Ch. III.] 
OF SEDIMENTARY DEPOSITION. 
29 
transported matter will at once cease in one region, and com- 
mence in another. 
Although the causes which occasion the transference of the 
places of sedimentary deposition are continually in action in 
every region, yet they are most frequent where subterranean 
movements alter, from time to time, the levels of land, and 
they must be immense during the successive elevations and 
depressions which must be supposed to accompany the rise of 
a great continent from the deep. A trifling change of level 
may sometimes throw a current into a new direction., or alter 
the course of a considerable river. Some tracts will be alter- 
nately submerged and laid dry by subterranean movements ; 
in one place a shoal will be formed, whereby the waters will 
drift matter over spaces where they once threw down their 
burden, and new cavities will elsewhere be produced, both 
marine and lacustrine, which will intercept the waters bearing 
sediment, and thereby stop the supply once carried to some 
distant basin. 
We have before stated, that a few earthquakes of moderate 
power might cause a subsidence which would connect the sea 
of Azof with a large part of Asia now below the level of the 
ocean. This vast depression, recently shown by Humboldt to 
extend over an area of eighteen thousand square leagues, sur- 
rounds Lake Aral and the Caspian, on the shores of which 
seas it sinks in some parts to the depth of three hundred feet 
below the level of the ocean. The whole area might thus sud- 
denly become the receptacle of new beds of sand and shells, 
probably differing in mineral character from the masses pre- 
viously existing in that country, for an exact correspondence 
could only arise from a precise identity in the whole combina- 
tion of circumstances which should give rise to formations 
produced at different periods in the same place. 
Without entering into more detailed explanations, the reader 
will perceive that, according to the laws now governing the 
aqueous and igneous causes, distinct deposits must, at different 
periods, be thrown down on various parts of the earth's surface, 
