chap. xii. of Conchiferons Deposits. 419 



a great thickness of superincumbent submarine strata, 

 yet it should be borne in mind that the lower fossili- 

 ferous strata with several of the same species of Mollusca, 

 the upper tufaceous beds, and the high summit-plain, 

 stretch for a considerable distance southward, and for 

 hundreds of miles northward ; seeing this uniformity 

 of structure, I conceive it may be fairly concluded that 

 the subsidence by which the shells at Santa Cruz and 

 St. Julian were carried down and covered up, was not 

 confined to these two points, but was co-extensive with 

 a considerable portion of the Patagonian tertiary forma- 

 tion. In a succeeding chapter it will be seen, that we 

 are led to a similar conclusion with respect to the 

 secondary fossiliferous strata of the Cordillera, namely, 

 that they also were deposited during a long-continued 

 and great period of subsidence. 



From the foregoing reasoning, and from the facts 

 just given, I think we must admit the probability of 

 the following proposition : namely, that when the bed 

 of the sea is either stationary or rising, circumstances 

 are far less favourable, than when the level is sinking, 

 to the accumulation of conchiferous deposits of sufficient 

 thickness and extension to resist, when upheaved, the 

 average vast amount of denudation. This result appears 

 to me, in several respects, very interesting : everyone is 

 at first inclined to believe that at innumerable points, 

 wherever there is a supply of sediment, fossiliferous 

 strata are now forming, which at some future distant 

 epoch will be upheaved and preserved ; but on the views 

 above given, we must conclude that this is far from 

 being the case ; on the contrary, we require (1st), a 

 long-continued supply of sediment ; (2nd), an extensive 

 shallow area; and (3rd), that this area shall slowly 

 subside to a great depth, so as to admit the accumula- 

 tion of a widely-extended thick mass of superincumbent 

 19 



