Stevenson.] «JV± [Aug. 15, 



As mentioned in my first report on the geology of southwestern Penn- 

 sylvania (that for L875), these lower benches ;ire merely shelves cut oui of 

 the icck, on which are spread thin deposits of irregularly or even wholly 

 unstratined debris. At some localities, the lower part shows little aside 

 from clay, while the upper part is made up almost wholly of sand, but or- 

 dinarily one of these occurs in pots within the other. 



Transported fragments, rounded or polished, appear with this series. 

 Those in the Monongahela benches have been brought from the south and 

 east, for no rocks are represented except such as occur in the mountains 

 crossed by the Youghiogheny and Monongahela or their tributaries ; and 

 no material belonging to the northern drift was found anywhere south 

 from the junction of the Ohio and Monongahela rivers. The detrital cov- 

 ering seems to be greatest on the third and fifth benches, where it is of no 

 small economical importance, the third yielding the glass-sand of Belver- 

 non, and the fifth, the glass-sand of Perryopolis and the fire-clay of New 

 Geneva. 



The upper limit of rolled stones shows some perplexing variations, 

 which can be accounted for only by supposing that the higher benches of 

 the series have been worn away from one side of the stream, or that, du- 

 ring the formation of two successive terraces, the channel-way held close 

 to one side. On Cheat river, near Tce"s ferry, at nine miles from the 

 Monongahela, the upper limit on the south side is the third bench ; near the 

 Line ferry, further down on the same river, no fragments occur on the 

 north side above the fourth, though they are numerous on the south side 

 up to the fifth ; the fourth terrace is the upper limit on the south side at 

 Point Marion, whereas fragments are abundant up to the fifth on the north 

 side. No fragments have been found anywhere above the fifth bench. 



After passing the third bench, the Monongahela river seems to have suf- 

 fered changes in its channel -way. Below Belvernon, there is distinct evi- 

 dence that it now flows along a line very different from that followed when 

 its bed was on the third bench. The change was more serious near Pitts- 

 burgh. The river had a direct course from Braddocksfield to the Alle- 

 gheny and that line is now followed by the Pennsylvania Railroad. The 

 present channel-way of the river is very tortuous. A similar change is 

 shown at Ohiopyle Falls on the Youghiogheny, where the old channel on 

 the fifth bench crossed the neck of a peninsula, while the new channel- 

 May makes a long and close bend around the peninsula. 



The conditions, then, are these : The area in which observations were 

 made covers more than 10,000 square miles ; embraces that part of Penn- 

 sylvania lying south from the Ohio and Conemaugh rivers and west from 

 the Alleghanies ; includes part of Maryland and West Virginia, lying on 

 both sides of the Alleghanies of Virginia ; and has the channel ways of four 

 great rivers, the Monongahela, Cheat, Youghiogheny and Potomac, lying 

 partly within it. Along all these streams are terraces, covered by detritus, 

 which contains many transported fragments polished by running water ; 



