262 Prof. Rogers's Address before the 



understand them correctly, to dissent. At the same time it would ap- 

 pear that in the Hudson and Champlain valleys and elsewhere, the ter- 

 tiary clay is covered with another stratum of drift " composed of coarse 

 gravel pebbles and bowlders," lying on its trenched and denuded sur- 

 face, and this latest drift, from the magnitude of its erratics, seems not 

 less indicative than the first, of the extent and energy of the transport- 

 ing agency, whatever that may have been. 



Produced in the interval of comparative tranquillity between the two 

 epochs of more vehement disturbance, what let us inquire, is the rela- 

 tive antiquity of this northern tertiary clay compared with the post pleio- 

 cene beds of the south, containing the Gnathodon cuneatus. These, as 

 we have seen, were contemporaneous with the Mastodon giganteum and 

 other large mammalia, and there can be little doubt that the mastodon 

 was posterior to the latest drift of the country, since no erratic deposit 

 covers its remains any where in the region of the drift. The northern 

 post pleiocene, is therefore older in all probability than the southern, by 

 at least the intervening period which produced the later drift. 



Reviewing now all the facts respecting the newer tertiary ages, we 

 are led to the following conclusions. That the whole period of the drift 

 was a prolonged one ; that the active dispersion of the far transported 

 matter was interrupted by an interval of comparative repose, when a 

 part of the northern country was lower than it now is by at least five 

 hundred feet, and low enough to admit the sea into its valleys ; that in 

 this interval the northern waters of this region were quite as cold as they 

 are at present in the same latitude ; that after the close of the drift period 

 there was a condition of temperature compatible with the general dis- 

 tribution of the mastodon on the land and with the existence in the wa- 

 ters, as far northward as Maryland at least, of certain shells of the Gulf 

 of Mexico, and that subsequently to this there was an expulsion south- 

 ward of these southern shells, a slight uplift of the Atlantic coast and an 

 extinction of the gigantic mammalia. Whether these deductions afford 

 any countenance to the hypothesis of some eminent geologists, that the 

 age of the di-ift was a period of great cold throughout the northern tem- 

 perate zone, or whether we may not account for all the vicissitudes 

 here recorded, by the simple theory of local modifications of climate 

 by changes in the oceanic currents, I will not here further discuss. 

 Pix»f. Mather in his report on the geology of New York, has with much 

 ingenuity treated of the possible conditions of the great oceanic currents 

 at the formation of some of our earlier strata and during the epoch of 

 the drift, and although I cannot assent to certain portions of his reason- 



/JiH^ 



