THE CAUSE OF THE GLACIAL PERIOD. 489 



Mr. Croll's theory are those pretty generally adopted at the 

 present time by the American geologists best qualified to in- 

 terpret the facts. Thus, among the more eminent American 

 geologists, Mr. GL K. Gilbert wrote, in 1883 : 



It deals with a series of physical laws and physical condi- 

 tions which interact upon each other in an exceedingly complex 

 way — in so complex a way that meteorologists, who have to 

 deal with only a portion of them, do not claim and scarcely 

 hope for a complete analysis of their combinations. The op- 

 portunities for arguing in a circle are most seductive, and the 

 a priori probability that important considerations have been 

 overlooked is not small. 



The only manner in which so comprehensive and intricate 

 an hypothesis can be established is by stimulating inquiry which 

 shall lead to corroborative evidence, and this is precisely what 

 Croll's hypothesis, after eight years of wide publicity, has failed 

 to do. If it is true, then epochs of cold must have occurred 

 with considerable frequency through the entire period repre- 

 sented by the stratified rocks ; and iceberg drift, if no other 

 traces, should have been entombed at numerous horizons. It 

 has not been found, however, and of the eight horizons claimed 

 by Croll to show evidence of glacial action, the treatise under 

 consideration [A. Geikie's "Text-Book of Geology"] mentions 

 only two with confidence, and two others with doubt. In the 

 two instances to which queries are not attached, the phenomena 

 appear to indicate local and not general glaciation. If the 

 hypothesis is true, the cold of the Glacial epoch must have 

 been many times interrupted by intervals of exceptional warmth, 

 but little has been added to the evidence adduced by Croll for 

 such an interruption, and in America, where there is now great 

 activity in the investigation of glacial phenomena, the evidence 

 of a single inter-glacial period is cumulative and overwhelming, 

 while there is no indication whatever of more than one.* 



With this agrees the opinion of President Chamberlin : 



The various astronomical hypotheses seem to be the worse 

 for increased knowledge of the distribution of the ancient ice- 



* " Nature," vol. xxvii, p. 262. 

 31 



