34. During preceding periods the solid crust of the globe, by reason of its thinness, presented, as has been said, a pretty feeble resistance to the action of the internal fire. This envelope, easily broken, permitted melted substances to be freely expelled to the surface of the Earth. After having acquired a certain thickness, this did not take place. Burning substances compressed on all sides, like boiling water in a closed vessel, would end in an explosion. The granite mass, violently broken at many points was riddled with crevasses, like a cracked vase. Upon the line of these crevasses the solid crust was raised and reformed, formed peaks, chains of mountains, and their ramifications. Certain parts of the envelope which were not rent where simply piled up, whilst upon other points excavations and depressions were produced.
The surface of the Earth became during the tertiary period very unequal. The waters, which until this time had covered in a nearly uniform manner the greater part of its extent, flowed down into the lowest places, leaving vast continents of dry land, or summits of isolated mountains, which formed islands.
Such is the great phenomenon which has been accomplished in the tertiary period, and which has transformed the aspect of the globe. It was not produced instantaneously or simultaneously at all points, but successively at epochs more or less remote from one another.