694 PROF, T. G. BONNEY ON [ Nov. 1902, 
Erstfeld, could have been but rarely exposed. But, it may be asked, 
does not this fact prove the crest at the watershed (now of crys- 
talline rocks) to have been higher by the thickness of the vanished 
sedimentaries? That, I think, would not follow, for we must 
remember that during this age the northern flank of the Central 
Alps would more closely resemble the corresponding part of the 
Tyrol at the present day: namely, an outer range of sedimentaries, 
like the Austro-Bavarian Alps, and a central one (the watershed) 
of crystalline rocks in which, however, remnants of the others were 
then more abundant. The Lepontine Alps, during the formation of 
the Nagelfiuh, may very well have been rather higher than at present, 
but probably not much; nor is such a difference necessary, since 
the Reuss can still transport similar materials to the level of the 
Lake of Lucerne, and even an addition of 2000 or 3000 feet to the 
altitude of the range would leave it inferior to the present Ober- 
land.’ The final uplift of this range,’ with its complicated folds and 
thrust-faults, and the elevation of the Nagelfluh to a frontier-zone of 
mountains, sometimes rising above the 5000-foot contour-line, may 
be ascribed to the series of movements which closed the Miocene 
Period and made a land-region of this part of Europe. That, of 
itself, would retard the outflow ; but as there is no more change in 
the physiography of the Alpine Rhine, Rhone, and Reuss than what 
the rock-structures themselves can explain, we may conclude that 
the rivers had already sufficient erosive power to keep open their 
outlets, and that the central axis of the Alps was not much higher 
than it is at the present day. 
The Alps, however, may have been lowered by actual subsidence ; 
and this objection is more difficult to meet. Indeed, though I see 
no reason for supposing that the difference in level between the 
crest of the Pennine Chain and the mouth of its lateral valleys, 
such as the Vispthal, has materially altered, I should think it 
probable that the slope of the great trunk-valleys, into which these 
discharge, such as the Rhone above St. Maurice, has been diminished, 
and that the great lakes, at any rate, lie in a zone of rather recent 
subsidence. ‘This movement, however, I think, has not so much 
affected the Alps as a whole as the relative level of certain parts, and 
is not likely, even on the former supposition, to have lowered their 
crest by more than 1000 or at most 1200 feet. If so, a correction of 
some 8° or 4° Fahr. must be applied to the temperatures which I 
have used. But as we do not know when, if at any time, to apply 
this correction, or, in other words, at what epoch the Alps reached 
their greatest altitude, it seemed to me simpler to reason from the 
existing state of things, instead of suggesting possible modifications 
of uncertain value. 
1 The Pizzo Rotundo is 10,490 feet above sea-level; the St. Gotthard Pass 
6935 feet. 
* The effects are conspicuous in the Mont-Blanc massif, and war it as far 
as Dauphiné. 
