THE REIGN OF MAMMALS. 199 



CHAPTER XVIII. 



THE EEIGN OF MAMMALS. 



ANOTHER cycle of eternity was past. The progress 

 of geological agencies had brought the crust of the 

 earth to a tension which was to be relieved by another 

 collapse. As the Paleozoic Time was closed by the sud- 

 den sinking of the beds of the Atlantic and Pacific Oceans, 

 and the corresponding protrusion of the ridges of the Ap- 

 palachians, so the Mesozoic Time was closed by a farther 

 progress in the same direction. The ever- shrinking nu- 

 cleus necessitated the ever -enlarging wrinkles of the en- 

 veloping crust. The furrows must deepen and the folds 

 must rise. The uplift which marked the close of Mesozoic 

 Time affected the whole continental body. It was not a 

 sudden uprising accomplished in a day. It may have ex- 

 tended through a century ; but it was an interval of move- 

 ments so much accelerated as to mark a pretty definite 

 boundary between two stages of continental development 

 and two great periods in the history of the world. During 

 the Cretaceous Age which had now just closed, the great 

 Mediterranean Gulf represented in Fig. 77 had been broad- 

 er along its eastern borders, and continuous to the Gulf 

 of Mexico. Through this, perhaps, the Gulf Stream had 

 coursed to the Frozen Ocean. Now, by an upheaval of the 

 central region, this gulf was severed in twain. On the 

 south it retreated to nearly the modern limits of the Mex- 

 ican Gulf, while northward remained an elongated body of 

 water, swelling out in the central portion of the continent, 

 in two places, to dimensions exceeding the Caspian and 



