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, 



