Ch. XVI.] 



CAUSE OF GLACIER MOTION 



369 



caverns or arches. The waters of these streams are always 

 densely charged with the finest mud, produced by the grind- 



movmar mass 



(See fig. 22.) 



The length of the 



Swiss glaciers is sometimes 



miles 



middle 



miles 



where they are broadest, occasionally two or three 

 their depth or thickness sometimes more than 600 feet. 

 When they descend steep slopes, and precipices, or are forced 

 through narrow gorges, the ice is broken up, and assumes 

 the most fantastic and picturesque forms, with lofty peaks 

 and pinnacles, projecting above the general level. These 

 snow-white masses are often relieved by a dark back-ground 

 of pines, as in the valley of Chamouni ; and are not only 

 surrounded with abundance of the wild rhododendron in full 



* 



flower, but encroach still lower into the region of cultivation, 

 and trespass on fields where the tobacco-plant is flourishing 

 by the side of the peasant's hut. 



The cause of glacier motion has during the last quarter of 

 a century been a subject of careful investigation and much 

 keen controversy. Although a question of physics, rather 

 than of geology, it is too interesting to allow me to pass 

 it by without some brief mention. De Saussure, whose 

 ' Travels in the Alps ' are full of original observations, as well 



>rehensive general views, conceived that 



com 



mierlit be sufficient 



slope of the valley, if the sliding motion were aided by the 

 water flowing at the bottom. For this c gravitation theory ' 

 Charpentier, followed by Agassiz, substituted the hypothesis 



of dilatation. 



most 



permeable 



innumerable 



tubes, often extremely minute. These tubes imbibe the 



aqueous fluid during the day, which freezes, it is said, in the 



cold of the night, and expands while in the act of conge- 

 lation. TTlP distp/nsion of t.TiP. wTioIp rrma« PYArfa n.n 



immense 



force, tending to propel the glacier in the direction of least 

 resistance — ' in other words, down the valley/ This theory 

 was opposed by Mr. Hopkins on mathematical and mechanical 

 grounds, in several able papers. Among other objections, 



VOL. i. 



B B 



