Hall.] 



438 



[Jan. 



sandstones forming prominent ridgeSj havinga miieli less dip and covering 

 greater areas. There are several cynclinals and antielinals, the axes of 

 which form an obtuse angle to the line of strike of Edge Hill. These folds 

 die down very rapidly to the eastward, but are marked by slight irregular- 

 ities in the line of strike of Edge Hill. 



I have been long inclined to consider the elevations of sandstone along 

 the north flank of the limestone valley, as having been caused by folds and 

 faults, but I do not see any proofs of faulting. But there is evidence of 

 unconformity between the two groups, as at Spring Mill, where the lime- 

 stone overrides the upturned Potsdam ; at a point west of the second cross- 

 ing of Sandy Run by the North Penn. R. R., just south of Port Washing- 

 ton, the limestone occurs in a bay in the sandstone, having a slighter dip. 

 Just south of Mogeetown, east of the Schuylkill river, near Norristown, 

 the limestones rest unconformably upon the Potsdam, to all appearances. 



The anticlinal ridges of Potsdam extending diagonally into the valley, 

 are flanked on both sides by limestone, and, in some cases, disappear be- 

 low the limestone which encircles it ; but in the most eastern fold I doubt 

 whether the limestone is connected. 



The marble is confined to the south side of the valley. The dip of the 

 limestone being to the south, it would place them at or near the top. Mar- 

 ble quarries are found in Chester county, close along the southern margin 

 of the limestone valley, and in close proximity to the slates. In Mont- 

 gomery county the same rule holds good, and very shortly after the disap- 

 pearance of the slates to the east of the Schuylkill, we find the marble is 

 missing as well. 



The rocks rise rapidly to the eastward. The slates of the South Valley 

 Hill are in regular succession upon the limestone. This may be seen where 

 the South Valley Hill ends at Gulf Mills. The structure is clearly proven 



by the succession of rocks, which 

 is marble, bastard marble, shale. 

 on the north side of the synclinal 

 and a double repetition on the anti- 

 clinal at Gulf Mills. The cove 

 made by the erosion of the anti- 

 clinal is just west of Gulf Mills, 

 west ofOonshohocken. Thetrans- 

 ition shaly limestones are repeated 

 several times by minor contortions, 

 and extend over a wide area from 

 Gulf Mills to the Schuylkill river. 



The alternations from the limestone into the slates is everywhere visible 

 along the southern margin of the Chester valley. 



The sUtes are traversed by a trap dyke, which crosses the Schuylkill at 

 Conshohocken. I do not think it marks the line of any disturbance of im- 

 portance. The synclinal and anticlinal of the slates do not cross the, 

 Schuylkill river. The slates flanking the marbles east of the Schuylkill 

 lie in a monoclinal, pitching to the south against the Potsdam, which was 



