HIGHEST LIMIT OF THE ERRATIC ZONE. 361 



tion, however, is hardly a fiftieth or sixtieth part of the 

 inclination of glaciers in extended situations. 



The inclinations of the uppermost limits of the erratic 

 zone, are intermediate between those of glaciers and navigable 

 rivers. They are of an inferior order to those of glaciers, 

 whilst they are of the same character as those of the most 

 impetuous torrents. These inclinations, without exception, 

 would be very considerable for rivers of several feet in depth, 

 and would be enormous for torrents of water of an equal 

 section with those which the limits of the Erratic Zone deter- 

 mine in the vallies of the Alps — sections often 800 to 1,000 

 metres in depth. With like inclinations and sections, cur- 

 rents of water would attain a fearful rapidity. 



This rapidity of any fluid increases with the inclination 

 of the surface, and the depth of its section; the rapidity 

 which all rivers acquire in the various stages of their increas- 

 ing size, is a good proof of this fact. It is, on the contrary, 

 to be doubted whether the motion of glaciers of great thickness 

 is governed by the same laws ; or whether they would move 

 with less difficulty on a slight incline, than thin glaciers. 

 This is a point of importance which must not be lost sight 

 of when comparing the two classes of motive agents. The 

 acquired rapidity in the case of these glaciers, amounts to 

 nothing. Such a difference exists between the laws regulat- 

 ing the movement of ice, and water courses, that, in order to 

 arrive at just conclusions with respect to erratic phenomena, 

 tables, constituted on the plan which I recommended, should 

 contain three comparative statements: the respective phe- 

 nomena of glaciers, water courses, and erratic phenomena. 

 — Annates des Sciences Geologiques, 



END OF VOL. I. 



