820 THE POPULAR SCIENCE MONTHLY. 



any were seriously affected by the cause specified. It must be admit- 

 ted, however, that the physical agencies which have already been 

 described seem competent to inaugurate a glacial epoch during any 

 period of high eccentricity of the terrestrial orbit, without any assist- 

 ance from the meteorological influences. These periods, recurring at 

 long and irregular intervals, must have alternately refrigerated and 

 revivified the circumpolar regions again and again during the immea- 

 surable ages whose lapse is so dimly recorded in the rocky strata. The 

 theory is, therefore, in the highest accord with the modern unif ormita- 

 rian doctrine which rejects all hypothetical explanations of phenomena 

 which are not in harmony with the jjresent course of nature. 



The present low temperature of the southern hemisphere as com- 

 pared with the northern is explained by the assertion that the warm 

 waters of the southern hemisphere are borne into the northern, while 

 the cold waters of the northern hemisphere are conveyed into the 

 southern. The argument supporting this conclusion is that of a radi- 

 cal advocate of the wind theory, and is as strongly opposed to the first 

 principles of the physical theory of climate as the most conservative 

 critic could wish. Space will not permit the discussion of this chapter. 



Chapter XVIII. contains a resume of the evidence of former glacial 

 eras which had been collected up to the time of the preparation of the 

 volume. There had already been placed on record more or less decisive 

 evidence of former glaciers, not only in the Quaternary but in the Mio- 

 cene, the Eocene, the Cretaceous, the Oolitic, the Permian, the Carbo- 

 niferous, the Old Red Sandstone, the Silurian, and even the Cambrian. 

 In some of these cases, notably in the formations of the Permian age, 

 the evidence is so voluminous, so distinct, and from such widely sepa- 

 rated localities, that it seems impossible not to conclude that our Pleis- 

 tocene ice age was but the homologue of long antecedent secular win- 

 ters. The reasons for the paucity of evidence regarding these early 

 glacial eras are summarized in the preceding chapter. 



A statement of the method of computing the eccentricity of the 

 terrestrial orbit, elaborate tables (laboriously computed by the author, 

 with the exception of about a dozen periods) showing the eccentricity 

 for 3,000,000 years in the past and 1,000,000 years in the future, and 

 conclusions as to the probable date of the glacial epoch, constitute Chap- 

 ter XIX. It has since been pointed out, by an undoubted authority in 

 such matters (Professor Newcomb), that Leverrier's formulae, which 

 were employed in making the computations embraced in the tables, 

 are defective, and hence that the figures given are not rigidly correct ; 

 but for the present these minor inaccuracies may be disregarded. The 

 dates given may, therefore, be assumed to be correct, though they are 

 undoubtedly only approximations. 



It has already been stated that periods of high eccentricity oc- 

 curred 210,000 and 850,000 years ago, respectively. Sir Charles Lyell, 

 the founder of the uniformitarian school of geology, was inclined to 



