PHYSICAL AND CLIMATAL CONDITIONS. 135 



occurretl in the Pleistocene age, and that we have not 

 now to account for anything so extreme as a polar ice-cap 

 or a continental <'lacier. I have alreatlv directed attention 

 in connection with this part of the subject to the views 

 which I expressed many years ago, and to which I still in 

 the main adhere. I do not ask any geologists, more 

 especially those still affected with the superstitions of 

 continental glaciers and ice-caps, to accept these causes 

 as sufficient to account for the climatic changes evitlenced 

 in geological time ; but I must ask that they should fully 

 exhaust the iniluence of known changes of distribution of 

 land and water, and differential elevation and depression 

 of continental masses, before invoking other causes, 

 whether of cold or heat. I must also insist on their 

 admitting, at least as primary conditions in glaciation, not 

 merely cold but heat, and not merely elevation but 

 depression of land. In (jther words, there must be 

 evaporation as well as condensation, and the former 

 depends on the application of heat to water-surfaces 

 adjacent to those of precipitation.* On the other hand, 

 evaporation being provided, there must, in order to 

 establish a lu'ceding-ground f(jr glaciers, and to permit 

 their existence, be a low mean temperature and high land 

 capable of aflbrding a condensing surface. The statements 



* To Ameiiciin geologists I would recoMiniend a course of reading 

 in " Whitney's Climatic (Jlianges," though I do not agree with the 

 author in all his eonelusions. In a paper in the Proceedings of the 

 Boston Society of Natural History (18!)0), Upham and Everett fully 

 admit the geographical causes of the glacial cold, and also the 

 existence of two periods of boulder distribution, separated by an inter- 

 glacial period, though they do not appear to see the bearing of these 

 and other admissions on the validity of the theory of continental 

 glaciation. .See also an important paper by Upham in the American 

 Geoloyint, December, 1890. 



