4 
W. J. Sollas — On Evolution in Geology. 
atmosphere, and what proportion is fumished 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 
band 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 amount 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 have 
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 betweeu 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 4T 
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 155°C., a 
temperature-difiference 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 elevation 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-difiference on which they depend, but in an 
increasing ratio ; and when to this we add the effects of an actual 
increase in the temperature-difiference 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 
copious evaporation of terrestrial waters, and a corresponding con- 
densation of aqueous vapour. 
Here we may notice in passing, that the rain would be warmer 
rain than the present, and it would fall upon a warmer terrestrial 
