/ 



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 gigantism 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 depos.t 

 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 dnfl 

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

 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 

 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 y 

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

 there was a condition of temperature compatible with the gen 

 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 t e 

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

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

 extinction of the gigantic mammalia. Whether these deductions a or^ 

 any countenance to the hypothesis of some eminent geologists, 

 age of the drift was a period of great cold throughout the northern tei 

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



here recorded, by the simple theory of local modifications ot c 



. . r J , ... . , .„ r.^w discus*. 



tudes 



treated 



geology 



:h 



currents 



ingenuity ireaiea 01 trie possible conditions 01 me great * ^ 



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



f his reason 

 the drift, and although I cannot assent to certain portions 01 



