from the Pre-Cambrian of Ketc Jersey. 361 



relation between the Van Xest Gap rock (Column 1) and 

 an average of four specimens of the dark-colored Byram 

 gneiss (Column 2). Column 3 gives the average mineral 

 composition of three varieties of the light-colored mem- 

 ber of this series, in Avhich the proportions of the compo- 

 nent minerals are seen to be notably different. The only 

 available chemical analysis of the Byram gneiss is of a 

 light-colored variety. As contrasted with the composi- 

 tion of the Yan Ne^^st Gap rock, which is typical of the 

 darker phase, the high percentage of silica and the lower 

 iron content are the outstanding features. 



Table II. 



12 3 



Quartz 7.93 10.50 27.33 



Orthoclase 1.25 0.66 



Microcline 9.25 11.25 52.00 



Microperthite 30.43 49.00 10.33 



Oligoclase 32.64 16.25 10.33 



Hornblende 15.77 8.50 0.50 



Angite 0.50 



Biotite 0.74 .... 1.33 



Accessories (magnetite, apatite, zircon, 



titanite) 3.15 2.75 2.83 



99.91 100.00 99.98 



1. Alkali quartz-syenite gneiss. Yan Nest Gap. N. J. 

 2 and 3. A. C. Spencer et al., U. S. Geol. Survey Atlas, 

 Franklin Furnace folio, 161, p. 5, 1908. 



Origin of the Byram gneiss. 



According to Bayley,"^ the Losee, Byram, and the 

 greater portion of the Pochuck gneisses are considered 

 to be slightly foliated igneous rocks which have been 

 intruded into a preexisting and now highly foliated pre- 

 Cambrian sedimentary terrane. "The linear structure 

 of the gneisses is regarded as the direct result of flowage 

 of the viscous magma and of the crystallization of some 

 of the minerals of the rocks under the influence of strains 

 produced by flowage.'' The gneissoid structure is not 

 due to later metamorphic effects which the region has 

 undergone during the progress of various orogenic move- 

 ments. 



^Bayley, W. S., Iron mines and mining in New Jersey; New Jersey Geol. 

 Survey, vol. VII, p. 126, 1910. 



