382 E. T. WHBKRY— PRECAMBRIAN OF PENNSYLVANIA 



A partial chemical analysis of a sample from specimen 1 in the above 

 table gave: SiOs, Q7'A; AI2O3 (including Fe.,0.^, TiO., and TitO^), 29.9; 

 alkaline earths, alkalies and water, by difference, 2.7 per cent. 



ORIGIN 



Quartz-sericite schists may so readily be formed by the shearing of 

 igneous rocks that it seems worth while to discuss at some length the evi- 

 dence from which the sedimentary origin of the present rock is inferred. 



The abundance of quartz and the high content of both silica and alu- 

 mina shown by analysis are characters more commonly associated with 

 sedimentary than, with igneous formations. The relatively large amount 

 of well defined crystals of sillimanite is also regarded as a good indication 

 of tlie sedimentary origin of the rock, for this mineral, while not unknown 



l^'iGURE 7. — Quartz-mica Schist shotoing Invasion liy Granite. (X %) 



Locality, 2 miles southeast of Freemansburg, AUentown quadrangle. Specimen 6493. 

 The dark layers of the schist are being separated by the paler granite, but retain ap- 

 proximate parallelism. 



as a constituent of igneous rocks, is far more characteristic of metamor- 

 phosed shales, originally high in aluminum minerals, such as kaolin. 

 Large round garnets with pale colors and abundant inclusions, such as 

 occur here, are also most frequently found in metamorphosed sediments. 

 The presence of numerous well rounded grains of zircon in a rock is, as 

 pointed out by Trueman,^^ strong presumptive evidence of sedimentary 

 origin ; and this feature is typically shown in the present rock. 



From the structural viewpoint, the evidence of the sedimentary origin 

 of this formation seems equally conclusive. The repeated alternation of 

 laminae high and low in silica and the extension of the lamination in 

 straight lines for long distances, both of which features it shows, are 

 characteristic of sediments. But the most significant relation is that 



lojourn. Geol., vol. 20, 1912, p. 257. 



