Davis.] 
350 
[January 18 , 
Very similar lakes are found on the lower course of the Dan- 
ube and are most probably of this origin, although Peschel 
considers them fragments of the Black Sea, cut off by the Dan- 
ube delta, and consequently as affording proof that lakes once 
salt may be washed fresh. 1 This latter point is true enough, but 
is not proven by the case in question. Many examples of these 
lakes will be found in alluvial plains. 
C. 2. lee Barrier Basins. Glacial lakes are now of little im- 
portance ; a few occur in the higher mountain chains, but they are 
trifling in size, and rank with many other species only as curiosities 
unless they become of disastrous importance in the valleys below, 
from the floods that follow a giving way of their barriers. Three 
subspecies are easily distinguished : first, when the advance of a 
glacier down a main valley closes the mouth of a lateral ravine ; 
second, when a glacier from a side valley obstructs the main 
stream ; third, when the great ice sheet of early quaternary times 
melted away so as to disclose the upper part of a valley slop- 
ing gently against it. 
The Merjelen-See in Switzerland serves as the type of the 
first subspecies ; it is held back by the Great Aletsch Glacier in 
a little valley behind the Eggischhorn, a favorite point of view, 
from which the lake below and the whole stretch of the ice 
stream in the main valley can be seen. Sometimes the waters 
find an outlet through the ice-barrier ; then the slow accumulation 
of months rushes out in a torrent, flooding the valley of the Massa 
below and leaving miniature bergs broken from the glacier 
stranded on the rocky bottom : subsequently, another motion of the 
glacier closes the outlet, and the basin slowly fills again. The high- 
est level of such a lake will be determined either by free escape 
across the ice, when it will have a variable maximum ; or by flow 
over a pass at the head of its lateral valley. The latter is the 
case with the Merjelen-See, and level of the pass is marked by a 
faint terrace and by a change of color on the rocks around the 
shore. 2 
Although rare at present, these lakes have had a considerable 
importance in the past. An extinct example was early recognized 
1 Phys. Erdk., ii, 314. 
2 L. Agassiz, Etudes sur les glaciers, 1840, 218, 220. Lyell, Principles of Geology, i, 
372; Antiquity of Man, 309. 
