384 E. T. AVHERRY PRECAMBRIAX OF PEXXSYLYAXIA 



degree, as in the specimen illustrated in figure T. Here feldspathization 

 must have been produced by thinly fluid emanations from the magmas. 

 In other instances the laminae nia}^ be spread widely apart, more or less 

 strongly deformed, and their identity largely destroyed; an instance of 

 this is shown in figure 8. At one locality where the magmatic material 

 was pegmatitic in character rude crystals of sillimanite as much as 1 

 centimeter in diameter and 3 centimeters long have been observed, the 

 blades of this mineral in the adjoining schist being, on the other hand, 

 rarely one-tenth as large; the mineral must have been dissolved and re- 

 crystallized by the magma. 



Counts of the areas occupied by the constituent minerals in thin sec- 

 tions of three of the rocks impregnated with feldspar are given here : 



Table 3. — Mineral Composition of feldspathized Quartz-mica Schist 



1 



Quartz 51 . 2 



Sericite 9.8 



Sillimanite IT . 6 



Feldspars 



Orthoclase 



Micropsrthite 18.6 



Oligoclase 



Accessories 



Garnet 



Ilmenite 2.1 



Zircon . 05 



Apatite 0.05 



Miscellaneous stains, etc 0.6 



2 



3 



4 



52.9 



48.5 



50 



4.0 



10.0 



5 



10.3 





10 

 25 



2.2 







23.9 



30.0 





2.2 





10 





8.5 



... 



0.10 



0.15 





0.05 



0.05 



. . . 



4.35 



2.8 



... 



Totals 100.00 100.00 100.00 100 



The specimens in table 3 were also collected on the Allentown quadrangle at 

 the following localities : 



1. One and one-half miles east of Mountainville (7323). 



2. Two miles southeast of Freemansburg. (The same place as No. 2 in 

 table 2.) (226.) 



3. One mile northwest of Springtown (253). 



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



Intimately associated with the rocks above described occur finely banded 

 gneisses which are believed to represent the extreme stage in the action 

 of the magma on the schist. In mineral make-up these rocks are essen- 

 tially identical with the igneous rocks of the region, but their ultimate 

 sedimentary origin is indicated by the following points : 



1. Close association with undoubted sediments. 



