37^ 



E. T. WHERRY PRECAMBRIAN OF PENNSYLVANIA 



In the following table average counts of the areas occupied by the 

 various constituent minerals in thin sections of the first three igneoiis 

 formations are presented for comparison with the tabulations of the sedi- 

 mentaries below : 



Table 1. — Mineral Compositions of igneous Rocks 



1 2 3 



Granitic gneiss Dioritic gneiss Gabbroic gneiss 



Quartz 



Feldspars : 



Orthoclase 



Micropertbite 



Oligoclase-anclesine . . 



Andesine-labradorite . 



Accessories 



Including : 



Augite 



Hornblende 



Enstatite-hyperstbene 



Biotite 



Minor minerals 



30 



15 



40 

 5 



10 



25 

 15 

 40 

 20 



40 

 50 



X 

 X 



Crystalline Limestone 

 character 



The limestone is normally a faintly stratified, coarsely crystalline rock, 

 containing apatite, diopside, graphite, phlogopite, titanite, tremolite, and 



Figure 1. — Crystalline Limestone showing Alteration to Amphibolite. (X ,%) 



Locality, 3 miles north of Boyertown, Pennsylvania. The portion below and to the 

 left is completely altered, and isolated grains of amphibole are growing in the midst of 

 the gray limestone above. 



other accessory minerals, and locally secondary serpentine and talc. It 

 shows interdigitation with all of the other sedimentary formations. 



