176 SPARKS FROM A GEOLOGIST'S HAMMER. 



conductibility of heat possessed by the rocky materials, 

 the earth's internal heat is so securely imprisoned that it 

 yields but one-fortieth of the actual temperature of the 

 world's surface. This is a calculation of Pouillet. Pro- 

 fessor Vogt estimates that if the earth were completely 

 cooled, its surface temperature would be eleven-twelfths 

 as high as at present. These figures show, at least, that 

 the present influence of internal heat upon the climates of 

 the earth is so slight that it may be neglected. 



But it is evident that in the earlier periods of crust- 

 formation the earth's internal heat was an important fac- 

 tor in climate. The condition of the atmosphere conspired 

 with the internal heat to raise the mean temperature of 

 the earth's surface. Charged with gaseous impurities, and 

 a superabundance of aqueous vapor, it served as a blanket 

 wrapped around the earth to arrest radiation. The first 

 general principle thus deduced in reference to geological 

 climates is that they have suffered a secular and continuous 

 depression of temperature. 



But we have much evidence of grand secular FLUCTUA- 

 TIONS of temperature. It would involve us in too great 

 detail to enter upon a general discussion of these fluctua- 

 ions, but I propose to offer an exposition of the most im- 

 portant, and, as we now undei'stand the subject, the most 

 regular of all the climatic fluctuations which our world 

 has felt. 



The northern hemisphere has been visited, at a period 

 geologically modern, by a remarkable depression of tem- 

 perature. "The Great Ice Age" had barely passed when 

 man first made his advent in Europe. The traces of a 

 geological winter repose everywhere throughout northern 

 America and Europe. The very hills of gravel and clay 



