146 
ELEMENTS OE GEOLOGY. 
erally wasting away by the incessant action of rain and. riv¬ 
ers, and in some cases by the undermining and removing 
power of waves and tides on the sea-coast. But the rate of 
waste is very unequal, since the level and gently sloping 
lands, where they are protected by a continuous covering of 
vegetation, escape nearly all wear and tear, so that they may 
remain for ages in a stationary condition, while the removal 
of matter is constantly widening and deepening the inter¬ 
vening ravines and valleys. 
The materials, both fine and coarse, carried down annually 
by rivers from the higher regions to the lower, and deposited 
in successive strata in the basins of seas and lakes, must be 
of enormous volume. We are always liable to underrate 
their magnitude, because the accumulation of strata is going 
on out of sight. 
There are, however, causes at work which, in the course 
of centuries, tend to render visible these modern formations, 
whether of marine or lacustrine origin. For a large portion 
of the earth’s crust is always undergoing a change of level, 
some areas rising and others sinking at the rate of a few 
inches, or a few feet, perhaps sometimes yards, in a century; 
so that spaces which were once subaqueous are gradually 
converted into land, and others which were high and dry 
become submerged. In consequence of such movements we 
find in certain regions, as in Cashmere, for example, where 
the mountains are often shaken by earthquakes, deposits 
which were formed in lakes in the historical period, but 
through which rivers have now cut deep and wide channels. 
In lacustrine strata thus intersected, works of art and fresh¬ 
water shells are seen. In other districts on the borders of 
the sea, usually at very moderate elevations above its level, 
raised beaches occur, or marine littoral deposits, such as 
those in which, on the borders of the Bay of BaiaB, near ]^a- 
ples, the well-known temple of Sefapis was imbedded. In 
that case the date of the monument buried in the marine 
strata is ascertainable, but in many other instances the ex¬ 
act age of the remains of human workmanship is uncertain, 
as in the estuary of the Clyde at Glasgow, where many ca¬ 
noes have been exhumed, with other works of art, all assign¬ 
able to some part of the Recent Period. 
Danish Peat and Shell-mounds or Kitchen-middens. —Some¬ 
times we obtain evidence, without the aid of a change of 
level, of events which took place in pre-historic times. The 
combined labors, for example, of the antiquary, zoologist, 
and botanist have brought to light many monuments of the 
early inhabitants buried in peat-mosses in Denmark. Their 
