

 it was all returned to the air, there would be seven times as much 

 carbonic acid in the atmosphere as at present." 



Cause of the climate of the coal. — Much speculative ingenuity has 

 been exhausted to little effect in attempts to account for the remark- 

 able climate of this period. We find here the same looseness of rea- 

 soning unfortunately so common among geologists when dealing with 

 physical subjects. The subject of most of this speculation has been 

 the cause of the supposed greater heat of the climate. There are two 

 principal methods of accounting for it. The first and most obvious 

 mode is by means ot the commonly received hypothesis that the earth 

 has cooled down to its present temperature from an original state of 

 incandescence. But although there is much independent evidence of 

 this original condition — and we think it extremely probable, therefore, 

 that the heat of the coal period was due, at least in part, to this cause — 

 yet, as Hopkins has shown, (Geol. Jour., 1853,) there are strong ob- 

 jections to this as the only cause. We have already said that the 

 surface temperature of the earth is due partly to internal and partly to 

 external causes. At present the surface temperature from internal 

 causes has become almost nothing, i. e. only one-twentieth of a degree 

 Fahrenheit. The increase of temperature below the surface is about 

 1° to sixty feet. Now, if we supposed the surface temperature from 

 this cause to be increased even to 1°, the increase for every sixty feet 

 of depth would be 20°. An increase of 10° surface temperature would 

 make 200° increase of temperature for every sixty feet. The springs, 

 except the most superficial, would all be boiling. Now, it will be 

 recollected that the winter temperature of Mellville, where coal is 

 found abundantly, is —20° Fahrenheit. It would, therefore, take near 



