Vol. 58. | SUBALPINE PLIOCENE CONGLOMERATES, 465 
Subalpine France and Switzerland, including the lake-basins, could 
not have been already eroded to their present depth, but were as 
yet in their initial stage of formation as shallow depressions, the 
valley-floors being in the lowlands from 100 to 200 metres, and in 
the upper parts as much as 400 metres higher than in our own day. 
It follows, further, that a very long inter-Glacial period of erosion 
must haye elapsed between the Upper Pliocene and the Middle 
Pleistocene or maximum glaciations, the latter of which found the 
valleys excavated, as is evidenced by the products of that glaciation 
which cover and fringe the slopes of, and are embedded in, the 
valleys. This last-named point is of decisive importance ; for if it 
be assumed that, towards the end of the Plocene Period, the 
valleys had already acquired their present outline and depth, it 
would follow that they must have been filled with the products 
of the Pliocene glaciation, to a depth of 100 to 400 metres-—a 
theory which is untenable in face of the now recognized fact that 
all the upper-terrace and valley-gravels are the products of the two 
younger glaciations. 
(2) True it is that the lowering of the valiey-floor of the Zurich 
and Geneva lake-basins by 400 metres (or 1300 feet) in the course of, 
possibly even later than, the Ice-Age, is a startling phenomenon. 
But it must be borne in mind that, apart from the greater volume 
and, owing to the greater fall, the enhanced erosive power of the 
then Alpine rivers as compared with those of our own day, the 
lake-basins per sé, as they appear at present, were formed not only 
by fluviatile erosion, and by subsequent banking-up at their lower 
ends, but by a zonal subsidence or settling along the base of the 
Alps, which followed upon the raising of the latter as a mechanical 
process. As is well known, this theory of a subsidence or ‘flexure,’ 
computed at about 400 metres on the northern side of the Alps, 
was first propounded by Prof. Heim ' on the evidence of the reverse- 
dip of the Molasse-terraces of erosion on both sides of the Lake of 
Zurich. itis turther demonstrated by the fact that the Molasse- 
floor of the valley itself exhibits a considerable reverse dip which, 
beginning near Baden, about 15 miles below Zurich, extends up the 
valley and the greater part of the lake: so much so, that, at the 
outlet of the lake at Zurich, the deepest borings have failed to 
reach the Molasse, and that, towards the upper end and at the 
deepest point of the present basin, the old Molasse-floor is not 
reached even at a depth of 140 metres (460 feet), and is, indeed, 
buried below lake-deposits at an unknown depth. And further 
proof is afforded by the considerable folding of the Molasse in the 
low hills between the Lakes of Zurich and Zug subsequent to the 
depesition of the Deckenschotter, which now appears at a level from 
300 to 400 metres lower than the deposits on the original plateau 
of the Albis or Uetli range (see fig. 3, p. 454). 
+ Beitrage z. geol. Karte d. Schweiz, vol. xxv (1891) pp. 475 & 476. 
