122 PROCEEDINGS OF THE ACADEMY OF [1890. 



North Valley Hill, 8-Conglomerate probably lower Potsdam, 9-Lau- 

 rentian. Nos. 2 and 8 are not on the map in C^ This would in- 

 dicate a broad synclinal, the Laurentian at the base, the hydromica 

 schists at the centre and the limestones of Cream Valley and of the 

 Chester Valley identical. 



There is one fact not easy of explanation. The South Valley Hill 

 on a line from Radnor station to the King of Prussia is over two 

 miles wide and is a broad elevated table-land. Close to this line two 

 valleys begin, draining northeastward, dividing the table-land 

 into three hills. Of these the middle and the northernmost end west 

 of the Gulf road, forming promontories projecting as it were into 

 bays. East of these promontories the whole region is limestone. 

 The southernmost division of the hill preserves its elevation and 

 crosses the Schuylkill at Conshohocken, but it is less than half a 

 mile in breadth. 



Now if the limestone were in strata nearly horizontal, we could 

 readily ex})lain this by supposing a deposition subsequent to the ele- 

 vation and denundation of the hydromica, but on the contrary all ob- 

 servable dips are vertical or nearly so. This seems opposed to the 

 theory of a simple synclinal. 



Is it possible that the limestone underlying the hydromica has 

 3Deen folded and crumpled up with it, in minute compressed anti- 

 clinals and synclinals like that exposed on the Schuylkill below Potts 

 Landing, that the hydromica has been removed by erosion, leaving the 

 ridges of limestone ? This is but conjecture. The structure, however, 

 is quite evident, and no other explanation seems to satisfy the con- 

 .ditions. 



In the cut of the Pennsylvania Schuylkill Valley Railroad through 

 the hydromica schist hill at Conshohocken, what appeared to be 

 decomposed hydromica schist was underlaid by limestone. 



In this connection may be mentioned the fact that the basin of 

 the Conshohocken Water Works, on this hill of hydromica schist, has 

 two or three times suddenly emptied itself through holes in the bot- 

 tom without visible outlet — easily explicable if the limestone under- 

 lies, but difficult otherwise. 



If the hydromica schist overlies the limestone, then the sandstone 

 X)n the north side of the hydromica hill, and on the south side of the 

 Chester Valley cannot be Potsdam, as Prof Rogers, Prof Lewis and 

 the writer supposed, but must be more recent. 



