Davis] 
380 
[January 18 , 
breaks in the rock ; and as evidence to support this view he 
instances many lake basins in Ireland that are placed where fault- 
lines intersect. That fissures and joints are as a rule lines of 
weakness, can hardly be disputed, but there are other conditions 
equally potent which the author neglects as much as he exagger- 
ates the effects of faults : the very imperfect correspondence of 
the irregularities of Lake Conga with the fissures considered as its 
cause (see his plate ii) point to the overrating of their power. 
A. 4. Warped Valley Basins. Valleys of erosion may be 
locally so elevated and depressed by transverse folding as to 
interrupt the continuous slope of their water line ; the depres- 
sions will then fill up and appear as lakes. It was to this expla- 
nation as applied by Lyell to Lago Maggiore that Ramsay 1 made 
objection ; but the objection holds good only if it be supposed 
that the valley sinks rigidly, in which case a depression of some 
twenty thousand feet would be needed in the Central Alps to 
cause the lakes at their southern base ; but if, as Lyell points out, 2 
the valley slope be changed by a folding, as is supposed in this 
species, the objection falls to the ground. For the Italian lakes 
some such folding offers a possible explanation, but it is not well 
proved and is not altogether necessary (see C. 3) ; at the north- 
ern base of the Alps the case is better made out. 
There is good evidence to show that the construction of the 
higher Alps and the cutting of transverse valleys were well 
advanced toward completion before the disturbance of the north- 
ern margin of Miocene rocks began, and it is very probable that 
the northernmost folds of the molasse were the latest made. In 
the upper parts of the transverse valleys, any lakes that may 
once have been formed during the growth of the Central Alps 
have long since been drained by the cutting down of their bar- 
riers, for such lakes are simply transient phases in the history of 
the valley; near the plain, where the disturbances of mountain 
growth are of comparatively recent geological date, the valleys 
have been interrupted by the growth of folds across their course 
1 Ramsay, Sir Charles Lyell and the Glacial Theory of Lake Basins, Philos. Mag., 
XXIX, 1865, 285-298. 
2 Antiquity of Man, 1873, 358. 
