79 8 REVIEWS 



What this height is habitually in this part of the Mississippi is not 

 known to the reviewer. The range between low and high water is 

 given by Abbott as twenty feet at St. Paul, thirty-five feet at the mouth 

 of the Missouri, and fifty feet at some points below. From the data 

 given in the paper, it would appear that in the natural river, before 

 influenced by damming, the low water was from twenty to twenty-five 

 feet below the main plain. If, therefore, an average flood stage were 

 applicable to this locality, the deepening of the channel since the river 

 floods rose to the plain could be estimated at only a few feet, but the 

 barrier formed by the slate renders an estimate of the time very uncer- 

 tain. If the slate were once much higher than now, it should have 

 kept the river longer within reach of the plain at flood time, so that 

 the hypothesis that the slate has been notably cut down by the river, 

 introduced to avoid the criticisms of Holmes and Hershey, is not with- 

 out its embarrassments in another direction. 



Now, it seems clear from the evidence presented in the paper that 

 the quartz chips were not spread over the plain while the clean strati- 

 fied gravels were being formed, nor while the river was meandering 

 over the plain in its transitional-adjustment stage, nor in its general 

 degradational stage, for at all of these stages, scour-and-fill should 

 have incorporated the chips in the stratified sands and gravels. The 

 chips were quite clearly introduced after the Mississippi had "with- 

 drawn into the narrower limits of an eroded stream bed" and while 

 only its flood stages overflowed the upper plain. This normally 

 occurred in the fourth stage sketched above. As the recent cutting 

 down of the channel has been slow on account of the slate barrier, a 

 very considerable period has probably elapsed since the Mississippi 

 last reached the upper plain even in its highest flood stages, except as 

 these might be made exceptional by ice jams and similar obstructions. 

 This gives the origin of the chips a respectable antiquity, but does not 

 offer any presumption that it fell within the glacial period, or even 

 very near its close. This seems to the reviewer to be the normal inter- 

 pretation of the evidence presented in the paper. 



T. C. C. 



