503 



A STUDY OF THE STRUCTURE AND EROSION OF BRUSH 

 MOUNTAIN, IN BLAIR COUNTY, PENNSYLVANIA (With a 

 Map). 



By J . P. Lesley, 



Professor of Geology in the University of Pennsylvania. 



At one of the meetings of the Society in the summer of 1873, a proof 

 copy of the Map of Brush mountain, bound into this number of the Pro- 

 ceedings, was exhibited to the members, and its most interesting features 

 pointed out. 



No detailed description of it seems necessary. Those who are conver- 

 sant with Topography in its application to Geology will find sufficiently 

 plain indications of the state of things at the Tyrone City Gap and along 

 the Little Juniata River, displayed by the ten-foot contour-lines of the 

 topography, and by the three small vertical sections placed near the 

 geographical points where they were made. 



The system of thick and thin contour-lines on the dark and light sides 

 of hills, to bring them into relief, is one which I invented about the year 

 1855, using it on a large scale for the first time in the construction of my 

 elaborate Maps of Fayette and Westmoreland Counties, made for the Penn- 

 sylvania Railroad Company. The effect upon the portraiture of five hun- 

 dred square miles of that strongly accidented country was very fine ; and 

 was obtained without in any measure sacrificing the scientific accuracy of 

 the work. I have pursued the same plan ever since, and teach it in the 

 Geological laboratory of the Scientific Department of the University. 



The Map here given is a heliotype reduction to one-third (linear) of the 

 original drawing, made by James Osgood & Co., of Boston. It is there- 

 fore a fac simile ; and every error is exhibited. The only one needing 

 designation is of a grave character, and I hope to present a correction of 

 it at some future time. The narrow, straight and deep little valley which 

 heads southwest between the crests of the mountain, and issues north- 

 east at the centre of the Gap, is not sti-aight at its mouth. It bends down 

 stream somewhat, allowing the southeast shoulder of the northwestern 

 ridge to project into the Gap, in the same style as does the southeast crest 

 ridge on the opposite side of the Gap ; only in a reverse direction ; that is, 

 down stream instead of up stream. This may be considered a venial error 

 of detail, but it is not ; for it casts a shadow on the demonstration that the 

 erosion of any one part of Apalachian land-surface relates itself harmoni- 

 ously with the general erosion of its district ; and yet is differentiated by 

 exceptional parts of structure. The short ravine splitting the mountain on 

 the northeast side of the Gap would have bent southward at its mouth, like 

 the long ravine opposite to it, but for a crush in the red rocks of IV., caused 



