CHAP. XXIV SUMMARY AND CONCLUSION 



537 



and the Himalayas, in Eastern North America and west 

 of the Rocky Mountains, in the Andes of South Temperate 

 America, in Tasmania, and in New Zealand, huge 

 moraines and other unmistakable ice-marks attest the 

 universal descent of the snow-line for several thousand 

 feet below its present level. If we reject the influence of 

 high excentricity as the cause of this almost universal 

 glaciation, we must postulate a general elevation of all 

 these mountains about the same time, geologically speaking 

 — for the general similarity in the state of preservation of 

 the ice-marks and the known activity of denudation as a 

 destroying agent, forbid the idea that they belong to 

 widely separated epochs. It has, indeed, been suggested, 

 that denudation alone has lowered these mountains so much 

 during the post-tertiary epoch, that they were previously 

 of sufficient height to account for the glaciation of all of 

 them; but this hardly needs refutation, for it is clear 

 that denudation could not at the same time have removed 

 some thousands of feet of rock from many hundreds of 

 square miles of lofty snow-collecting plateaus, and yet 

 have left moraines, and blocks, and even glacial striae, 

 undisturbed and uneffaced on the slopes and in the 

 valleys of these same mountains. 



The theory of geological climates set forth in this 

 volume, while founded on Mr. Croll's researches, differs 

 from all that have yet been made public, in clearly 

 tracing out the comparative influence of geographical and 

 astronomical revolutions, showing that, while the former 

 have been the chief, if not the exclusive, causes of the 

 long-continued mild climates of the Arctic regions, the 

 concurrence of the latter has been essential to the 

 production of glacial epochs in the temperate zones, as 

 well as of those local glaciations in low latitudes, of which 

 there is such an abundance of evidence. 



The next question discussed is that of geological 

 time as bearing on the development of the organic world. 

 The periods of time usually demanded by geologists have 

 been very great, and it was often assumed that there was 

 no occasion to limit them. But the theory of development 

 demands far more ; for the earliest fossiliferous rocks 



