1879.1 0\)J [Stevenson. 



valleys for a considerable distance from the National road towards the river, 

 and when followed out they are found to he continuous with those previ- 

 ously crossed ou the road. 



Age and Origin of the Horizontal Benches. The surfaces of these benches 

 are still level, and in many places the escarpments are so nearly perfect 

 that one has difficulty in believing that the flat topped hills are not ancient 

 fortifications. The detrital covering is well preserved, though it consists 

 only of loose sand with occasionally a little clay. 



Some may suppose, that, in measure at least, these benches are creatures 

 of the imagination and that they are merely such as must occur in a region 

 where the rocks are of unequal hardness and are horizontally stratified. 

 But such a supposition would be wholly erroneous. The rocks are not hori- 

 zontally stratified and the benches bear no relation whatever to the dip of 

 the rocks. Thus in Springhill township of Fayette county, on the west 

 slope of Chestnut Ridge, the rocks dip at from 4 to 6 degrees toward the 

 north-west, but the fifteenth bench is as level there as it is on the side of 

 Brush Ridge, eight miles away, where the clip is but one degree, or at 

 Uniontown, ten miles away, where the dip is three degrees, or at any 

 other locality within the whole region. That no relation exists between 

 the stratification and the benches is a fact which cannot be stated too 

 positively. * 



It has been suggested that these benches may be regarded as marking 

 base levels of erosion, such as have been described by Maj. Powell. In de- 

 fining the term, that author uses the following language : "We may con- 

 sider the level of the ocean sea to be a grand base-level, below which the 

 drylands cannot be eroded ; but we have for local and temporary purposes, 

 other base-levels of erosion, which are the levels of the principal streams 

 which carry away the products of erosion." 



As Major Powell states, the term " level " is used with some freedom in 

 reference to stream-beds, and the term base-plane might have been more 

 apt. Major Powell notes the fact, that, for all practical purposes, a stream 

 ceases to deepen its channel-way long before the bed has reached the level 

 of the lower end of the stream. If I understand the doctrine aright, it is, 

 that after the corrasive energy of the stream has reached its minimum, it is 

 less rapid in its effect than the wasting away of the adjoining surface ; so 

 that eventually the whole region will be worn down to a slightly inclined 

 plane, having about the same altitude with the base-line of erosion or the 

 bed of the stream. 



It is with some hesitation that I venture to disagree with those who 

 would explain the phenomena on this hypothesis, for they have had oppor- 

 tunity for very extended study of surface geology; but the explanation is 

 insufficient here, and is open to serious objections, some of which are 

 almost insuperable. 



The wide-spread horizontally of the benches seems to conflict with any 

 such explanation. One can conceive that all the streams feeding the Alle- 



PROC. AMER. PI1IL0S. S0C. XVIII. 101. 2N. PRINTED NOV. 5, 1879. 



