1880.] ^OJ [Hall. 



upturned along the line from Spring Mill to Edge Hill P. O., probably prif* 

 to their deposition. This is evident from the fact that the limestones lap 

 over the rocks of the second group at West Conshohocken. 



It may be that the great unconformity of the measures west of the Schuyl- 

 kill river cannot he explained without a fault along the line of junction. 

 The southern margin of the slates of the South Valley Hill is somewhat 

 irregular, they come in contact with the rocks of the second group as far 

 west as the east branch of the Brandywine creek, or that neighborhood, 

 where they are succeeded and overlaid by micaceous garnetiferous rocks 

 with limestone of the sixth group. 



The southern boundary of the South Valley Hill slates or Lower Hud- 

 son river slates, is about on a line from West Conshohocken to a point 

 about four miles north-west of West Chester, and gradually diverges from 

 the line of the Chester valley as we proceed westward. This divergence 

 is caused partially by the flattening of the measures, and partially by their 

 increased thickness. 



The sixth group which flanks the South Valley Hill rocks on the south 

 and overlies them, increase rapidly in thickness to the west of the Brandy- 

 wine creek. This group lies upon the rocks of the second group and en- 

 circles the western end of that area. The East Branch of the Brandywine 

 creek cuts along close to the line of junction, between these groups. The 

 schists in places are found on both sides of the creek, while at other points 

 the syenitic rocks of the second group extend west of the line. The lime- 

 stones of this group are well exposed in the neighborhood of Doe Run. 

 The most easterly exposures are at Caleb Cope's and Copesville on the 

 Brandywine; also at Brinton's Bridge on the Brandywine, and two local- 

 ities east of Brandywine creek, one in Birmingham and the other in Thorn- 

 bury township. 



The locality at Caleb Cope's place, north- wtst of West Chester, is a simi- 

 lar deposit to the one at Brinton's Bridge, a thin bed of impure crystalline 

 limestone between beds of schistose rock. 



The first locality is on a line with the locality at Cope's Mill, and as can 

 be seen by the map, is also on a line with a number of limestone quarries 

 west of it. I have satisfied myself by walking over the ground that they all 

 belong to one horizon, and if not absolutely continuous they are only broken 

 by the thinning out of the limestone itself. The locality at Brinton's 

 Bridge is on a line with the quarries of East Marlborough and London 

 Grove townships, and although it is a small bed, it is flanked on both sides 

 by the same rocks which flank the heavier beds further west. The fact of 

 it being a thin bed bears me out in the assertion that all these beds thin 

 rapidly towards the east. 



I am not prepared to say whether or not there is more than one horizon 

 of limestone in this portion of Chester county. I am forced to the conclu- 

 sion as to its superposition to the South Valley Hill slates, as the succession 

 is clearly seen along the East Branch of the Brandywine Creek. 



The rocks of this group seem to vary considerably, and it is not im- 

 probable that they may have to be subdivided, as I have included all the 



