. Bulletin of the Geological Society of America. Geology. 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 sedim
. Bulletin of the Geological Society of America. Geology. 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 rega
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