390 E. T. AYHERRY PRECAMBRIAN OF PENNSYLVANIA 



ORIGIN 



As noted above, under the heading "Cr3-stalline limestone/'' amphibo- 

 litic and pyroxenitic gneisses have localh^ been developed by metamor- 

 phisni of that rock, but there is no evidence that any considerable propor- 

 tion of the basic gneisses of this region have been thus formed. The 

 primary rock in the majority of cases was more probably shale. The 

 most important |)oint to be decided, however, is not what sort of sediment 

 they may have arisen from, but whether they are of sedimentary origin 

 at all ; for the gneisses produced by the recrystallization of igneous rocks, 

 such as gabbro, may be similar in some respects. In the Piedmont belt 

 of Precambrian rocks the basic gneisses are regarded as almost exclusively 



metamorphosed gabbro.^* But 



"T the relations in the northern 



])elt are believed by the writer 



^ to indicate the ultimate sedi- 



" mentary origin of the bulk of 



— these rocks. In addition to the 

 3 reasons for the conclusion that 



~ much of the corresponding Po- 

 _ chuck gneiss is a metamorphosed 

 ^ sediment, given in the New Jer- 

 6 sey folios above cited, the fol- 



— lowing points are worthy of 

 note : 



,o„.^. ,. ., ,. The mineraWical evidence in 



Figure 13. — Basic Gneiss showing Alternation , ° 



of dark and lUjiit Bands. {X V2) favor of a sedimentary origin of 



Locality, same as figure 12. The right-liand these rocks is not as Complete 



side of this specimen was sawed oflf and thin ^g ^^^ ^^le Case of the formations 

 sections made all the way across ; counts of . 



the minerals in these are given in table 9, the previously described, since Only 



numbers marked on the figure corresponding to feldspathized phaseS are knoWU. 

 those in the table ; No. 5 is a sill of granite. 



The rounded zircons which can 

 be found in these basic gneisses are, however, as pointed out above, an 

 excellent evidence of sedimentary origin. The presence of augite is not 

 regarded as opposing such an origin, for augite frequently forms during 

 the metamorphism of sediments, especially if they are somewhat cal- 

 careous. 



The original minerals having been extensively, if not completely, re- 

 crystallized and rearranged, it is unsafe to base any conclusions on the 

 present mineralogical features of the rock as a whole. But the structural 

 relations shown in many instances are similar to those of the previously 



" F. Bascom and others : Philadelphia Folio, U. S. Geol. Survey, No. 162. 



