150 Review of the Controversy Eegarding the Motion of Glaciers. 



superficial layer of ice or snow, is at or below the freezing point.* All 

 inward transmission of liquid pulses is impossible. 



One point more may be touched upon. Perhaps the most puzzling 

 of all glacial 2:)henomeua to those who have attempted to find a true 

 and sufficient cause of the motion, is the behavior of the secondary and 

 tertiary glaciers, and of those miniature neves and ice streams, which, 

 from their triangular form, have been named by Agassiz, snow^-trowels. 

 This is directly opposed to what would naturally be expected, and to 

 what every theory, ]Mr. CroU's included, requires. The Alpine glaciers 

 were arranged by De Saussure in two orders : "Glaciers of the first 

 rank are inclosed in valleys, which, though elevated, are commanded 

 on all sides by hills." " Glaciers of the second rank are not confined 

 to valleys, but are spread out on the slopes of the higher peaks. "f 



"To complete Saussure's description I will add, that glaciers of the 

 first rank lie, in genera], on very gentle slopes, varying from 3° to 10°, 

 while glaciers of the second rank have a much steeper inclination, 

 ranging from 15° to 50'^, and_ even higher. ''4! 



In the years 1841-5-6, careful measurements Avere made by Mr. 

 Desor, to determine the rate of advance of these secondary glaciers. 

 ►Several of the tributaries of the Aar were chosen for the purpose, and 

 among them that of the Zinkenstock, whose slope varies from 30° to 

 50°. These measurements, wdiich others have since confirmed, show, 

 that while the Mer de Glace, with an inclination of 5° to 8°, advances at 

 the maximum rate in summer of about 36 inches a day, this secondary 

 stream, on a slope of 41° only, crawls down at an inch and a half during 

 that interval. 



At the same time the advance of the smaller masses (snow-trow'els) 

 was also determined, and found to be even less than that of the second- 

 ary streams, though lying on slopes of equal or greater inclination. 



These facts, due to the care and perseverance of Agassiz and his 

 companions, have proved great difficulties in the way of theories of 

 glacier motion. If the glacier slid down, it should slide faster on 

 a slope of 50° than on one of 5°; if it was thrust down by the expansion 

 of freezing water, or crept down by dilitation, as Mosely supposed, the' 

 same result should follow. If it flowed by the force of gravitation, 

 helped by an inherent viscosity, it must flow down a steep slope more 

 rapidly than over a nearly level surface ; and, finally, if it trickles down 

 by steps infinitesimally small, during moments of liquefaction infinitesi- 



* It is hardly necessary to point out how much more stronj;ly this reasoning applies to the 

 glaciers of the I'olar ref^ion than to those of Switzerland, v.hich are here directly alluded i\^. 

 Vet, the constant formation of icebergs is satisfactory proof of their movement. 



■j- Voyages dans Ics Alpes. 



X Agassiz's S^steme Glaciaire, p. 3. 



