274 



accumulated during a long 'period of subsidence of the ocean-floor, 

 varied hy many and long pauses and upicard oscillations. 



Prof. Rogers added a brief notice of the structure of the mid- 

 dle secondary belt of sandstones of the Middle States as a good 

 example of the successive accumulation of shore-dei)Osits previously 

 referred to. He showed that the uniform and gentle slope of all 

 the strata in one direction, and the nature of the materials com- 

 posing them, concur in indicating the directions from which they 

 have been derived ; and he pointed out the manner in which 

 the successive parallel strata in their downward extension ter- 

 minate against the ancient floor, which in some cases is so shallow 

 as to be partly exposed, with the abutting strata, in the natural 

 sections of the country. 



Prof. Agassiz replied that he admitted the shallowness of the 

 ocean in which these strata were deposited ; during a local up- 

 heaval of the shore, the whole bottojii was, in his opinion, sub- 

 siding from the shrinking caused by the cooling of the earth's crust. 

 He alluded to the transverse rents across the New York strata in 

 a direction north and south, and mentioned the Schoharie valley, 

 in which the strata increase in thickness from north to south, 

 indicating that the seaside was to the south and the shore-line to 

 the north. He could detect no si£:ns of denudation in these an- 

 cient strata of New York. The Potsdam sandstone is both geo- 

 logically and topographically the lowest. 



Prof. Rogers replied that he had listened with extreme sur- 

 prise to the statement just made by Prof. Agassiz, disclaiming 

 the theory of the rising of the ocean-floor during the formation of 

 our northern paleozoic strata. According to his recollection, the 

 discussion was commenced by Prof. Agassiz's denying the cor- 

 rectness of the views of Darwin and others of the extensive de- 

 struction of strata and their fossils during a period of slow up- 

 heaval, and urging as an insuperable objection the great extent 

 and completeness of the paleozoic series of New York, which he 

 maintained had been deposited during a period of upheaval. As, 

 however, Prof. Agassiz has now stated that he recognizes the 

 subsidence of the ocean-bed as essential to the theory of their 

 formation, Prof. Rogers thought it of no importance in this con- 

 nection how that depression may have been brought about, or 



