386 E. T. AVHERRY PRECAMBRIAN OF PENNSYLVANIA 



Table 5. — Mineral Composition of Quartzite 



12 3 4 



Quartz 78.5 78.2 57.0 70 



Graphite 10.7 21.2 0.1 10 



Accessories 20 



Biotite 0.5 10.6 



Sericite 3.9 



Feldspar, altered 0.9 2.2 



Garnet 3.7 30.0 



Pyrite and limonite 1.8 



Zircon 0.05 0.05 0.05 



Apatite 0.45 0.05 0.05 



Totals 100.00 100.00 100.00 100 



The above sections were made on specimens from : 



1. One mile east of Vera Cruz station (7842). 



2. Three-fourths mile southwest of Vera Cruz station, near southwest corner 

 of the Allentown quadrangle (7788). 



3. Two miles southwest of Lower Saucon (6887). 



4. The rounded-off average of 1, 2, and 3. 



ORIGIN 



The quartzitic character of this phase of the graphite-bearing forma- 

 tion makes its sedimentary origin seem above question. Confirmatory 

 evidence is fomid in the presence of the graphite itself, in the occasional 

 appearance of notable amounts of apatite, which may perhaps be regarded 

 as implying the animal derivation of the carbon, and in the rounded zir- 

 cons. Finall}^, the inferences drawn from lamination and relation with 

 igneous rocks in the case of the quartz-mica schist hold with equal force 

 here. The rock is totally unlike the products of shearing of the igneous 

 rocks, both mineralogically and structurally, and shows feldspathization, 

 injection, and assimilation by the granite rather than transition, which 

 would result from its origin by the shearing of the latter rock.^^ 



MAGMATIC MODIFICATION 



Introduction of feldspar and other constituents by the invading mag- 

 mas has been even more extensive in the case of this graphite-bearing 

 formation than in that of the schist, but instead of the orthoclase and 

 plagioclase feldspars showing microperthitic intergrowth they tend to 

 appear separately, as brought out in the following table of mineral com- 



12 In the Raritan folio reasons are given for the assignment of an igneous origin to 

 some of the grapliite-bearing rocks, and some of the schists are believed to result from 

 the shearing of these. No corresponding occurrences have been found in the Pennsyl- 

 vania area thus far. 



