58 



MAN AND THE GLACIAL PERIOD. 



is difficult to tell whether a boulder has suffered any ex- 

 tensive transportation or not. 



One of the most interesting and satisfactory demon- 

 strations of the distribution of boulders by glacial ice was 

 furnished by Gruyot in Switzerland in 1845. His observa- 

 tions and argument 

 will be most readily 

 understood by refer- 

 ence to the accom- 

 panying map, taken 

 from Ly ell's clear de- 

 scription.* The Jura 

 Mountains are sepa- 

 rated from the Alps 

 by a valley, about 

 eighty miles in width, 

 which constitutes the 

 main habitable por- 

 tion of Switzerland, 

 and they rise upwards 

 of two thousand feet above it. But large Alpine boulders 

 are found as high as two thousand feet above the Lake 

 Neufchatel upon the flanks of the Jura Mountains beyond 

 Ohasseron (at the point marked G- on the map), and the 

 whole valley is dotted with Alpine boulders. Upon com- 

 paring these with the native rocks in the Alps, Guyot in 

 many cases was able to determine the exact centres from 

 which they were distributed, and the distribution is such 

 as to demonstrate that glacial ice was the medium of dis- 

 tribution. 



For example, the dotted lines upon the map indicate 

 the motion of the transporting medium. On ascending 

 the valley of the Ehone to A, the diminutive representa- 

 tive of the ancient glacier is still found in existence, and 



Fig. 24.— Map showing the outline and course of 

 flow of the great Eh6ne Glacier (after Lyell). 



Antiquity of Man, p. 299. 



