518 THE FEESH- WATER LOCHS OF SCOTLAND 
influence of the winds in producing currents, and the greater or less 
abundance of salts in solution, also affect the temperature conditions, 
as well as the quantity of atmospheric gases absorbed from the air 
at the surface and distributed by currents and convection throughout 
the mass of water in the lake. 
Deposits. The deposits in lakes consist of gravels, sands, marls, clays, and 
muds, the variations in these depending largely on the geology of the 
country in which the lakes are situated. Usually the muds in the 
deeper parts of lakes contain a large amount of organic matter, which 
is chiefly of vegetable origin, and in this respect they differ from 
marine deposits. It occasionally happens that diatomaceous deposits 
are formed in lakes, especially where the detrital matter from the 
seen in lava-flows, which build great dams across valleys : the marshes around 
the edge of the Snake Eiver lava-sheets seem to be lakes of this sort, verging on 
extinction : crater lakes are associated with other forms of eruption. Accidents 
of the cold kind are the glacial invasions : we are perhaps disposed to overrate the 
general importance of these in the long history of the world, because the last one 
was so recent, and has left its numerous traces so near the centres of our civiliza- 
tion ; but the temporary importance of the last glacial accident in explaining our 
home geography and our human history can hardly be exaggerated. During the 
presence of the ice, especially during its retreat, short-lived lakes were common 
about its margin. . . . We owe many prairies to such lakes. The rivers running 
from the ice-front, overloaded with sand and silt, filled up their valleys and 
ponded back their non-glacial side-streams ; their shore-lines have been briefly 
described in Ohio and Wisconsin, but the lakes themselves were drained when 
their flood-plain barriers were terraced ; they form an extinct species, closely 
allied to the existing Danube and Red River type. As the ice-sheet melts away, 
it discloses a surface on which the drift has been so irregularly accumulated that 
the new drainage is everywhere embarrassed, and lakes are for a time very 
numerous. Moreover, the erosion accomplished by the ice, especially near the 
centres of glaciation, must be held responsible for many, though by no means for 
most, of these lakes. Canada is the American type, and Finland the European, 
of land-surface in this condition. The drainage is seen to be very immature, but 
the immaturity is not at all of the kind that characterized the first settlement of 
rivers on these old lands : it is a case, not of rejuvenation, but of regeneration ; 
the icy baptism of the lands has converted their streams to a new spirit of lacus- 
trine hesitation unknown before. We cannot, however, expect the conversion to 
last very long : there is already apparent a backsliding to the earlier faith of 
steady flow, to which undisturbed rivers adhere closely throughout their life. 
"Water-surface is, for the needs of man, so unlike land-surface, that it is 
natural enough to include all water-basins under the single geographic term 
' lakes.' Wherever they occur, — in narrow mountain-valleys or on broad, level 
plains ; on divides or on deltas ; in solid rock or in alluvium, — they are all given 
one name. But if we in imagination lengthen our life so that we witness the 
growth of a river-system as we now watch the growth of plants, we must then as 
readily perceive and as little confuse the several physiograj)hic kinds of lakes as 
we now distinguish the cotyledons, the leaves, the galls, and the flowers, of a 
quickly growing annual that j)roduces all these forms in appropriate order and 
position in the brief course of a single summer." 
