INTRODUCTION 377 



Between 1908 and 1913, while connected with the geology department 

 of Lehigh University, which is situated in South Bethlehem, Pennsyl- 

 vania, on the north flank of this belt, the writer made a study of the Pre- 

 cambrian rocks of that vicinity.® High-alumina mica-schists, such as are 

 common in the Piedmont belt, but 'had not been previously reported from 

 this northern one, were discovered at a number of widely separated points, 

 and evidence was obtained which strongly confirms Doctor Fenner's con- 

 clusion that the banding of a by no means inconsiderable proportion of 

 the gneisses is a remnant of original lamination. 



General Geology 



The rocks of the region which are believed to be of sedimentary origin 

 are crystalline limestone, quartz-mica schist, graphite-bearing quartzite, 

 the gneisses derived from the last two by magmatic action, and basic 

 (amphibolitic) gneiss. The limestone is thought to be the exact equiva- 

 lent of the Franklin of New Jersey, while the other rocks are regarded as 

 representing sediments of diverse character which were deposited more or 

 less contemporaneously with the limestone. The areas occupied by these 

 rocks are approximately as follows, in square miles : 



Crystalline limestone 0.5 



Quartz-mica schist and derived gneiss 5.5 



Graphite-bearing quartzite and gneiss 9.0 



Basic (amphibolitic) gneiss. 35.0 



Total (about one-tenth of the area of the belt) 50.0 



Subsequent to tlie deposition of these sediments the region was invaded 

 by igneous magmas, the sedimentary formations being extensively meta- 

 morphosed and no doubt partly assimilated. Five igneous formations 

 can be recognized — granitic gneiss, the Byram of New Jersey, made up 

 chiefly of quartz and orthoclase or microperthite feldspar ; dioritic gneiss, 

 the Losee of New Jersey, similar to the preceding, but containing con- 

 siderable plagioclase feldspar ; gabbroic gneiss, the igneous portion of the 

 New Jersey Pochuck, high in augite and containing basic plagioclase; 

 granitic pegmatite; and diabase.'^ All of these formations are overlain 

 unconformably by the basal Cambrian quartzite. 



« While most of the data presented herein were obtained on the Allentovvn quadrangle, 

 in which South Bethlehem is located, a few visits to the southwestern extension of the 

 belt on the Boyertown and Reading quadrangles showed the formations there to be iden- 

 tical. No detailed studies were made by the writer of the Precambrian in these two 

 quadrangles, because this was to be mapped by Misses Bliss and Jonas, under the direc- 

 tion of Miss Bascom ; that mapping has since been completed. 



■^ Described by Anna L .Tonas : Bull. Am. Mus. Nat. Hist., vol. 37, 1917, pp. 173-181. 



