ON COAL. 145 



being both a bad absorber and bad radiator of heat^ botb heating very 

 slowly and cooling very slowly, it is evident that a great predominance 

 of that element would produce, also, a very uniform climate. The 

 difference of temperature between pole and equator, and between winter 

 and summer, would be less than at present. 



Some geologists think, with Mr. Lyell, that this uniformity and 

 humidity of climate is sufficient to account for the coal vegetation 

 without the necessity of a higher mean temperature than now exists. 

 If the present mean temperature was distributed more equably both 

 over the earth surface and over the year, the effect would be to pro- 

 duce cooler equator, it is true, but also much warmer high latitudes, 

 and particularly the winters of high latitudes would be much less 

 severe. The evidence is, however, it seems to me, in favor of some 

 elevation of the mean temperature also. It is difficult to conceive how 

 any uniformity of distribution of the present mean temperature, such 

 as would be produced by the predominance of water, could raise the 

 winter temperature of Mellville island to the point necessa,ry for the 

 luxuriant growth of tree ferns. Some increase of temperature from 

 internal cause seems to be necessary. I suppose, therefore, that if the 

 temperature of the earth from internal causes was slightly elevated, 

 say 10°, so that the mean temperature from 60° should become 70°, 

 and then this mean temperature distributed over the earth surface as 

 uniformly as possible, by means of a wide extent of ocean, we should 

 have all the conditions necessary to produce the phenomena of coal 

 vegetation. It will be recollected, too, that we have much indepen- 

 dent evidence of the cooling of the earth from an original very high 

 temperature. 



With reference to the highly carbonated condition of the atmo- 

 sphere, we may suppose this to be the result of the greater activity 

 of carbonic acid producing causes, or else we may refer it to the 

 original constitution of the air — the natural process by which car- 

 bonic acid is given to the air, decomposition, combustion, respiration 

 of animals, and volcanoes, carbonated springs, &c. It will be admitted 

 by all that the first three may be neglected, since they return to the 

 air only what had been previously taken from it. The carbonic acid 

 supplied to the air by volcanoes and carbonated springs, according to 

 Bischoff, is so inconsiderable that, unless we suppose these sources 

 much more active than now, it would take millions of years to affect 

 materially the constitution of the air. But even this refuge is taken 

 away, when we recollect that volcanoes and springs derive their car- 

 bonic acid from carbonates, and chiefly from carbonate of lime, or com- 

 mon limestone. But limestones, according to the testimony of all who 

 have carefully studied them, and particularly according to the recent 

 microscopic observations of Sorby, are entirely of animal origin, i. e. 

 entirely made up of broken fragments of shells, corals, crinoids, some- 

 times recognizable under the microscope, sometimes reduced to impal- 

 pable powder. This carbonate of lime is evidently derived from 

 sea- water. Whence, then, does sea-water derive its carbonate of lime? 

 The lime is derived, beyond doubt, from igneous rocks, the carbonic 

 acid probably from the atmosphere, through the animal and vegetable 

 kingdoms, since lime exists in igneous rocks not as a carbonate but as 

 10 s 



