But the
great spread of vegetation and the rise of land in the later
Jurassic and the Cretaceous would reduce this density of the
atmosphere, and help to lower the temperature.
It is clear that the cold would at first be local. In fact, it
must be carefully realised that, when we speak of the Jurassic
period as a time of uniform warmth, we mean uniform at the same
altitude. Everybody knows the effect of rising from the warm,
moist sea-level to the top of even a small inland elevation.
There would be such cooler regions throughout the Jurassic, and
we saw that there were considerable upheavals of land towards its
close. To these elevated lands we may look for the development of
the Angiosperms, the birds, and the mammals. When the more
massive rise of land came at the end of the Cretaceous, the
temperature would fall over larger areas, and connecting ridges
would be established between one area and another. The Mesozoic
plants and animals would succumb to this advancing cold. What
precise degree of cold was necessary to kill the reptiles and
Cephalopods, yet allow certain of the more delicate flowering
plants to live, is yet to be determined.
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