360 N. E, A. Hinds — An Alkali Gneiss 



Plagioclase 32.64 



Microperthite 30.45 



Microcline 19.25 



Quartz 7.93 



Hornblende 15.77 



Biotite 0.74 



Magnetite 2.15 



Apatite 0.42 



Zircon . 0.58 



99.91 



The position of the rock in the new quantitative miner- 

 alogical classification, proposed by Johannsen,^ is in 

 Class 2, Order 2, Family 9 (Granodiorite). 



The specific gravity of the Van Nest Gap gneiss is 

 2.841. As calculated from the norm, according to the 

 method recently suggested by Iddings,^ the value is 

 2.803. The difference between the two results is due to 

 the fact that the hornblende has a higher specific gravity 

 than its two normative equivalents, diopside and hyper- 

 sthene, which Iddings gives as 3.28 and 3.33, respectively. 



Belations of the Yan Nest Gap gneiss to the By ram gneiss. 



Bayley' has described the Byram series as including a 

 number of granitoid gneisses lithologically related by the 

 presence of potash feldspar as one of the chief mineral 

 components. This feldspar may be either orthoclase or 

 microcline, the latter occurring to the exclusion of the 

 former in the Van Nest Gap rock. The first main type of 

 the Byram gneiss is a dark-gray, moderately coarse- 

 grained rock, composed essentially of microperthite, 

 microcline, orthoclase, quartz, hornblende, or a little 

 pyroxene, magnetite, and occasionally biotite. Oligoclase 

 is usually subordinate, but may equal the other feldspars, 

 as in the case in hand. Fresh rock of the second principal 

 variety is pink, light-gray or white in color, and differs 

 from Type I in the presence of much more quartz and in 

 the paucity of the dark constituents. 



The following table shows the close mineralogical 



^ Johannsen, A., A quantitative mineralogical classification of igneous 

 rocks, Jour. Geol., vol. 28, pp. 38-60, 158-177, 1920. 



® Iddings, J. P., Eelative densities of igenous rocks calculated from their 

 norms, this Journal, vol. 49, pp. 363-366, 1920. 



^ Bayley, W. S., U. S. Geol. Survev Atlas, Passaic Folio 158, 1908. 



