488 CENOZOIC TIME. 



In the Rocky Mountain region, some Cretaceous species and genera 

 continue on, if the coal series is Tertiary ; and yet the number now 

 known is less than half a dozen. The vast majority of the species, 

 and nearly all the characteristic genera, disappear. 



The facts do not authorize the inference that extermination was so 

 complete as is implied in the above statement, although establishing 

 that it was remarkable for its universality and thoroughness. It has 

 been found that, in the bottom of the Atlantic, a living species of Tere- 

 bratula ( T. caput-serpentis) is probably identical with one of the Cre- 

 taceous species (T. striata), and several genera of corals, known hith- 

 erto only among Cretaceous fossils, have their species in the Atlantic 

 depths, some of which differ but little from those of the Cretaceous. 

 Such facts prove that the deep ocean was beyond the reach of the 

 agencies that produced extermination over the Continental seas. 



Cause of the Destruction of Life. — The general extermination of 

 species at the close of the Cretaceous period was probably connected 

 with changes of level, which took place at the time over the higher 

 latitudes of America, Europe, and Asia, bringing on an era of un- 

 usual cold, and sending cold Arctic currents southward over the Conti- 

 nental seas. In North America, there are no mai"ine Tertiary beds 

 known north of southern New England, on the east, and none in the 

 Arctic regions, — indicating, apparently, that the whole area was above 

 the sea then, as now. This cause would have been sufficient to pro- 

 duce all the effects mentioned ; and it appears to be the only cause that 

 would be sufficiently complete and universal in its action. It is there- 

 fore most probable that the destruction was due (1) to the more or 

 less complete emergence of the continents, especially their northern 

 portions ; and (2) to the change of climate and oceanic temperature 

 thus occasioned, — both aerial and oceanic currents being rendered 

 colder than in the Mesozoic era. This source of destruction would not 

 have acted over the bottom of the Atlantic and other deep oceans ; 

 and hence species even of the Cretaceous era may survive there. 



IV. CENOZOIC TIME. 



It has been observed that, before the close of the Mesozoic, the 

 mediaeval features of the era were already passing away. The Cycads 

 had begun to give place to the Sassafras, Tulip tree, Willow, Maple, 

 Oak, and Palm ; the ancient type of Ganoids, to Salmon, Perch, and 

 Herring ; and the Corals, Echini, and Mollusks, were in a great degree 

 allied to those of existing seas, though of extinct species. But, not- 

 withstanding these progressing changes, the Mesozoic aspect continued 



