392 



E. T. WHERRY PRECAMBRIAX OF PEXXSYLYANIA 



adjacent ones remain straight, and in other similar ways. It is thereby 

 rendered certain that the basic bands can in no way represent dikes of 

 gabbro or an3i:hing of that sort penetrating the granite ; they must have 

 been solid and laminated before the granite came in. This fact alone, as 

 Doctor Fenner cautiously points out, is not a proof of the sedimentary 

 origin of the basic l^ands, for they might be igneous rocks which had been 

 laminated by metamorphism prior to the granitic intrusion: taken in 

 connection with the evidence favoring a sedimentary origin derived from 

 other relations, however, it has strong confirmatory value. 



From these basic gneisses, in which the dark minerals are so large in 

 amount as to render the whole rock dark in color, there exist all grada- 

 tions to acidic igneous 

 rocks which are mainly 

 light in color, but show 

 lines and streaks of 

 dark minerals ; a speci- 

 men of one of these is 

 shown in figure 14. 

 To what extent the 

 latter represent pri- 

 mary alignment of the 

 t-abular minerals, on 

 the one hand, and rem- 

 nants of preexisting 

 laminated rocks which 

 have been melted up 

 in the magmas with- 

 out losing their structure entirely, on the other, it is impossible to decide 

 from the limited exposures in this region. The association with basic 

 gneisses which these occurrences show indicates, however, that the second 

 relation holds in many cases. The original extent of sedimentary Pre- 

 cambrian formations in this region was therefore, no doubt, even greater 

 than that described in tliis paper. 



'i:^^'^^^: 



^m^$. 



Figure 14. 



of dark Minerals. 



-Granite shoxcing Streaki 

 (X ¥2) 

 Locality, near South Bethlehem, Allentown quadrangle. 

 Specimen 222. The dark bands are believed to represent 

 laminae of basic gneiss, which has been melted up and 

 largely assimilated by the granite. 



SUMMAEY 



In the foregoing pages descriptions have been presented of several types 

 of rocks occurring in the northern belt of Precambrian in Pennsylvania 

 which give evidence of ultimate sedimentary origin. The evidence is 

 partly mineralogical, partly structural, and partly derived from relations 

 to rocks of igneous origin. These sediments have been extensively feld- 

 spathized and more or less completely assimilated by the magmas, but 

 enough of them is preserved to indicate the existence in this region of a 

 considerable bodv of Precambrian sedimentarv formations. 



