


422 STRUCTURAL AND FIELD GEOLOGY 
are confined to regions which are proved by other evidence 
to have been formerly occupied by snow-fields and glaciers. 
And those glaciated regions are pre-eminently the lake-lands 
of the world. In Europe, for example, very few lakes occur 
outside of the glaciated areas over which ice-sheets and 
glaciers formerly extended—the more notable exceptions 
being the volcanic basins of Auvergne, the Eifel, and Central 
Italy. In North America the same remarkable distribution 
of lakes may be seen. Throughout the extensive regions 
lying north of the glacial boundary, they are exceedingly 
numerous, while south of that line they are almost unknown. 
Of the few lakes which occur in regions which have never 
been subjected to glaciation, the more important occupy 
tectonic and volcanic basins. 
Unless they be very capacious and extensive, basins soon 
become obliterated. Erosion and sedimentation are too 
active to permit of their prolonged duration. Exceptionally, 
basins which occur in dry and practically rainless tracts 
where erosion and deposition are at a minimum, may persist 
for lengthened periods of time. The saline and alkaline 
lakes of such regions are in many cases visibly drying up, 
and wind-blown sand is encroaching upon their desiccated 
floors. Again, tectonic basins may long outlive the land- 
surface upon which they first appeared. Should the floor of 
such a basin, occupied by a great lake, continue to subside at 
approximately the same rate as it is being filled up with 
sediment, and the effluent river cannot in the meantime 
succeed in draining the water away, it is obvious that the 
lake may persist for a very long time. A vast thickness of 
sediment might come to accumulate upon its floor, although 
the depth of the lake might never have exceeded a few 
hundred feet. The lake would in such a case form the base- 
level for all the surrounding region, the surface of which, 
perhaps mountainous to begin with, would be gradually 
lowered, and might pass through a complete cycle of erosion 
before the lake ceased to exist. In a word, a great lake or 
inland sea may become the burial-place of the high grounds 
that drain towards it, for it bears the same relation to these 
as an ocean to a continent. Relatively few lakes, however, 
occupy tectonic basins. Of these the majority, as we have 
