1890.] NATURAL SCIENCES OF PHILADELPHIA. 93 



from the reports of the Second Geological Survey of Peunsjlvauia 

 will show some of these conclusions. I have added to each the 

 letter indicating the outcrop referred to in the list above given. 



C* p. 84 (B). " The southern edge of the Soutli Valley hill belt of talc mica 

 slates is defined upon the map, hy a cham of dots and stripes of two colors repre- 

 senting outcrops of terpentine and outcrops of crystalline limestone. 



Were these outcrops ranged in more than one line ^ the task of explaining their 

 appearance at the surface would be much easier. But a siiig/e line of them 

 necessarily places them either at the top or at llie bottom of the talc mica schist 

 formation ; Mr. Hall choosing the former, Mr. Rogers the latter alternative. 



The case is complicated by the fact that the outcrops along this one line are some- 

 times serpentine, sometimes limestone. It looks as if the serpentine might be a 

 subsequent modification of the limestone; or else that one and the same original 

 magnesian sediment was heavily charged with carbonate of lime in some places, 

 and was a non-calcareous silicious mud in others." 



C ■* p. 85 (B). " The subject is crowded with embarrassment; and all the more 

 seeing that the serpentine-limestone range along the south edge of the talc-mica 

 belt ought to hold some discoverable relation with other ranges of serpentine 

 crossing Delaware, Chester and Lancaster counties; as well as with the famous 

 serpentine outcrop between limestone and gneiss on the Delaware river, north of 

 Easton, in Northampton county." (A.) See Report D^, 1883. 



C* p. 85 (A). '' The fact that the Northampton county serpentine underlies the 

 Great Valley limestone formation in connection with Potsdam sandstone supports 

 Mr. Rogers' views in Chester county, and strongly opposes Mr. Hall's conjecture 

 that the Chester serpentine may be Upper Silurian. On the other hand the fine 

 serpentme outcrops on Lake Memphremagog, at the Canada Vermont line, are 

 clearly Upper Silurian, but they are far removed from any gneissic region. Mr. 

 Hall after examining the Easton serpentines, considered them altered calcareous 

 slates lying at the base of the limestone formation, over the Potsdam sandstone." 

 (F(?ot note.) 



C •* p. 87 (B). " It is evident that even a synclinal belt of serpentine 2,000 feet 

 wide, or even 4U0 feet wide, can mean nothing else than a great thickness of the 

 talc-mica schist formation metamorphosed more or less completely into serpentine. 

 And a good cause for such alteration is present in an extensive outburst of trap 

 close by ; and everybody familiar with the surface of Delaware and Chester counties 

 knows how almost invariably its trap and serpentine appear together. ^ This of it- 

 self eftectually divorces the serpentine outcrops of Chester county from the lime- 

 stones." 



C^ p. 26 (F). " The southern belt of serpentine extends from Chestnut Hill 

 tothe Delaware county line, crossing the Schuylkill river below Lafayette station. 

 It has not been proven to be a contmuous belt; and the color indicates the steatite 

 or serpentine where its existence is beyond dispute. This range or belt of ser- 

 pentine belongs, undoubtedly, to one horizon, although the deposits may be in len- 

 ticular masses. 



C^ p. 26 (C). "The northern belt of serpentine extends from a point a short 

 distance east of the Schuylkill river to the neighborhood of the Delaware county 

 line, north of Bryn Mawr. This belt seems to be a repetition of the southern belt, 

 being on the north side of a synclinal basin. 



East of the Schuylkill the extent of the outcrop is not definitely known. The 

 outcrops designated are the only ones positively known to exist. If there was no 

 faulting along the northern edge of the belt, between the Schuylkill and Chestnut 



1 In Radnor Township they are in two lines. T. D. R. 



2 My observations do not confirm this. Proc. Acad. Nat. Sci. 1883, p. 241. 



