THE ORANGE AND MONSON AKEA. 57 



town and across Greenwicli and t^nlield — tliat is, for 2() miles sontli — it 

 forms the bottom of a very peculiar, deep valley, in the center of which 

 rise strange, isolated peaks which have in some unexplained wa)- es(;aped 

 the general erosion. 



Continuing south across Ware and Palmer, the gneiss band narrows 

 somewhat and the valley is interrupted, though this is in part only apparent, 

 being due to the filling of the valley by the abundant glacial-lake deposits 

 in this latitude. Farther south the valley reestablishes itself in Monson 

 and continues far beyond the limits of the map (PI. XXXIV). 



Across Orange the newer rocks dip toward the gneiss from all sides, 

 forming a fan structure. Across the remaining area it forms the center of 

 a closely appressed anticline, slightly overturned to the east, as the dips are 

 all 70°-80° W. 



GENERAL DESCKIPTION. 



The Monson gneiss is a cleai'-gray, friable biotite-gneiss, in mass made 

 up of small angular grains of quartz and orthoclase, equally limpid and 

 colorless, and so loosely joined that there are many interstices, and one can 

 sometimes rub a fragment into powder between the fingers. Scattered 

 through this aggregate are grains of shining l>lack mica, whose parallel 

 arrangement produces the more or less clearly marked foliation visible upon 

 cross fracture and the equally marked "stretching" seen upon the foliation 

 face. The foliation is produced by the concentration of the black biotite 

 in bands which have between them long linear or elliptical si)aces that 

 appear white upon the gray ground, and in which the knots of feldspar 

 mostly occiir when the rock becomes subporphyritic. The stretching is 

 manifested upon the foliation faces by the greater concentration of the 

 biotite along broad, imperfectly marked parallel bands; and oftentimes 

 when by incipient decomposition the feldspar has been rendered opaque 

 white or flesh-colored it is seen to have the same linear parallel arrange- 

 ment. This stri;cture often obtrudes itself more readily upon the attention 

 than the foliation itself There is, however, little or no greater tendency in 

 the quarry slabs to split parallel to this structural feature, and Ijlocks are 

 frequently gotten out with the "stretching" running diagonally across their 

 broad faces. 



The biotite, although so important for the color and structure of the 

 rock, is present in rather inconsiderable amount. 



