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



whole area is occupied chiefly by gneisses, representing 

 in general a few recurring lithological types. '^ 



The Pre-Cambrian of this region is composed of a 

 series of metamorphosed sedimentary and igneous rocks, 

 with certain additional elements of doubtful afiSliations. 

 The sedimentary foliates, of w^hich the Franklin limestone 

 is the most important, include coarsely crystalline lime- 

 stone, marble, quartzite, conglomerate, breccia, slate, and 

 schists and gneisses of various types. The orthogneisses 

 are divisible into three groups, the Pochuck, the Losee, 

 and the Byram. Associated with these rocks are many 

 non-foliated granitic and pegmatitic intrusions. 



The exact geological relations of the Van Nest Gap 

 gneiss are unknown, but, as will be shown later, its chemi- 

 cal and lithological characters very closely resembles 

 those of certain phases of the Byram gneiss, and, for 

 this reason, the rock is tentatively assigned to that group. 



Spencer-^ states that the Byram gneiss includes ^^ several 

 varieties of granitoid gneiss which are lithologically 

 related by the presence of potash-bearing feldspars 

 among their principal mineral components. As thus 

 defined, the formation includes the ^^ Hamburg'', **Sand 

 Pond'', and '^Edison" gneisses, which were separately 

 mapped by Wolff in the Franklin Furnace district; the 

 ** Oxford type" of gneiss, described by Nason; and the 

 gneissoid granite of Breakneck Mountain on the Hudson, 

 described by Merrill. ' ' 



The relations of the Byram gneiss to the other members 

 of the Pre-Cambrian in this region are rather obscure. 

 It is reasonably certain that the contacts with the 

 Franklin limestone and with the Pochuck gneiss are intru- 

 sive ; hence these two members are older than the Byram 

 phase. The Losee and Byram gneisses appear to be 

 * * approximately contemporaneous. ' ' 



Petrography. 



Megascopic. — The Van Nest Gap gneiss is a rather 

 coarse, even-grained rock of dark grayish-green color, 

 and of fresh, unweathered appearance. It exhibits a 

 roughly parallel linear structure, due to the partial segre- 

 gation of the hornblende into pencil -like stringers. Sur- 



* Spencer, A. C, et al., U. S. Geol. Survey Atlas, Franklin Furnace Folio 

 161, 1908. 



