CLIMATES OF GEOLOGIC TIME. 



287 



most marked dm-ing times of mountain-making, i. e., during the "critical periods" at 

 the close of the eras and the less violent movements at the close of the periods, we should 

 expect ice ages, or at least considerablj^ cooled cUmates, occurring here also. Let us see 

 how the facts agree with this hypothesis. 



Of the "critical periods" at the close of the Paleozoic, Mesozoic, and Cenozoic eras, 

 we know that the first and last were accompanied by glacial climates, but the Mesozoic, 

 though a time of very extensive mountain-m.aking and great and prolonged volcanic 

 activity in North America, did not close with a glacial, but only with a slightly cooled cli- 

 mate. Not only this, but we find that volcanism was renewed in the Cordilleras of North 

 America throughout much of the Eocene, and yet there was developed no glacial cUmate 

 at this time.* In the same way the marked temperature reduction at the close of the 

 Cenozoic in the Pleistocene was subsequent to the Miocene and Pliocene movements of 

 this period and not coincident with them, while that of the Paleozoic appears to fall in with 

 the rise of the Urals and Appalachians, though but httle volcanism seems to have accom- 

 panied the movements in North America. It should also be said that equally extensive 

 movements were going on in Europe in the rise of the European Alps during the geologic 

 times before and after the Permic glaciation, and that the earlier movements did not 

 appreciably affect the chmate. 



Again, there was decided mountain-making toward the close of the Siluric in the forma- 

 tion of the Caledonian Mountains all along western Europe from Spitzbergen to Scotland, 

 with marked volcanic extrusions during the Siluric and early Devonic in Maine, the 

 Maritime Provinces of Canada, and Europe. Yet we have no glacial climate at these 

 times, certainly not in the northern hemisphere; rather it seems that the temperature was 

 mild the world over. It is possible, however, that the Table Mountain tillites of South 

 Africa may coincide with this time, and if so a colder temperature affected the soulhem 

 hemisphere only locally. 



On the other hand, the "life thermometer" indicates a cooled period at the close of the 

 Triassic and the following Liassic, but this reduction of temperature, again, is geologically 

 subsequent to, rather than coincident with the marked volcanic activity of the Triassic 

 in many widely separated places. 



Finally, there were earth movements of considerable magnitude at the close of theLower 

 Cambric, Ordovicic, and Jurassic that were not accompanied by glacial climates. At all of 

 these times there appears, however, to have been a drop in temperature, slight for the two 

 first-mentioned periods and more marked for the third one, for here we find in the austral 

 region, during earUest Cretacic times, winters alternating with summers. 



We may therefore conclude that volcanic dust in the isothermal region of the earth 

 does not appear to be a primary factor in bringing on glacial chmates. On the other 

 hand, it can not be denied that such periodically formed blankets against the sun's radiation 

 may have assisted in cooling the chmates during some of the periods when the continents 

 were highly emergent. 



It has long been known that during times of intensive mountain-making and more or 

 less cooled climates there was great destruction and alteration of hfe. The first effects 

 of the environmental changes occurred among the organisms of the land, while the cUmax 

 of alteration among the marine hfe appeared later. This is especially well seen in the 

 Permic glaciation, which first blotted out the cosmopolitan Upper Carbonic flora and the 

 insects, while the life of the sea continued wdthout marked change into Middle Permic time. 

 In the later Pernuc, in the northern equatorial waters of Tethys, occurred the final destruc- 

 tion of many stocks that had long dominated the Paleozoic seas. The explanation of these 

 facts appears to be that on the lands the change of chmate takes immediate effect on the 

 organisms, while in the oceans a longer time is consumed in cooling down the warm and 



* See footnote, page 283. 



