Ch. XII.] FORMATION OF LAKE-BASINS. 173 



lake-basin may result as above stated. If there be no ice, the forma- 

 tion of a lake will depend on the relation of two forces : the rate at 

 which the land is raised or sunk, and the rate at which the river can de- 

 posit sediment in the new depressions. Should the movement be very- 

 slow, the river may fill the incipient cavity with mud, sand, and pebbles, 

 as fast as it is formed, and having levelled it up may afterward cut 

 through the new stony barrier at the lower margin of the depressed 

 area ; but if the capacity of the new basin increases at too great a 

 rate, the river will only be able to encroach partially upon it by form- 

 ing a delta at its higher extremity. If the change takes place in a 

 glacial period, the thickness of the ice will augment from century to 

 century, not in consequence of erosion, but simply because the con- 

 tour of the valley, is becoming gradually more basin-shaped. The 

 mere occupancy, therefore, of cavities by ice, by preventing fluviatile 

 and lacustrine deposition, is one cause of the abundance of lakes 

 which will come into existence whenever the climate changes and the 

 ice melts. 



In Switzerland there' are lacustrine formations of the Post-pliocene 

 period, which show that the Lake of Zurich, and some other Swiss 

 lakes, were formed before the erosive power of ice had been exerted 

 in that country ("Antiquity," p. 314). In Scotland, also, there is 

 evidence that some of the main valleys by which the drainage now 

 takes place were in existence before the Glacial epoch. But although 

 most of the valleys of the Alps and some of the lakes were pre-glacial, 

 there seems ground for suspecting that not a few of the valleys were 

 converted into lake-basins during the long series of ages in which ice 

 prevailed. In support of this view, many good observers affirm that 

 below the present outlet of the great lakes in Switzerland and Italy, 

 an ancient fluviatile alluvium may be seen, on which the moraines of 

 the great glaciers which once traversed the lakes repose. The pebbles 

 in these old alluviums comprise all those varieties of rocks which 

 belong to the upper course of the valley above, or to tributary valleys 

 in the same higher region. The phenomenon here alluded to would 

 be in perfect accordance with the theory that the rivers were once 

 continuous, or not intercepted by lake-basins destined to be filled and 

 traversed by glaciers. It is unnecessary to resort to M. de Mortillet's 

 hypothesis, that each basin was first filled up with alluvium sometimes 

 above 2000 feet thick, and that this was afterward cleared out by a 

 glacier, for such removal would imply a capacity of erosion which we 

 are not warranted to assume, and which, if granted, might have 

 enabled the ancient glacier of the Rhone to excavate the basin of the 

 Lake of Geneva out of the miocene molasse. Dr. Falconer, Mr. Ball, 

 and other writers, have pointed out that the form of several of the 

 great Italian lakes, such as Como, Maggiore, and Garda, is by no 

 means in harmony with the hypothesis that they have been hollowed 

 out by great glaciers which once passed through them. 



From the analogy of flowing water, we have reason to suspect that 



