517 



[Fra/.er. 



(9.) There should ])e evidence of Potsdfun south of the belt of lime- 

 stones striking witli that of Doe Run to the east, but thei*e is not. 



(10.) Tliere sliould be evidence that the Doe Run limestone is above tlie 

 Potsdam to the south, but the former appears to dip under the latter. 



This limestone as well as the small detached bodies just alluded to seem 

 to be analogous to that between Scottsville and Rockville in Bucks 

 count}-. 



(11.) There are small tongues and isolated patches of Lanrentian rocks 

 occurring in the midst of these southern schists. One comes into Chester 

 county from the east in Eastown and Treddyfrin townships, and another 

 occupies a small area near West Chester. These patches are bordered on 

 all their sides by these schists with no intervening rocks. The bordering 

 rocks therefore cannot belong to a group above the Potsdam and the 

 lower Silurian limestone. 



(13.) Several localities in Kennett Square and New Garden townships 

 exhibit areas of Potsdam rocks surrounded by these schists with no inter- 

 vening limestone. The schists therefore cannot belong to an horizon 

 sitperior to the latter. 



These are some of the reasons which are opposed to the structure sug- 

 gested by Mr. Hall. 



The section on Mr. Hall's p. 32 is so different from the same section 

 which the writer made in 1880, and the conclusions which Mr. Hall draws 

 from his section, are so important, that a rough copy of the writer's section 

 is herewith subjoined, on an approximate scale of 1425 feet = 1 inch. The 

 direction of the section is about that of the average dip or S. 12° E. It is 

 necessary to explain that the first group of dips is projected on the line of 

 section at Henderson's Station from the road west of that point, and the 

 Primal must lie west of where this section begins. 



If this junction be accepted, however, from Mr. Hall's observations, it 

 will not affect the important conclusions which suggest themselves. 

 First, of a possible fault between the limestone with part of its underlying 

 schists and the mica-schists to the S E. ; and secondlj^ the synclinal char- 

 acter of the limestone near Conshohocken, with an anticlinal of the un- 

 derlying schists to the south-east cut by a trap dyke. 



