1887.] Professor Sacco on Origin of Great Alpine Lakes. 279 
debouched upon the plains, their path therefore lay over a region 
more or less thickly covered with gravelly deposits, and we need 
not wonder therefore at the great thickness attained by the con- 
glomerates which we now meet with underneath the great terminal 
moraines of Piedmont, &c. The occurrence of these conglomerates 
has long been well known, ever indeed since attention was first 
directed to them by Martins and Studer some forty years ago. 
The “ morainic amphitheatres ” and the underlying and associated 
diluvial gravel, &c., are the characteristic accumulations of the 
glacial period, and correspond, in my opinion, to the similar accumu- 
lations which, according to Swiss geplogists, belong to what they 
term the ‘‘second glacial epoch.” From all the Alpine valleys at 
this period powerful streams and torrents descended, and their pro- 
ducts occur not only opposite the mouths of the greater Alpine 
valleys which contained large glaciers, but spread out also into the 
plains, and low grounds opposite mountain-v alleys in which no 
glaciers appear to have existed. To these deposits various names 
have been assigned, such as “Quaternary alluvia;” “ fluvio-lacus- 
trine alluvia,” “diluvium,” “cones de dejection,” “Areneano” = 
(gravelly sands with remains of Elephas primigenius , Megaceros , 
Cervus euryceros , (fee.); “Ferretto,” <fec. 
It is unnecessary, however, to pause longer over this period, the 
general conditions of which, so far as they relate to the Alps, are 
sufficiently well known. The eventual decadence of the great 
glaciers was, in my opinion, brought about by the gradual and 
general elevation of the continent, and the consequent disappear- 
ance of many wide marine and lacustrine areas. By the gradual 
disappearance of those water areas evaporation was progressively 
diminished, until the atmospheric precipitation on the Alps was 
reduced by one-tenth, consequently the glaciers, for lack of aliment, 
gradually retreated, and the great troughs in the mountain valleys 
became lacustrine basins. (Probably, also, the gradual lowering of 
the temperature of the globe may have had something to do with 
diminished evaporation and precipitation.) Now, the lakes are 
being gradually filled up by the sediment washed into them by 
streams and rivers, so that the geologist can foresee a time when they 
will become in this way entirely silted up. In Post-glacial times the 
streams were not of such importance as those of the Glacial period, 
