186 
ELEMENTS OF GEOLOGY. 
tent of 3000 feet, and Swiss geologists have long been accus¬ 
tomed to attribute their lake basins, in part, to those con¬ 
vulsions by which the shape and course of the valleys may 
have been modified. Our experience, in the lifetime of the 
present generation, of the changes of level witnessed in New 
Zealand during great earthquakes is entirely opposed to the 
notion that the movements, whether upward or downward, 
are uniform in amount or direction throughout areas of indefi¬ 
nite extent. On the contrary, the land has been permanently 
raised in one region several feet or yards, and the rise has been 
found gradually to die out, so as to be imperceptible at a dis¬ 
tance of twenty miles, and in some areas is even exchanged 
for a simultaneous downward movement of several feet. 
But, it is asked, if such inequality of movement can have 
contributed towards the production of lake basins, does it 
not leave unexplained the comparative rarity of lakes in trop¬ 
ical and subtropical countries. In reply to this question it 
may be observed that in our endeavor to estimate the effects 
of subterranean movements in modifying the superficial ge¬ 
ography of a country we must remember that each convul¬ 
sion effects a very slight change. If it interferes with the 
drainage, whether by raising the lower or sinking the high¬ 
er portion of a hydrographical basin, the upheaval or de¬ 
pression will only amount to a few feet at a time, and there 
may be an interval of years or centuries before any further 
movement takes place in the same region. In the mean 
time an incipient lake if produced may be filled up with 
sediment, and the recently-formed barrier will then be cut 
through by the river, whereas in a country where glacial 
conditions prevail no such obliteration of the temporary lake- 
basin would take place; for however deep it became by re¬ 
peated sinking of the upper or rising of the lower extremity, 
being always filled with ice it might remain, throughout the 
greater part of its extent, free from sedin>ent or drift until 
the ice melted at the close of the glacial period. 
One of the most serious objections to the exclusive origin 
by ice-erosion of wide and deep lake-basins arises from their 
capricious distribution, as for example in Piedmont, both to 
the eastward and westward of Turin, where great lakes are 
wanting,* although some of the largest extinct glaciers de¬ 
scending from Mont Blanc and Monte Rosa came down from 
the Alps, leaving their gigantic moraines in the low country. 
Here, therefore, we might have expected to find lakes of the 
first magnitude rivalling the contiguous Lago Maggiore in 
importance. 
* Antiquity of Man, p, 813. 
