DISPLACEMENT OF THE LOESS AT VICKSBURG. 475 



fication from loess above to gravel below. I dwell upon the point because 

 the relation is not the one commonly seen. In the neighborhood of Vicks- 

 burg on the banks of the Mississippi, where the bluffs are two hundred feet 

 high, the loess commonly appears to rest unconforinably on the gravel. The 

 former is charged with fossils down to a plane of contact as smooth as a 

 floor for hundreds of square yards ; and below that plane there is nothing 

 but gravel — stratified and cross-bedded gravel, which President Chamberlin 

 has well described as consisting of chert with no far northern material. But 

 the apparent unconformity has been produced — and the statement is made 

 with hesitation, because it sounds incredible — by movements within the body of 

 the formation since it was laid down ; and in some of the better sections in 

 the neighborhood of Vicksburg the character of the movements is illustrated. 

 At one extremity of the best section about Vicksburg (half a mile south of 

 the National Cemetery), the loess and gravel intergraduate as already de- 

 scribed ; while at the other extremity of the section the usual unconformity 

 appears — the loess resting upon the smooth surface of a gravel bed ; but at a 

 point between, a line of fracture cuts off the transitional beds of sand, silt, 

 loam, and fine gravel normally lying between the loess above and the gravel 

 below, indicating either that the stratified beds have been squeezed out, or 

 that the loess has slipped down upon the gravel surface from a higher level. 

 In short, about Vicksburg, there have been landslips of enormous extent, 

 and these landslips have produced the prevailing unconformity. The struc- 

 ture finds expression in a wide-spread but peculiar surface configuration : 

 There are many areas of plane surface one to three miles in extent which re- 

 mind the geologist at once of fluviatile or littoral terraces; but no two of the 

 planes rise to the same level, and, while all are inclined more or less, no two 

 incline in the same direction or with the same slope ; consequently there is 

 a series of unrelated terraces sculptured into hills and ravines yet retaining 

 indications of original attitudes, running over great areas. Thus the whole 

 structure of the Pleistocene dejmsits in the vicinity of Vicksburg and Grand 

 Gulf, and the whole topography as well, are affected by a series of landslips. 

 The point of present importance is the fact that the loess and gravels to- 

 gether constitute a distinct structural unit. The loess graduates downward 

 into the gravels, and these gravels are Pleistocene; and both represent glacial 

 action, unquestionably during the earlier ice invasion. 



President Chamberlin : I think I understand what Mr. McGee refers to. 

 The same phenomena may be seen at Randolph, at Fort Pillow, and on 

 Crowley's ridge. I referred to it hastily, and it is not strange that Mr. 

 McGee should have misunderstood me. At Fort Pillow and at Randolph 

 there are beautiful sections. There are the Tertiaries at the bottom, and 

 then these gravels 8 to 10 feet, more or less, in depth. These graduate, as 

 Mr. McGee has said, up into a silt. This silt ranges up to 8 or 10 or more 



