﻿4 W. J. Sollas — On Evolution in Geology. 



atmosphere, and what proportion is furnished by that constant 

 carbonic anhydride generator — decaying vegetable matter. 



That directly dissolved would increase with the increased amount 

 of carbonic anhydride present in the air, but it would on the other 

 hand be diminished by the higher temperature which, as we shall 

 show, the earlier rain-water possessed ; and as the subterranean 

 waters were also in the earlier periods warmer than they now are, 

 the quantity of carbonic anhydride absorbed by them would also 

 be for this reason less. It is thus impossible to say whether the 

 presence of an excess of carbonic anhydride in the air would do 

 more than very partially compensate for its diminished solubility in 

 water warmer than the present. 



2. Denudation by rain and rivers. — As the rate of this depends 

 ceteris paribus on the amoimt of rainfall, we have now to inquire 

 how far this quantity would differ in the earlier from that of the 

 later geologic times. 



The rainfall is the result of the difference in temperature met 

 with in passing from the equator to the poles and from the surface 

 of the earth to the higher regions of the atmosphere. 



Owing to increased radiation from the sun, our earth must haA'e 

 possessed a higher temperature all over its surface in the earlier 

 geologic periods, and in addition to this a more extreme difference 

 in temperature must have existed between the poles and the equator. 

 The ratio of this difference would of course remain the same, since 

 that depends on an unvarying cause, but the actual difference 

 would be much greater, as also would be the difference between the 

 temperature at the surface of the earth and in the higher regions of 

 the atmosphere. The general elevation of a constant temperature- 

 difference to a higher temperature level would of itself be sufficient 

 to cause increased evaporation and greater rainfall. As an illustra- 

 tion of this let us mix together 1 cubic inch of air saturated with 

 aqueous vapour at 100° C. with another cubic inch saturated at 60° C, 

 a difference of temperature of 40° C. The result will be that 4-1 

 grains of water will be precipitated. Now let us take 1 cubic inch 

 saturated at 65*5° C. and mingle it with 1 cubic inch at 15"5°C., a 

 temperature-difference greater than in the first case, viz. 50° C. The 

 condensed aqueous vapour will now weigh only 1 grain ; one quarter 

 the quantity of the former instance. 



Thus a merely equable elcA'ation of the earth's temperature would 

 suffice of itself to produce greatly increased rainfalls, —greater not 

 simply in proportion to the elevation to a higher temperature level 

 of the temperature-difference on which they depend, but in an 

 increasing ratio ; and when to this we add the effects of an actual 

 increase in the temperature-difference itself, we shall obtain a rain- 

 fall which will very forcibly indicate that the increased radiation 

 from the sun must have had its full influence in producing a more 

 coidIous evaporation of terrestrial waters, and a corresponding con- 

 densation of aqueous vajDour. 



Here we may notice in jDassing, that the rain would be warmer 

 rain than the present, and it would fall upon a warmer terrestrial 



