chap, viii.] THE CAUSES OF GLACIAL EPOCHS. 
155 
Probable date of the Glacial Epoch. — 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 difference 
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. 
