510 



LDec. 1.: 



The Horizon of the South Vidleij Ilill lioclcs ia Pinnsyhuoda. Brj Dr. 

 Pvrsifm- Frazer. 



(Bead before the American Philosophical Society, December 15, 188i?.) 



The regions of the State in which the above rocks occur having been in- 

 dependeiitly studied by difterent observers, their labors have been brought 

 to contact, and it is found that a difference of theory ahnost as oUl as 

 geological investigation in tliis country, exists in the respective views 

 of the workers. 



The substance of one of these tiieories has just been issued in the Re- 

 port Cg, of the Second Geological Survey Reports, of which the subject 

 is, "Philadelphia County and the southern parts of 3Iontgomery and 

 Bucks, by ^Ir. Charles E. Hall." -- 



The first argument advanced to prove the formation of the schists of the 

 South Valley Hill subsequently to the Chester limestone is, that all the 

 dips of the latter are southward or under the former. That this is so in 

 the majority of cases (though with dips differing both in direction and 

 intensity), is undoubtedly true, but there are exceptions to this rule in 

 Sadsbmy, Cain, East Cain, West Whiteland, East Whiteland and Tred- 

 dyfrin ; in other words, in six out of the seven townships in which this 

 contact occurs in Chester county. [See table on page 108 of Memoir on 

 the Geology of S. E. Pennsylvania, by writer.] 



These exceptions to the general rule arc just of such a character as one 

 would expect if a fault had traversed a region of higli but generally re- 

 versed dips, f 



»I:i the introduction to tliis volmin", I'rof. Lesley nieiitious the Serpentine of 

 Bryn Mawr as tinning south towards the town of Chester, and not continuing 

 in its south-west course tlirongh DeUiware and Chester counties. The evidence 

 of this did not appear from a somewhat rapid search through Mr. Hall's vol- 

 ume. On page S8 he gives the course of the Serpentine as far west as to a point 

 a little south of Bryn Mawr, and on pp. 25 and '26 he speaks of the outcrops as 

 belonging to one deposit, and clearly indicates his belief that they are of syn- 

 clinal structure though apparently scattered. 



It is difficult to believe that the Serpentine at Bryn Mawr is not connected 

 Avith that north of Iladnor, &c., and does not belong to the belt which traver- 

 sing Chester county with a breadth between the extreme lines of isolated out- 

 crops of from five to eight miles, becomes very largely developed in West Not- 

 tingham and the neighboring townships of Chester and Lancaster. 



t It is of course a slip of the pen when Prof. Lesley saj-s that the presence ot 

 Hudson River plant-forms is shown in Prot. Frazer's lleport C... Co is devoted 

 to Adams and part of Franklin counties, &c. Xor is any such statement in C~. 



There was in the collection of specimens at the Lincoln University a fossil 

 said to have been found in one of the Peach Bottom slate quarries wlilch was 

 determined to be Bulhotrephis flexuosa. All ettorts, however, to find this fossil 

 in place were unsuccessful. Besides this, even if the Peach Bottom slates were 

 determined to be ot Hudson Uiver age, it would be very far from proving that 

 the great mass of the South Valley Hill schists was of this age. Pains were 

 taken in the description of the Susquehanna Section, pp. 140-141, to show that 

 the structure below Fishing creek, and especially near Peter's creek, was not by 



