INFLUENCE OF LIMESTONE UPON THE ATMOSPHERE 619 
testimony of geological history. It was in the Middle Ordo- 
vician, the Middle Silurian, the Middle Carboniferous, the Middle 
Cretaceous and the early Tertiary that life of the warm temper- 
ate types prevailed in the arctic lands. And these seem to be 
periods of base leveling and of wide incursions of lime-deposit- 
ing seas. It cannot at present be asserted, on the other hand, 
that there were intervening periods when warm temperate life 
did not prevail there, and could not because of low temperature. 
In the very nature of the hypothesis here entertained, such 
periods would be coincident with relative land elevation, and 
their record would be absent, or so obscurely indicated that 
only critical investigation directed to the point could detect it. 
But in the lower latitudes we find evidence of aridity and of 
cold temperatures intervening between at least some of the 
periods of extensive limestone formation, as, for example, the 
saline deposits which took place between the great limestone- 
forming epoch of the mid-Silurian period and the similar epoch 
of the Devonian; or, again, the saline and gypsum deposits 
and red sediments of the Permian and Triassic age between the 
limestone epoch of the Carboniferous and that of the Jurassic. 
In this gap also falls perhaps the glaciation of India, Australia, 
and South Africa. The Pleistocene glaciation and the measura- 
ble aridity of recent times, falling between the limestone-forming 
epoch of the early Tertiary and a possible limestone-forming 
epoch of the future which should theoretically follow upon the 
degradation of the continents, if crust movements remain in 
abeyance, may form another example. 
In considering the antithetical epochs where limestone forma- 
tion is at a minimum, and rock disintegration is at a maximum; 
it may be noted that at the close of the lime-depositing epoch 
the ocean is low in calcium bicarbonate and rich in carbonic acid, 
and is not then predisposed to deposit lime chemically, but is, 
on the other hand, in a condition to receive and hold the cal- 
cium bicarbonate sent down from the land. When, therefore, an 
epoch of special earth shrinkage and of readjustment to accumu- 
lated stress ensues, and the ocean is more fully drawn into the 
