MESOZOIC TIME — CRETACEOUS. 877 



fossils give information, and, with a single doubtful exception, of the Am- 

 monites ; and, among other Mollusks, of the genera Exogyra, Diceras, 

 Requienia, Hippurites, Eadiolites, Pterinea, Inoceramus, and others. Part 

 of the change had been accomplished before the time of the catastrophe, 

 for decline had made much progress in the Cycads, Ammonites, Belemnites, 

 and in the Reptilian tribes. But still the destruction was great, world-Avide, 

 one of the most -marvelous events in geological history. Among the larger 

 land animals the species most likely to have escaped extermination are the 

 Mammals ; for many of them had no doubt already accustomed themselves to 

 the higher lands or ridges of the continents, and their covering of fur would 

 have made adaptation to a colder climate easy. The Birds also would have 

 been to a large extent tenants of the interior and denser forests of the con- 

 tinent of the time. The Pterosaurs might have had, perhaps, an equal 

 chance with the Birds, but for the absence of a coat of feathers. 



As to the cause of the epochal disappearance of species, the remarks on 

 the like event after the Appalachian revolution, on page 735, apply also here. 



The Laramide orogenic disturbance in America passed with no marked 

 contemporary movements in Europe, none sufficient to account for the 

 thoroughness of the disappearance of species. Change by modification had 

 its marked eifects, for it has always been in progress ; but extermination 

 must have been the more prominent method of bringing about the great 

 result. 



Causes of extermination. — Since the destructions were to a very large 

 extent marine, the oceanic circulation was probably one means of destruc- 

 tion. The world, by the end of the Cretaceous period, had become more 

 diversified than ever before in its zones of temperature. The emergence 

 from the ocean of a third of North America had taken place, and probably 

 of as much of South America, and of large portions also of the other con- 

 tinents, and this would have determined some lowering of the earth's mean 

 temperature, cooling both the air and oceanic waters. The cooling, during 

 the Cretaceous period, it is certain, was great enough to drive Cycads from 

 the Arctic regions to latitudes that are now at the middle of the Temperate 

 Zone. If the change had made the Arctic waters only 15° F. colder than 

 they were during the Cretaceous period, the polar waters, as they flowed 

 southward, would probably have been exterminating to the greater part of the 

 life of coast regions along the shallower waters, and down to such depths as 

 the cold current reached. Such a cause might make a complete break in the 

 succession of species in a region, without any break in the succession of beds, 

 as happened in ISTew Jersey (page 821). Its action would have been least on 

 the western coast of North America, because of the shallowness of Bering 

 Strait. Moreover, under these circumstances temperature would have 

 worked similarly over the land, forcing Cycads southward, and putting 

 unfavorable conditions into the old haunts of Reptile life. 



The other most probable cause of destruction to life is that from earth- 

 quake waves. The making of a mountain system along the whole length of 



