Hall.] 440 [Jan. 2, 



schistose rocks "with limestone from the South Valley Hill slates along the 

 East Branch of the Brandywine to Chadd's Ford orthe Maryland line, and 

 west at least as far as Avondale, Chester county. From their position above 

 the slates of the South Valley Hill, which are Hudson river, they belong to 

 a limestone group above the Hudson river group. Inasmuch as no fossils 

 have been found as yet, it is difficult to assign them to any particular age, hut 

 I am inclined to think that they may be Silurian and possibly Helderherg. 



There may be an unconformity between these schists and sandstones 

 and the slates below, hut as yet I have not been able to determine the area 

 of the upper group accurately, and before this is done it is impossible to 

 state decidedly what the relation is. 



In Eastern New York, south-west of Albany, we find the Hudson river 

 shales and sandstones overlaid by the Niagara in thin beds of concretionary 

 limestone, often not recognizable, followed by the Helderherg limestone, 

 the Oneida conglomerate, Medina sandstone, the Clinton group, and Onon- 

 daga shales, all having died out east and north before reaching the Helder- 

 herg escarpment. May we not have similar structure here? 



The limestones of the Chester Valley extend in an almost straight line 

 from the Schuylkill river to the neighborhood of Quarry ville, in Lancaster 

 county, where the straight valley ends and connects with the great lime- 

 stone valley of Lancaster county at Camargo P. O. 



At Camargo P. O., according to Prof. Frazer's map, a tongue of slates 

 connects across the limestone and is colored the same as an area of rock 

 north of the limestone valley, extending to the county line south and east 

 of the Gap P. O. 



The limestone valley from the Schuylkill to Quarryville or thereabouts 

 isamonoclinal, the beds all pitching to the southward, followed by Chloritic 

 schists, Hydromica schists and Mica schists, which overlie the limestones. 

 This is clearly demonstratediat the eastern end of the South Valley Hill, 

 which is formed by these slates. The marbles, which are always confined 

 to the southern edge of the valley, mark the horizon of the Trenton lime- 

 stone with its alternations of slate and slaty limestone, passing by alterna- 

 tions into shale and slate of the South Valley Hill or Hudson river group. 

 Just north of Gulf Mill a synclinal of the slates dies out, and at Gulf Mill 

 we have a double repetition of the alternations of impure limestone and 

 slate found flanking the synclinal on the north, which proves Gulf Mill to 

 be on an anticlinal axis. (See wood cut.) 



The dips, as a rule, are pretty high towards the Schuylkill river, varying 

 from 50° to 85°. But as we proceed westward they become somewhat less, 

 which may in part account for the widening of the slate area before spoken 

 of, hut there is a marked thickening in the beds to the southwestward, 

 which also must be taken into consideration. 



Owing to the short time I have for the preparation of this paper, I will 

 proceed to carry these determinations of horizons south-westward. 



At Camargo P. O., we would have an anticlinal of slates overlying the 

 limestones, which anticlinal would be on a line of the axis of the Tocquan 

 creek anticlinal recognized by Prof. Frazer on the Susquehanna. 



