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The ocean, also, as it became colder, would absorb larger and larger quantities of carbon-dioxide. Thus the Pleistocene atmosphere, gradually relieved of its vapours and carbon-dioxide, would no longer retain the heat at the surface. We may add that the growth of reflective surfaces ice, snow, cloud, etc. would further lessen the amount of heat received from the sun.
As we saw, that view implies that, as the heavier elements penetrated centreward in the condensing nebula, the gases were left as a surrounding shell of atmosphere. When the water-vapour settled as ocean on the crust, the atmosphere remained a very dense mixture of oxygen, nitrogen, and carbon-dioxide to neglect the minor gases.
The warmth of the Jurassic period is generally attributed to the low relief of the land, and the very large proportion of water-surface. The effect of this would be to increase the moisture in the atmosphere. Whether this was assisted by any abnormal proportion of carbon-dioxide, as in the Carboniferous, we cannot confidently say.
Now it is quite certain that the proportion of carbon-dioxide was greatly reduced in the Pleistocene. The forests of the Tertiary Era would steadily reduce it, but the extensive upheaval of the land at its close would be even more important. The newly exposed surfaces would absorb great quantities of carbon.
Then follows this remarkable passage: "Carbon-dioxide, because of its greater specific gravity, would also be in relatively greater amount so far as this cause is considered. For the planet would part, caeteris paribus, with its lighter gases the quickest.
The Permian cold which, however, is universally admitted is more or less entangled in that controversy; the Cretaceous cold has no connection with it. Whatever excess of carbon-dioxide there may have been in the early atmosphere was cleared by the Coal-forests. We must set aside all these theories in explaining the present facts.
However that may be, there was a considerable lessening of the carbon-dioxide of the atmosphere, and this in turn had most important effects. First, the removal of so much carbon-dioxide and vapour would be a very effective reason for a general fall in the temperature of the earth. The heat received from the sun could now radiate more freely into space.
All these sources of supply are admittedly absent from Mars, which has no permanent bodies of water, no rain, and tropical regions which are almost entirely desert. Many writers have therefore doubted the existence of water in any form upon this planet, supposing that the snow-caps are not formed of frozen water but of carbon-dioxide, or some other heavy gas, in a frozen state; and Mr.
This condition lasted until the rocks and the forests of the Carboniferous age absorbed enormous quantities of carbon-dioxide, cleared the atmosphere, and prepared an age of chill and dryness such as we find in the Permian. But the planetesimal hypothesis has no room for this enormous percentage of carbon-dioxide in the primitive atmosphere.
The enormous expansion of the sea a great authority, Neumayr, believes that it was the greatest extension of the sea that is known in geology and lowering of the land would of itself tend to produce this condition, and it may be that the very considerable volcanic activity, of which we find evidence in the Permian and Triassic, had discharged great volumes of carbon-dioxide into the atmosphere.
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