1890.] NATURAL SCIENCES OF PHILADELPHIA. 89 



and crystalline limestone. The mica schists of the series often con- 

 tain garnets, staurolite, andalusite, fibrolite and cyanite, while in 

 the granitic veins which traverse the series are found tourmaline 

 beryl and cassiterite. The total thickness of the Montalban is ap- 

 parently much greater than that assigned to the Huronian, upon 

 which it sometimes rests unconformably, or in the absence of the 

 Huronian, as is often the case, directly upon the Laurentian." 



Mr. Hall's views have already been given, viz., that the Phila- 

 delphia gneisses are the lowest except the Laurentian, the ^Nlanayunk, 

 schists next, the Chestnut Hill schists above and the serpentines the 

 highest.^ He further states : — 



" I think there is no possibility of assigning the rocks of the 

 Philadelphia Manayunk and Chestnut Hill groups (the First and 

 Second belts of Prof. Rogers) to any place below the slates of the 

 South Valley Hill or Hudson River groups. But to what horizon 

 in the palaeozoic column they belong must remain for the present un- 

 certain." C p. 9. 



This generalization is derived chiefly from the finding of Potsdam 

 and No. 2 Limestone between the Laurentian and the schists. Prof. 

 Lewis described itacolumite southeast of the Laurentian, east of the 

 Schuylkill and the writer found a similar rock west of the Schuyl- 

 kill.- !Mr. Hall's observation has been doubted, but it would seem 

 without reason except that he gave no exact locality to enable others 

 to confirm his observation. A full description may therefore be of 

 interest. 



Passing southeastward along the North Pennsylvania Railroad 

 from Edge Hill Station, Ave pass through the well-known deep cut 

 through the Potsdam of the Barren Hill belt. We then pass 

 for a mile throuo-h a vallev, and then come to the garnetiferous schists 

 of the Chestnut Hill belt at Jenkintown. 



In this valley, a mile northeastward of the railroad, Laurentian? 

 is abundant in the form of large masses on the surface. Southwest- 

 ward, high ground is soon reached on which the fields are full of' 

 Laurentian fragments. About .6 miles W. 5° S. of Glenside station is. 

 a W'Ood in which is a small quarry exposing about 50 feet vertically 

 of Laurentian rocks, some N. 30° E. dipping 55° N. W. in the 

 southeasterly part of the quarry. In the northwesterly part the 

 rock is much jointed, the joints irregular. 



1 C5 pp. 2, 59, etc. 



2 2nd. Geol. Survey of Pa., An. Rep. 188G. Part IV, p. 1595. 



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