274 Proceedings of Royal Society of Edinburgh. [june 20 , 
Miocene times, the sea retreated from Switzerland, and hence the 
succeeding deposits consist, not of marine, hut terrestrial and lacus- 
trine beds. These I take to he representative of the Tortonian and 
Messinian of Italy. During the following Piacentian epoch the sea 
invaded the valley of the Rhone, and reached as far as Lyons, hut 
did not approach nearer to Switzerland. 
It is to be noted in this connection that the invasion of the 
Piacentian Sea was not general, even for the south side of the 
Alps, for deposits of that age are wanting in Yenetia, east of Lake 
Garda. It woidd appear, therefore, that the post-Messinian sub- 
sidence was not nearly so well marked in this particular region as in 
that which lay further to the west. Thus the Venetian districts, 
with their continental deposits of Pliocene age, show phenomena 
analogous to those met with on the northern side of the Alps. 
Towards the middle of the Pliocene period, a movement of eleva- 
vation was again initiated. This appears to have been somewhat 
rapid in certain regions, for we find in places blue marls, with a 
deep-sea fauna, overlaid directly by yellow sands charged with fossils 
of littoral habitats. In other places the same deep-sea strata are 
covered by continental accumulations, pointing in like manner to a 
more or less rapid upheaval. In yet other places, however, we find 
evidence of a gradual change from deep-sea to shallow-water con- 
ditions, showing that the elevation may, after all, have been rather 
protracted than rapid. 
The distribution of the arenaceous marine deposits of the Astian 
along the base of the Apennines (where they are widely and 
almost continuously spread), and here and there also at the foot of 
the Alps (such, for example, as the marly beds of the Piacentian), 
leaves one in no doubt as to their stratigraphical position. At the 
foot of the Alps, however, or at a little distance from these moun- 
tains, we encounter certain gravelly deposits, generally quite un- 
fossiliferous, and having a prevalent fluviatile character. These 
gravels, according to some geologists, correspond in age to the yellow 
marine sands of the Astian ; by others they are regarded as Quater- 
nary accumulations. And so in Italy, as in Switzerland, there is 
the same difficulty as to the precise stratigraphical position of these 
deposits. But while in Switzerland their horizon has been variously 
assigned to any stage — from the Messinian to the Quaternary — in 
