Association of American Geologists and Naturalists. 259 



cided in date with that formation, and this confirms, I think, my view 

 of their gradual disappearance. But it has been clearly shown that 

 contemporaneously with these very fossils, existed the Gnathodon and 

 other shells of the Mexican Gulf, indicative of a Avarm climate in the 

 so called post pleiocene period. Does this not suggest the conclusion, 

 that the expulsion southward of the Gnathodon and its tropical asso- 

 ciates, was unconnected with the northern drift and its assumed cold, 

 and occurred in all probability contemporaneously with the extinction 

 of the mastodon, at an era subsequent to that which witnessed the 

 strewing of the northern erratics. Whether the other cause I have 

 proposed, viz. the withdrawal from our immediate coast of the Gulf 

 Stream, that great tepid river in the ocean, could produce a sufficient 

 cooling of the climate of the adjacent continent and its coast, to effect 

 at length the extinction of the higher animals and the disappearance of 

 the more susceptible testacea, is a suggestion which those will best be 

 abl^ to weigh, who have studied the influences which that warm cur- 

 rent, remote as it is, even now exerts in controlling the climate of the 

 United States.* 



POST PLEIOCENE OF THE NOKTH. 



Turning to the northern districts of the United States, we meet with 

 another formation referable to the post pleiocene period, which is much 

 more widely dispersed than that containing the Gnathodon, and which 

 also sheds additional light on the epoch of the drift. This is the great 

 blue clay deposit which fringes so many of the rivers and lakes from the 

 coast of Maine to Michigan, and from the parallel of the mouth of the 



* Since this address was read, some very instructive facts connected with the 

 subject of the climate of the post pleiocene period, have been presented to the Asso- 

 ciation by Dr. Amos Binney, as part of a valuable report on the land shells of 

 North America. From this report, it appears that in Indiana and some of the ad- 

 jacent western states, there exists a shallow deposit of clay, first noticed by Dr. 

 Owen, which abounds in fossil land shells. Among them a southern species of 

 Helecina, now rare in the middle latitudes, occurs in the greatest profusion, being 

 as far as the evidence of a single species can reach, an indication of a somewhat 

 warmer temperature. But the value of this discovery is much enhanced by the fact, 

 that the same land mollusks underlie the remains of the mastodon and other large 

 mammalia at Big Bone Lick in Kentucky. Later than the deposition of the drift 

 which they overlie, and earlier than the epoch of the extinction of the mastodon, 

 these fossilized land shells promise when more fully investigated, to furnish a 

 record of an intermediate period, when according to the view of Dr. Binney, a 

 series of shallow lakes existed in the West, and when, as we may conjecture, the 

 temperature of the region was at least as high as during the immediately succeeding 

 era of the mastodon. 



