CHAP. XXXIV.] DEPTH OF FRESHWATER STRATA. 247 



such earthquakes as that of 1811-12, in the territory 

 of New Madrid, we shall be prevented from em 

 bracing the theory implied in the language of those 

 who talk of &quot; the epoch of existing continents.&quot; In 

 treating of deltas, they are in the habit of assuming 

 that the present mass of alluvial matter which has 

 been thrown into the sea at the mouths of great 

 rivers, began to be deposited in all the great hydro- 

 graphical basins of the world at one and the same 

 fixed period namely, when the formation of the 

 existing continents was completed ; as if the relative 

 levels of land and sea had, during that time, remained 

 stationary, or had been affected to so inconsiderable 

 an amount, as to be unimportant in their influence 

 on the physical geography of each region, in com 

 parison with the changes wrought by the rivers, in 

 converting sea into land. But what we already 

 know of the deltas of the Po, Indus, Ganges, and 

 other rivers, leads to a very different conclusion. 

 The boring of an artesian well at Calcutta, was 

 carried to the depth of 481 feet, the greater part of 

 the section being below the level of the sea, and yet 

 all the beds pierced through were of freshwater 

 origin, without any intermixture of marine remains. 

 At different depths, even as far down as 380 feet, 

 lacustrine shells, and a stratum of decayed wood, 

 with vegetable soil, which appears to have supported 

 trees, was met with.* These appearances may readily 

 be accounted for, by assuming that there was a 

 gradual subsidence of the ground for ages, which 



* See &quot; Principles of Geology,&quot; Seventh Edition, 1847, p. 266. 



M 4 



