N. E. A. Hinds — An Alkali Gneiss 355 



Aet. XXIV. — An Alkali Gneiss from the Pre-Camhrian of 

 Neiu Jersey; by Norman E. A. Hinds. 



Introduction. 



Geology. 



Petrography. 



Megascopic 



Microscopic. 

 Chemical composition. 



Relations of the Van Nest Gap gneiss to the Byram gneiss. 

 Origin of the Byram gneiss. 

 General relations. 

 Summary. 



Introduction. 



The rarity of foliated alkali igneous rocks of primary 

 origin is well known to petrographers. A few occur- 

 rences have been cited by Kosenbusch^ and Washington's- 

 latest compilation of rock analyses has added new types, 

 but, as compared with the known volume of alkali igneous 

 rocks, their metamorphic equivalents have been found on 

 an extremely small scale. This paper adds a further 

 example in the form of an alkali quartz-syenite gneiss 

 from the pre-Cambrian complex of New Jersey. The 

 rock was collected by Dr. J. E. Wolff from a tunnel cut 

 through Van Nest Gap, near the town of Oxford Furnace, 

 in the west central part of New Jersey, for the Delaware, 

 Lackawanna, and Western Eailroad. 



The writer wishes to express his thanks to Dr. Wolff 

 for many helpful suggestions in the preparation of this 

 paper, and for the chemical analyses which he kindly 

 made. 



Geology. 



According to Wolff and Brooks,^ the pre-Cambrian 

 series in New Jersey occupies a highland belt ' ^ about 20 

 miles wide, which runs across the State, and continues 

 northeastward into New York and southwestward into 

 Pennsylvania. With the exception of a few longitudinal 

 valleys, in which the younger Paleozoic rocks occur, the 



^ Rosenbusch, H., Elemente der Gesteinslehre, 1910, p. 620. 



^ Washington, H. S., Chemical analyses of igneous rocks : U. S. Geol. Sur- 

 vey, Prof. Paper 99, 1917. 



'^ Wolff, J. E., and Brooks, A. H. The age of the Franklin white limestone 

 of Sussex County, New Jersey, TJ. S. Geol. Survey, 18th Ann. Kept., Pt. II, 

 p. 431, 1898. 



