CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 151 



would certainly produce a vv onderful ameliorating effect on 

 the climate of tlie east coast of North America, and might 

 raise the temi^erature of Labrador to that of Scotland. 

 Now these two changes have almost certainly occurred, 

 either together or separately, during the Tertiary period, 

 and they must have had a considerable effect either in 

 aiding or checking the action of the terrestrial and astro- 

 nomical causes affecting climate which were then in 

 operation. 



It would be easy to suggest other probable changes 

 which would produce a marked effect on climate ; but we 

 will only refer to the subsidence of the Isthmus of Panama, 

 which has certainly happened more than once in Tertiary 

 times. If this subsidence were considerable it would have 

 allowed much of the accumulated warm water which 

 initiates the Gulf Stream to pass into the Pacific ; and if 

 this occurred while astronomical causes were tending to 

 bring about a cold period in the northern hemisphere, the 

 resulting glaciation might be exceptionally severe. The 

 effect of this change would however be neutralised if at 

 the same epoch the Lesser and Greater Antilles formed a 

 connected land. 



Now, as such possible and even probable geograjDhical 

 changes are very numerous, they must have produced im- 

 portant effects ; and though we may admit that the astro- 

 nomical causes already explained were the most important 

 in determining the last glacial epoch, we must also allow 

 that geographical changes must often have had an equally 

 important and perhaps even a preponderating influence on 

 climate. We must also remember that changes of land 

 and sea are almost always accompanied by elevation or 

 depression of the pre-existing land : and whereas the 

 former produces its chief effect by diverting the course of 

 warm or cold oceanic currents, the latter is of not less 

 importance in adding to or diminishing those areas of con- 

 densation and ice-accumulation which, as we have seen, 

 are the most efficient agents in producing glaciation. 



If then Sir Charles Lyell may have somewhat erred in 

 attaching too exclusive an importance to geographical 

 changes as bringing about mutations of climate, his critics 



