J. Croll — Evidence of former Glacial Periods. 71 



jeet carefully, we find that there is actually no just ground to 

 conclude that it has not. For, in all probability, throughout 

 the strata to be eventually formed out of the destruction of 

 the now existing land-surfaces, evidence of ice-action will be 

 as scarce as in Eocene or Miocene strata. 



Did the stratified rocks forming the earth's crust consist of a 

 series of old land-surfaces iustead (as they actually do) of a se- 

 ries of old sea-bottoms, then traces of many glacial periods 

 might be probably detected. Nearly all the evidence which 

 we have regarding the Glacial period has been derived from 

 what we find on the now existing land-surfaces of the globe. 

 But probably not a vestige of this will exist in the stratified 

 beds of future ages, formed out of the destruction of the pres- 

 ent land-surfaces. Even the very arctic shell-beds themselves, 

 which have afforded to the geologist such clear proofs of a 

 frozen sea during the Glacial period, will not be found in those 

 stratified rocks ; for they must suffer destruction along with 

 everything else which now exists above the sea-level. There 

 is probably not a single relic of the Glacial period which has 

 ever been seen by the eye of man that will be treasured up in 

 the stratified rocks of future ages. Nothing that does not lie 

 buried in the deeper recesses of the ocean will escape com- 

 plete disintegration and appear imbedded in those formations. 

 It is only those objects which lie in our existing sea-bottoms 

 that will remain as monuments of the Glacial period of the 

 Post-tertiary era. And, moreover, it will only be those 

 portions of the sea-bottoms that may happen to -be upraised 

 into dry land that will be available to the geologist of future 

 ages. The point is this : Is it probable that the geologist of 

 the future loill find in the rocks formed out of the now exist- 

 ing sea-bottoms more evidence of a glacial epoch during Post- 

 tertiary times than we now do of one during, say, the Miocene, 

 the Eocene or the Permian period ? Unless this can be 

 proved to be the case, we have no ground whatever to con- 

 clude that the cold periods of the Miocene, Eocene and Per- 

 mian periods were not as severe as that of the Glacial period. 

 This is evident, for the only relics which now remain of the 

 glacial epochs of those periods are simply what happened to 

 be protected in the then existing sea-bottoms. Every vestige 

 that lay on the land would in all probability be destroyed by 

 subaerial agency and carried into the sea in a sedimentary 

 form. 



The question of the existence of former glacial periods is 

 one on which paleontology can afford but little really reliable 

 information. One of the main characteristics of a glacial pe- 

 riod is the scarcity or comparative absence of plant and ani- 

 mal life. He certainly would be a bold geologist who would 



