CLIMATIC VARIATIONS : THEIR EXTENT AND CAUSES.^ 



By Prof. J. W. Gregory, F. K. S. 

 University of Glasgow. 



Introduction. 



The past variation of climate is an attractive study, as it controls 

 so many questions in geology, geogi'aphy, and meteorology. But the 

 subject is of especial difficulty, as it deals with the action of complex 

 chemical and physical processes working under conditions and on 

 materials which can be estimated only by the freest speculation. 

 The question may be approached a priori by consideration of the 

 evolution of the atmosphere, as suggested by general chemical prob- 

 abilities; or we may determine from the sedimentary rocks the 

 strength and nature of the geographical agencies that formed them ; 

 or we may examine the indirect evidence given by fossils as to the 

 climates under which they lived. The fact of marked local varia- 

 tions in climate is abundantly proved ; and it will probably be equally 

 agreed that there is no evidence known to the geologist of any 

 progressive refrigeration of the earth. The idea of the secidar cool- 

 ing of the earth is deeply impressed on our terminology ; but geolog- 

 ical principles are independent of the theory. The terms suggested 

 by it may always be retained from their historic interest and con- 

 venience, as we still speak of the rising of the sun. Responsibility 

 for the belief in the secular cooling of the earth rests with the 

 astronomers and physicists, from whom geologists have accepted it. 



Local variations in climates are abundantly established by the 

 former glaciation of temperate regions, the once greater extension 

 of glaciers in tropical regions, and the frequent growth of reef- 

 building corals outside their present geographical limits. But we 

 need not unnecessarily increase the difficulties of the problem by ac- 

 cepting the world-wide range of great climatic changes without con- 

 vincing evidence. Doctor Ekholm takes as the starting point of his 

 valuable paper the ground that " the inquiries of modern geology 



" Reprinted by permission from Congr^s Geologique International. Compte 

 Rendu de la dixi&me session, Mexico, 1906. Mexico City, 1907, pp. 407^26. 

 (Printer's proofs not seen by author.) 



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