236 
THE GEOLOGIST. 
greater than its cohesion to the surface on which it rests, when- 
ever the lower surface is iu a state of disintegration. It was 
perfectly consistent with this conclusion to assign to the glacier 
whatever degree of plasticity might be necessary to account for 
the relative motions of its central and longitudinal portions under 
the enormous pressure to which, according to his theory, he 
showed it might be subjected. Such relative motions, however, 
were probably facilitated more by the dislocation than the plas- 
ticity of the mass. Sufficient, he trusted, had been advanced to 
prove that the sliding theory assigned a cause adequate to the pro- 
duction of all the observed phenomena of glacial movements. 
With respect to the transport of erratic blocks and detritus from 
the Alps to the Jura, Mr. Hopkins observed that the greatest 
height which glaciers had formerly attained in the valley of the 
Rhone (whence a large portion of the erratics had been derived), 
appeared to be well defined by lateral moraines and polished rocks, 
while the greatest height at which these blocks had been de- 
posited on the Jura was also well defined. Thus, according to 
M. Charpentier, the Rhone glacier must have risen, at the 
mouth of the valley, to about 2,500 feet above the existing sur- 
face of the Lake of Geneva ; while the highest band of detritus 
on the Jm-a was stated to rise to a still higher leveL It was in- 
conceivable, therefore, that such detritus should have been 
lodged at its present elevation by former glaciers. The only 
way in which it appeared possible to obyiate the mechanical diffi- 
culties of the subject, was to suppose the transport to have been 
effected when the Jura was at a lower level relatively to the Alps, 
and the whole district lower relatively to the surface of the ocean. 
In such case, the space between the Alps and the Jura may have 
been occupied by the sea, and the ice, with its transported ma- 
terials, may have passed from the former to the latter chain, 
partly with the character of a glacier, and partly with that of an 
iceberg. This hypothesis is perfectly consistent with the sup- 
position of the general configuration of the surface of the Jm-a 
having been the same at the epoch of the transport as at the 
