QUARTZ-MICA SCHIST 383 



shown toward the igneous rocks of the region, which is discussed in the 

 following paragraph. 



The quartz-mica schists are frequently in contact with granite, and in 

 every case the field relations show that the latter is the subsequent rock. 

 It either cuts across laminae or forces them apart, giving rise to handed 

 rocks, such as the ones illustrated in figures 7 and 8. On the other hand, 

 sheared granites, observable at several places in the region, present a 

 totally different picture. The quartz and feldspar grains show progres- 

 sively undulatory extinction, then shattering, a good example of which is 

 shown in figure 9, then alteration to sericite, the final result being a 

 pseudo-porphyritic mass of compact sericite with wavy and irregular 

 lamination, dotted with rounded fragments of the original constituents. 



Figure 8. — Quartz-mica Schist shotcing Invasion hy Granite. (X %) 



Locality, 5 miles west of Boyertown. The layers of schist at the hottom are con- 

 torted, and have evidently been practically melted by the granite, which permeates them. 



These rocks are the opposite of the quartz-mica schist in every respect. 

 Quartz is small in amount, sillimanite and garnet absent, and zircon 

 angular. Across the lamination the variation is gradual ; along the strike 

 rapid. And the igneous rock grades into the schist, never cutting sharply 

 across nor penetrating it along laminae. 



MAGMATIG MODIFICATION 



As far as can be determined, feldspar is absent or present only in very 

 subordinate amount in the normal schist. But where emanations from 

 the granite magmas have acted on the rock, feldspar has been extensively 

 introduced ; it is chiefly orthoclase showing microperthitic intergrowth of 

 oligoclase. This feldspar impregnates the rock more or less uniformly, 

 surrounding the grains of the normal constituents. The lamina? may be 

 but slightly separated, moderately deformed, and obscured to a negligible 



