64 REPORT—1843. 
that the temperature of their lower surfaces could not generally be less than 32° Fahr. 
Such, the author stated, must necessarily be the case, unless the conductive power of 
ice was greater than it was deemed possible that it could be. He considered the 
sub-glacial currents as powerful agents in the disintegration of the lower surfaces of 
glaciers, especially near their lower extremities. The results of Prof. Forbes’s obser- 
vations on the motion of the Mer de Glace of Mont Blanc, afforded, as regards that 
glacier (and, by inference, as regards all other glaciers), a complete refutation of the 
theories which attribute glacial movements to any expansion or dilation of the ice. 
The Professor had, however, put forth a new theory, which agreed with that offered 
by Mr. Hopkins in attributing glacial motion to the action of gravity, but differed 
from it entirely as a mechanical theory in other respects. The Professor appeared to 
reject the sliding theory of De Saussure on account of the difficulties already men- 
tioned (which were now removed by the above experiments), and assigned to the 
mass of a glacier the property of plasticity, or semifluidity, in a degree sufficient to 
account for the fact of its descending down surfaces of such small inclination. Ac- 
cording to this theory, the motion was due to the small cohesion of one particle of 
glacial ice to another. Mr. Hopkins stated his conviction that the internal cohesion 
of the mass was immensely greater than its cohesion to the surface on which it rests, 
whenever the lower surface is in 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 plasticity of the mass. Sufficient, he trusted, had been advanced 
to prove that the sliding theory assigned a cause adequate to the production 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 
deposited 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 2500 feet 
above the existing surface of the Lake of Geneva; while the highest band of detritus 
on the Jura was stated to rise to a still higher level. It was inconceivable, therefore, 
that such detritus should have been lodged at its present elevation by former glaciers. 
The only way in which it appeared possible to obviate the mechanical difficulties 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 materials, 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 
supposition of the general configuration of the surface of the Jura having been the 
same at the epoch of the transport as at the present time; and Mr. Hopkins believed 
it would be found equally so with all the observed phenomena of that region. 

Mr, Murchison exhibited a Relievo Map of England and Wales, prepared by 
Messrs. Dobbs and Co., and coloured geologically under his own direction. He 
pointed out the accordance between the physical features of the country and the 
boundaries of particular strata, and stated generally the dependence of geographical 
contour upon the geological structure of any region and the mechanical forces to 
which the rocks had been subjected. 
Mr. E. Hall exhibited his Maps and Sections illustrating the geological structure 
of Derbyshire and the Lancashire Coal-field. 
Prof. E, Forbes exhibited a Map of Lycia, by Lieut. Spratt. 
a eee b 

