CHAP, yiii.] THE CAUSES OF GLACIAL EPOCHS. 155 



Frohable date of the Glacial E2)och. — The state of extreme 

 glaciation in the northern hemisphere, of which we gave a 

 general description at the commencement of the preceding 

 chapter, is a fact of which there can be no doubt whatever, 

 and it occurred at a period so recent geologically that all the 

 mollusca were the same as species still living. There is clear 

 geological proof, however, that considerable changes of sea and 

 land, and a large amount of valley denudation, took place during 

 and since the glacial epoch, while on the other hand the surface 

 markings produced by the ice have been extensively preserved ; 

 and taking all these facts into consideration, the period of about 

 200,000 years since it reached its maximum, and about 80,000 

 years since it passed away, is generally considered by geologists 

 to be ample. There seems, therefore, to be little doubt that in 

 increased excentricity we have found one of the chief exciting 

 causes of the glacial epoch, and that we are therefore able to 

 fix its date with a considerable probability of being correct. 

 The enormous duration of the glacial epoch itself (including 

 its interglacial, mild, or warm phases), as compared with the 

 lapse of time since it finally passed away, is a consideration of 

 the greatest importance, and has not yet been taken fully into 

 account in the interpretation given by geologists of the physical 

 and biological changes that were coincident with, and probably 

 dependent on, it. 



Changes of the Sea-level dependent in Glaciation. — It has been 

 pointed out by Dr. Croll, that many of the changes of level of 



produce a more severe winter climate on the west than on the east of the 

 Atlantic during the glacial epoch, and though the first of these — the Gulf 

 Stream — was not nearly so powerful as it is now, neither is the difEerence 

 indicated by the ice-extension in the two countries so great as the present 

 difference of winter-temperature, which is the essential point to be con- 

 sidered. The ice-sheet of the United States is usually supposed to have 

 extended about ten, or, at most, twelve, degrees further south than it did 

 in Western Europe, whereas we must go twenty degrees further south in 

 the former country to obtain the same mean winter temperature we find 

 in the latter, as may be seen by examining any map of winter isothermals. 

 This difference very fairly corresponds to the difference of conditions 

 existing during the glacial epoch and the present time, so far as we are 

 able to estimate them, and it certainly affords no grounds of objection to 

 the theory by which the glaciation is here explained. 



