OBJECTIONS TO THE FLUVIATILE HYPOTHESIS. 547 



dence of old banks and bottoms of channels. Careful search has not 

 been made, but general examination has not yet revealed them. Some • 

 of the later channels upon the surface may have become early occupied 

 by later streams, and so have been cut away. A glance at the map sug- 

 gests that the Osage may have at one time passed along the line of the 

 Big Muddy, in eastern Callaway county, to the south branch of the Salt 

 river near Mexico ; that the Kansas may have entered it in Callaway 

 county, and that Grand river mav have crossed in the vicinit}^ of Moberly. 

 Moreover, the absence of vegetation and the rapid deposition of muddy 

 material may in part account for the absence of traces of old banks in 

 the structure of these formations. A more careful study of such plains 

 as that of the Hoang-Ho and Nile is advisable before we reject this 

 h3q)othesis for these reasons. 



How may the present location, size and depth of the troughs of the 

 principal streams be explained ? We have already, in sketching our 

 hypothesis, suggested a reason for their locations. Probably the most 

 formidable objection to the Lacustrine and Fluviatile theories is the ex- 

 cavation of the troughs of the Missouri and Mississippi since the depo- 

 sition of the loess. The}^ are 250 to 300, even in places 400 feet in dej^th 

 (50 to 100 feet below the waterlevel) and from one and a half to 10 miles 

 in width, varying inversel}^ as the resistance of the strata traversed. Time 

 does not allow us to discuss the matter exhaustively, and we must con- 

 tent ourselves with a few of the more important considerations. In dis- 

 cussion we shall refer principally to the Missouri because of our greater 

 familiarit}^ with it. 



The change of baselevel was great, probably 200 to 300 feet, when first 

 the Missouri broke over the Gasconade divide, and the erosion was of 

 loose formations, similar to those now covering the uplands to the north. 



Soon the sill of Silurian rock was reached, but it is not of a massive 

 sort like the Niagara limestone. It easih^ breaks into small blocks. If 

 the current was strong enough to wield the blocks, it must have been 

 rapidly torn away. This, it should be remembered, is a most important 

 factor in corrasion. If the strength of a current is not sufficient to move 

 the separable units composing a stratum, it acts slowly, mainly by so- 

 lution ; but the moment that the current is able to pickup and transport 

 blocks of average size, the larger the blocks the more rapid the erosion- 

 We shall see that the current in this case must have been very strong. 

 The divide between the Osage and Gasconade was not more than 12 

 miles in width. 



But the most important consideration is the great volume of the Mis- 

 souri at that time, ^^^e may estimate the area of land-ice draining into 

 that stream durinoj the accumulation of the first or outer moraine as 



LXXVI— Bui-L. Geol. Soc Am., Vol. 5, 1893. 



