BASIC (aMPHIBOLITE) GNEISS 391 



described formations. For instance, rapid alternation in composition 

 across the bands, but persistence of individual bands for considerable 

 distances along the strike, are more suggestive of sedimentary than of 

 igneous origin. To bring out the manner in which alternation occurs, a 

 specimen 8 centimeters across — shown in figure 13— in which six bands 

 could be recognized, was sawed through and thin sections of its entire 

 cross-section prepared; counts of areas occupied by the several minerals 

 are recorded in table 9. 



Table 9. — Mineral Composition of a 'banded basic Gneiss 



1 2 3 4 5.6 



Width of band, centimeters. 2.0 1.7 1.4 1.1 0.5 1.3 



Color gray black gray black white black 



Quartz 11.4 11.0 20.4 20.6 19.7 3.6 



Feldspars : 



Orthoclase 54.2 32.3 57.8 35.6 .... 49.7 



Oligoclase 7.2 2.4 5.2' 7.0 5.6 



Microperthite 80.2 .... 



Augite 15.2 .... 8.5 



Hornblende 6.6 50.0 3.0 32.7 36.8 



Accessories : 



Magnetite 5.35 4.25 5.05 4.05 0.05 4.25 



Zircon 0.05 0.05 0.05 0.05 0.05 0.05 



Totals 100.00 100.00 100.00 100.00 100.00 100.00 



Locality — Three miles northwest of Boyertown. 



Band 5 is shown by its macroscopic aspect and its mineral content to 

 be a tiny sill of granite, but the other bands show no evidence of igneous 

 origin, except in so far as feldspathization has been produced in them by 

 emanations from magmas. 



The relations to the other rocks of the region are equally significant. 

 The basic gneisses are almost always intimately associated with the other 

 sedimentary formations here described. They are in particular frequently 

 interbedded v^ith the graphite-bearing quartzite, and they may always be 

 found to a greater or less extent adjacent to the limestones and to the 

 quartz-mica schist. 



When the basic gneisses occur in the vicinity of the granites, many 

 features indicate the subsequent age of the latter. In this respect the 

 relations described by Doctor Fenner in the paper above cited are dupli- 

 cated repeatedly in the present region. Basic laminae often come to an 

 end against typical granite, and angular inclusions of basic rocks are fre- 

 quent in the latter. Evidence that the granite magma has softened the 

 basic rock is found in the pinching and swelling of some layers while 



XXX — Bull. Geol. Soc. Am., Vol. 29, 1917 



