388 E. T. WHERRY PRECAMBRIAN OF PENNSYLVANIA 



position. The i^henomena of feldspathization are in every respect like 

 those described in connection with the quartz-mica schist above. When 

 graphite is present in the granitic bands it is, as a rule, coarser than in 

 the quartzitic ones, showing that recrystallization of this mineral has 

 taken place (as with sillimanite in the other rock). Indeed, when the 

 magma has been pegmatitic the graphite plates are at times 3 to 5 centi- 

 meters across, whereas in the original rock they are measured in as many 

 millimeters. 



Table 6. — Mineral Composition of feldspathized Graphitc-J)Caring Quartzite 



12 3 4 



Quartz 48.9 43.4 34.9 40 



Feldspars .... 45 



Orthoclase 18.7 26.8 35.4 



Oligoclase 14.6 17.7 20.5 



Graphite 14.9 3.4 4.6 10 



Accessories 5 



Biotite 5.5 4.3 



Garnet 0.1 1.4 0.2 



Zircon 0.05 0.05 0.05 



Pj-rite and limonite 2.7 1 . 75 



Apatite 0.05 0.05 



Totals 100.00 100.00 100.00 100 



The localities of the above are all on the AUentown quadrangle, as follows : 



1. One-fourth mile southwest of Shimer station (which is wrongly located on 

 the topographic map) (5217). 



2. One-fourth mile east of Limeport (7947). 



3. One mile southwest of Lower Saucon (6854). 



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



Basic (amphibolite) Gneiss 



character 



The nature of the original materials from which the basic gneisses^^ of 

 this region have been derived can only be conjectured, since none but 

 rocks resulting from profound magmatic modification are now known. 

 These feldspathized phases are characterized by the abundance of quartz, 

 hornblende (or biotite or augite), and of both orthoclase and oligoclase 

 feldspars; a typical section is shown in figure 11. Ilmenite or magnetite 

 is a prominent accessory, and rounded zircons can sometimes be found, 



13 In Canada and the Adirondack region rocks like these are called amphibolite. The 

 origin here assigned corresponds to one of those described in Adams and Barlow's much 

 quoted paper : Can. Dept. Mines, Geol. Survey, Memoir 6, 1910, p. 87, etcetera. 



