THE MODE OF FORMATION OF CERTAIN GNEISSES 

 IN THE HIGHLANDS OF NEW JERSEY 



CLARENCE N. FENNER 

 Geophysical Laboratory, Carnegie Institution of Washington 



PART I. OBSERVATION OF FIELD PHENOMENA 



In Northwestern New Jersey a broad belt of pre-Cambrian 

 crystalline rocks extends across the state and is prolonged north- 

 easterly into New York, forming there the Highlands of the Hud- 

 son. On the southwestern side it is continued into Pennsylvania. 

 The rocks which compose this belt, are of considerable variety. 

 Gneissic rocks of several types predominate, but beds of crystalline 

 limestone are associated with them, and masses of rock of granitic 

 and dioritic character, showing at times almost no foliation, are 

 also present. The general trend is northeast and southwest, with 

 steep dips, usually toward the southeast. 



The geological character of the region as a whole and of various 

 portions of it has been frequently described. As in other areas 

 of similar foliated gneisses in various parts of the world, the ques- 

 tion of origin of the various types has been a most puzzling one. 

 Certain observers have held them to be highly metamorphosed 

 sediments; others have favored the view that they represent the 

 partial differentiation of a still fluid magma, which by continued 

 movement has pressed out the differentiated portions into broad 

 sheets; and a third view would attribute their structure to the 

 shearing and recrystallization of an already solidified mass. 



In a number of visits which the writer has made to various 

 portions of the area, certain features have been observed in several 

 places which were believed to be of considerable significance in the 

 interpretation of the mode of origin of the rocks in which they 

 occurred, and during the past summer (19 13) especially favorable 

 conditions for observation were found at a quarry which is being 

 newly opened up at Pompton Junction. The quarrying operations 



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