674 
moraines, as contrasted with comparatively angular blocks of the 
pre-existing drift which have not been in contact with the glacier. 
I refer you to the work of M. Godeffroy for the explanation of the 
manner in which he supposes the surface of the advaneing or re- 
treating glacier was subjected to lateral overflows or “ écroulemens ” 
of stones, gravel, and earth, and also for his theory of medial mo- 
raines ; but I now bring to your notice his ingenious effort to solve 
one of the very difficult climatological problems in the Alps. Having 
shown how the lower valleys must, from year to year, become more 
and more encumbered with detritus, he seizes this fact to explain 
by it alone, both the well-known retreat of the glaciers and the fact 
brought forward by Venetz and other observers; viz. that roads 
which existed in certain former passes of the High Alps are now quite 
choked up with snow and ice-—a fact which has been supposed to 
indicate a sensible decrease of temperature within the historic era. 
M. Godeffroy contends, that in ancient times, when the gorges were 
more open, and the heaps of detritus at the entrance intb the lower 
valleys were less in size and fewer in number, and when consequently 
the glaciers easily extended to greater distances, the continual and 
unrestricted supply of snow and ice from many affluents more than 
countervailed the loss through atmospheric action; but that as the 
obstacles increased at some distance above the terminal moraine, the 
lower ends of the glaciers not being so fed as to regain in one sea- 
son the melting losses of the previous year, the inevitable result 
was a successive shrinkage and retrocession of the mass. ‘The in- 
crease of snow and ice in the upper passes, and the blocking up of 
the roads, are explained by the same agency ; for as soon as the de- 
scent of the glacier from the higher to the lower Alps was im- 
peded, it would follow, that the frozen matter of the higher re- 
gions, deprived of its previous exit, must find its way into the ad- 
jacent upper depressions, and there form those mers de glace which 
have obstructed the road-ways or passes of our ancestors. Thus is 
the supposed anomaly explained without recurring to any change 
of climate*. 
In that part of our own country to which the glacial theory has 
been applied, Mr. Charles Maclaren, already known to you by ex- 
cellent geological treatises, has recently published a well-condensed, 
small work explaining the views of Agassiz. The phenomena of 
glaciers and the general doctrines derived from their study being 
explained, Mr. Maclaren proceeds to analyze those cases of trans- 
ported detritus in the neighbourhood of Edinburgh to which the 
theory had been supposed to apply. 
*T hoped to have been able to quote the opinions of Professor J. 
Forbes on this vexata questio, because it is well known that he was a 
companion of Professor Agassiz in the Alps during the last summer, but 
this distinguished cultivator of physical science has not yet published his 
views on the action of glaciers as affecting the surface of the earth, though 
he has given to the public a very ingenious sketch, descriptive of a peculiar 
parallel striation in the solid ice of glaciers.—Edinburgh New Philoso- 
phical Journal, January, 1842, 
