224 SCIENCE PROGRESS. 



whilst the lake is drained through a rock gorge, whose head 

 once formed a col at a lower level than that of the drift 

 barrier. A similar instance of the formation of lakes, 

 accompanied by a marked deflection of drainage, has been 

 described by M. Delabecque in the case of Lakes Longemer 

 and Gerardmer in the Vosges district (7). This method of 

 formation of lakes was described in 1894 by the late 

 Professor Carvill Lewis, in the work already cited. He 

 states that " if the moraine is compact, an outlet for the 

 lake is often cut through the rock, and a post-glacial gorge 

 is formed". He gives Lake Constance as an example, 

 and observes that "this lake is dammed on its north side by 

 the great terminal moraine of the Rhine glacier, and owes 

 its large size to the flatness of the region in which it lies. 

 Its present outlet to the west by way of Schaffhausen, 

 is a post-glacial one cut through rock. Before it was 

 obstructed by the moraine, the pre-glacial Rhine flowed 

 northward into the Danube." Lakes blocked by moraines, 

 whether the outlet is now over the moraine or over solid 

 rock, are spoken of by Professor Carvill Lewis as " inter- 

 morainic lakes ". 



Before quitting the class of lakes produced by the 

 formation of a dam across a valley, allusion must be made 

 to those due to a lava-flow obstructing the drainage of a 

 valley, as happened in the case of the lava-flow descending 

 from Etna in 1603, which blocked the valley of the Simeto. 

 Lakes must frequently have been produced in this manner 

 in volcanic regions. 



(iii.) Lakes in areas which have been depressed by earth- 

 7novement. — Differential movement of portions of the earth's 

 crust has long been known to have produced lake-basins 

 both in volcanic and non-volcanic regions. A good example 

 of a lake of this character in an old volcanic region is Lake 

 Balaton (the Platten See) in Hungary, which has been 

 described by Professor Judd (8), and the instances de- 

 scribed by Dr. Gregory along the " Great Rift Valley " 

 of Africa are now familiar to all readers (9). The proofs 

 of the formation of lakes by earth-movement are particularly 

 clear in the case of the ancient lakes Bonneville and Lahon- 



