100 W. UPHAM — THE SUCCESSION OF PLEISTOCENE FORMATIONS. 



appears by the changes of its marine moUuscan faunas to have been 

 vastly longer, having comprised perhaps between two and four million 

 years, of which the Pliocene period would be a sixth or eighth part, thus 

 exceeding the whole of the ensuing era of great epeirogenic movements 

 and resulting glaciation. 



DISCUSSION. 



W J McGee : Mr Upham's communication is a highly interesting and 

 suggestive one ; but, in speaking of the Columbia and Lafa3^ette forma- 

 tions as intimately related, if not essentially a unit, he falls into a 

 serious error, though an error, perhaps, not surprising on the part of one 

 who has not seen the formations in question. If there is one pre- 

 dominant general fact in the geology of southeastern United States, it 

 is the fact of the unconformity between the Columbia and Lafayette 

 formations, the greatest unconformity of the entire Coastal plain series, 

 extending in time from the Cretaceous to the present, and in space from 

 the Hudson to the Rio Grande. This unconformity represents erosion 

 approaching, if not reaching, 1,000 feet in depth in the lower Missis- 

 sippi region, and from 300 to 500 or more feet in depth at the embou- 

 chures of the other rivers of the Coastal plain, save, perhaps, off cape 

 Hatteras, where the depth may perchance not exceed a hundred feet. 

 The unconformity is represented by widespread erosion over areas indi- 

 vidually embracing thousands of square miles, from which the entire 

 formation has been removed, as in the Mississippi embayment area of 

 150 by 600 miles ; it is represented not only by the removal of fully half 

 of the original volume of the Lafayette formation, but by the degrada- 

 tion of an equal or greater volume of subjacent formations of Neocene, 

 Eocene, and Cretaceous age beneath. The Columbia and Lafayette 

 formations are by no means the deposits of a single period , they were 

 not at all closely connected in time; the unconformity between them, 

 representing erosion to an average depth of some hundreds of feet 

 vertical over an area of some hundred thousand square miles, is the 

 most impressive record of coastal plain history. 



R. D. Salisbury : Our interpretation of Lafayette and Columbia in 

 New Jersey must still be regarded as in some sense tentative ; but, if 

 we have interpreted correctly, the break between the Lafayette and the 

 Columbia, instead of being a minor one, is one of great magnitude, 

 representing a great time interval. It probably exceeds in importance 

 any subsequent break in the geologic series. 



