380 E. T. WPIERRY PRECAMBRIAN OF PENNSYLVANIA 



As this formation has ]iot been recognized heretofore^ certain of its 

 features may be described in greater detail. In thin section the quartz 

 occurs in interlocking grains, shoMdng marked wavy extinction and often 

 traversed by sericite-filled cracks. The mica has the usual flaky struc- 

 ture, its crystals varying from submicroscopic size (pinite) through the 

 minute grains of sericite to the coarse plates of muscovite. The silliman- 

 ite occurs in long slender prisms, mostly curved or bent slightly and in 

 subparallel arrangement ; it is easily recognized by its high relief, medium 

 double refraction, straight extinction, and positive sign. Many of its 

 crystals exhibit a mesh structure resembling that of serpentinized olivine, 

 owing to alteration into pinite along cracks ; this is well shown in figure 

 3. The garnet varies considerably in amount and form, crystals 1 centi- 

 meter in diameter, filled with inclusions of quartz and mica, being some- 

 times found, although it is usually much smaller. A thin section of one 

 of these, with typical inclusions, is shown in figure 4. In color it is 

 always pink or pale red. The zircon occurs in minute grains inclosed 

 in the quartz mosaic; and these are usually rounded in outline, as shown 

 in figure 5, in marked contrast with the angular outlines of crystals from 

 the igneous rocks of the region, one of which is illustrated in figure 6. 



Counts of the areas occupied by the various minerals in typical thin 

 sections gave the results set forth in table 2 : 



Table 2. — Mineral Composition of the Quarto-mica Schist 



] 



Quartz 55 



Sericite 



Sillimanite 35 



Accessories 



Feldspar (altered ) 



Biotite 



Ilmeulte 



Pyrite (and limonite) 



Zircon 



Apatite 



Tjials 100.00 100.00 100.00 100 



The specimens, on sections of which the above counts were made, were col- 

 lected at widely separated points on the Allentown quadrangle, as follows : 



1. One mile west of Seidersville (206). 



2. Two miles southeast of Freemansburg (217). 



3. East edge of quadrangle, one mile south of Lehigh River (6396). 



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



1 



2 



3 



4 



55.4 



52.6 



49.7 



50 



5.9 



40.0 



15.7 



20 



35.1 



6.8 



24.3 



25 

 5 



.... 



0.4 



5.0 





1.6 



0.1 



.... 



. . . 



1.95 



0.05 



5.2 



... 



0.05 



0.05 



0.05 









0.05 



... 



In the Ilaritan, New .Tersey. folio several schistose rocks were noted to occur in the 

 Precambrian, hut they appear to be somewhat different from the material here described. 

 However, similar rocks are described in New Jersey in a forthcoming report on the Dela- 

 ware W^ater Gap quadrangle. 



