249 



indicate it, but rather an upheaval. Before the Potstlam sand- 

 stone there is no evidence of high mountains. During upheaval, 

 there is dislocation at the junction of the beds, forming a series 

 of abrupt vertical cliffs, as seen in the succession of the strata of 

 New York ; in the case of denudation, the surface would be a 

 series of general curves, which is not the case in New York. 

 During upheaval the level of the sea may be actually less, from 

 the contraction of the earth's crust while cooling ; in consequence 

 of this contraction the ocean would always remain at a certain 

 depth, sufficient for these depositions of thousands of feet of strata 

 during the upheaval of the land. The study of the fossils, also, is 

 opposed to the theory of subsidence and denudation, and we do 

 not find those of the primary carried into the secondary beds. 



Prof. Rogers replied that the subsidence of the ocean-bottom 

 is equivalent to an increase between the bottom and the level of 

 the sea ; when this distance is increasing, the deposits are thereby 

 sealed in by the subsequent deposits ;.when this distance is les- 

 sening, thereby successively exposing all the margins of the strata 

 to the action of the surf, they are worn down and destroyed. The 

 change of relative distance is the important point ; how this is 

 brought about is of little consequence to the present discussion. 

 The perfect conformity and parallelism of the bank-like strata 

 adverted to by Prof. Agassiz, and the terrace form, depend on 

 the dip of the strata, and on the planes of cleavage being nearly 

 perpendicular to the stratification ; and the present aspect of 

 these surfaces is continually changing. 



Prof Agassiz replied that, though of course these terraces 

 have undergone great changes, he meant to convey the idea that 

 they represented, to all intents and purposes, the original surface 

 of the country. 



Dr. Hayes submitted a letter from the interior of Li- 

 beria, Africa, in which the writer says that there is no 

 occurrence there of native iron, as stated by him in Vols. 

 V. p. 250, and VI. p. 279, of the Proceedings. 



The piece there described was smelted by the tribe among 

 whom the writer resided, who keep the art to themselves, as they 



