THE CAUSE OF THE GLACIAL PERIOD. 481 



the greater the less a given place is under the influence of the 



seas. * 



We may test the theory still further by an appeal to geo- 

 logical facts. According to Mr. Croll, there must have been 

 a succession of glacial periods in the past, and it would seem 

 that numerous indications of such epochs, if they occurred, 

 must exist in the successive geological strata. If such indi- 

 cations are not found in requisite amount, the advocates of 

 Mr. Croll's theory are bound to give a satisfactory explana- 

 tion of the failure. 



To a consideration of this evidence Mr. Croll devotes the 

 seventeenth and eighteenth chapters of his book on "Climate 

 and Time,'' and at the outset confesses that " the facts which 

 have been recorded as evidence in favor of the action of ice 

 in former geological epochs are very scanty indeed." To ac- 

 count for this deficiency of evidence, he adduces, first, " the 

 imperfection of the geological records themselves; and, sec- 

 ond, the little attention hitherto paid toward researches of 

 this kind." 



Mr. Croll's presentation of the reasons, from the nature 

 of the case, why the evidence of glaciation in the earlier geo- 

 logical periods should be in large degree obliterated, is prob- 

 ably as strong as can be made. He argues that the present 

 land-surfaces in nearly all cases represent former ocean-beds, 

 hence sedimentary strata deposited during the Glacial age 

 must consist of the water-worn material which had been car- 

 ried out from glacial streams into the bordering seas and 

 oceans, so that the most distinct signs of glacial action which 

 we could expect to find in sedimentary strata would be de- 

 posits of pebbles, forming conglomerate rocks, and the occur- 

 rence in these conglomerates of occasional angular fragments, 

 such as could only be transported on ice. 



Mr. Croll, also, very naturally, dwells upon the extent to 

 which the land-surfaces exposed between two geological 

 epochs must have suffered from denudation. Erosive agen- 



* Woeikoff in " American Journal of Science," vol. cxxxi, pp. 169, 172. 



