270 Chamberlin and Lever ett — Studies of the 



charged northwesterly ; that the evidence relative to the mid- 

 dle Allegheny makes it very probable that this also discharged 

 northwesterly ; that the evidence relative to the lower Alle- 

 gheny and the upper section of the Ohio river also favors a 

 northerly discharge, but is, as yet, too incomplete to justify a 

 firm opinion. 



Whether this weighing of the evidence is entirely correct or 

 not, it is manifest that the preglacial erosion of these sections 

 must be determined severally and independently if any safe 

 conclusions are to be drawn. If they were separate basins, the 

 depth of erosion of one does not determine that of the others, 

 which were in no way perhaps directly connected with it. 

 There would be a certain harmony, so far as like conditions 

 prevailed, and the argument of analogy would have a proper 

 application. In applying it, however, it is necessary to ob- 

 serve that the united sections were unlike in size, in direction, 

 and in distance from the major basins into which they emp- 

 tied. More particularly must it be noted that the united parts 

 represent different fractions of their systems. Some are head- 

 water sections, some body sections ; and these have each their 

 characteristic differences. 



A few of the salient features of the old fluvial floors are all 

 that space will permit here. 



Old Fluvial Floors. 



The old fluvial floor of the uppermost section finds its low- 

 est point, on the line of the present Allegheny, at Steamburg, 

 where it is about 1050 feet A. T. From this point, the old 

 floor rises both up stream and down stream. Going up 

 stream, the rock floor rises so as to become the bottom of the 

 present stream only far up towards the head-waters, in Potter 

 county, Pennsylvania. Going down stream, it rises so as to 

 come near the surface at Kinzua, where the old col was cut 

 across. The present floor here stands at about 1170 feet A. T. 

 The old floor was considerably higher, but we have little data 

 for an estimate of how much. 



In the second of the united sections, we have, in the track 

 of the Allegheny, only a short segment crossing the upper 

 part of the old basin. The old floor appears to drop down 

 from the old col at Kinzua to 1150 feet A. T., and possibly to 

 1100 feet, and then to rise toward the old col at Thompson's 

 reaching about 1220 feet A. T., as well as we can estimate it, 

 — possibly more. The lowest point of this segment, it will be 

 seen, is 50-100 feet higher than the lowest point in the upper 

 section. It is more than a hundred feet higher than the low- 

 est point in the adjoining section to the south, and about 100 



