392 - -  -- DYNAMICAL GEOLOGY. —  [Boox TIT. 
now on the sides of hills, and now over broad plains. They lie for — 
the most part in rock-basins, but many of them have barriers of 
detritus. Their connection with the operations of the glacial period 
will be afterwards alluded to. In the mountainous regions of — 
temperate and polar latitudes, lakes abound in valleys, and are 
connected with main drainage lines. In North America and in © 
Equatorial Africa, vast sheets of fresh water occur in depressions of — 
the land, and are rather inland seas than lakes, 
The distribution of temperature in lakes is a question of consider- | 
able geological interest in regard to which careful measurements are 
much needed. The observations of Sir Robert Christison at Loch 
Lomond in Scotland, show that in this sheet of water, which lies 25 
feet above the sea-level, with a depth of about 600 feet, and is in 
great measure surrounded with high hills, a tolerably constant 
temperature of about 42° Fahr. is found to pervade the lowest 100 
feet of water. Again in the Lake of Geneva the surface temperature 
in autumn is 78° Fahr., while the bottom water at a depth of 950 
feet was found to mark 41°7’. The Lago Sabatino near Rome has a 
temperature of 77° at the surface, but one of 44° at a depth of 490 
feet. Similar observations on other deep lakes in Switzerland and 
Northern Italy indicate the existence in all of them of a permanent 
mass of cold water at the bottom. _ The cold heavy water of the 
surface in winter must sink down, and as the upper layers cannot be 
heated by the direct rays of the sun, save to a trifling and superficial 
extent, the temperature of the deep parts of these basins is kept 
permanently low. 
Geological functions.—Among the geological functions dis- 
charged by lakes the following may be noticed: 
Ist. Lakes equalize the temperature of the localities in which 
they lie, preventing it from falling as much in winter and rising as 
much in summer as it would otherwise do. The mean annual 
temperature of the surface water at the outflow of the lake of 
Geneva is nearly 4° warmer than that of the air. 
2nd. Lakes regulate the drainage of the area below their outfall, 
thereby preventing or lessening the destructive effects of floods. 
3rd. Lakes filter river water and permit the undisturbed accu- 
mulation of new deposits, which in some modern cases may cover 
thousands of square miles of surface, and may attain a thickness 
of nearly 3000 feet (Lake Superior has an area of 32,000 square 
miles; Lago Maggiore is 28U0 feet deep). How thoroughly lakes 
can filter river-water is typically displayed by the contrast between 
the muddy river which flows in at the head of the Lake of Geneva, 
and the “blne rushing of the arrowy Rhone,” which escapes at the 
1 Winds, by blowing strongly down the length of a lake, sometimes considerably 
increase for the time being the volume of the outflow. If this takes place coincidently 
with a heavy rainfall, the flood of the escaping river is greatly augmented. These features 
are noticed in Loch Tay (D. Stevenson, “ Reclamation of Land,” p. 14). Hence, though, 
on the whole lakes tend to moderate floods in the outflowing rivers, they may, by a 
combination of circumstances, sometimes increase them, 
