GEOLOGY OF MONADNOCK MOUNTAIN 5 



is due to iron pyrites. Little scales of graphite are characteristic 

 of this phase of schist, and are sometimes abundant enough to 

 give a gray color to the unrusted rock. Fibrolite was not observed 

 with the graphite, though in the more micaceous, or first phase of 

 the schist, fibrolite and graphite may sometimes be seen together. 

 This rusty phase occurs over quite an area in the southeastern 

 part of the region represented in our map, and also in the north- 

 western part, and on the southeastern slope of Gap Mountain. 

 In all of these areas there is no well-defined border between the 

 first or fibrolitic phase and the third or rusty, graphitic phase; 

 there is a blending of one into the other, and they are equiva- 

 lents. The second phase also is only a variant of the third. 



Included in the area of the accompanying geological map, 

 though not a part of the mountain, are granite masses which are 

 closely connected with the rock structure of the mountain, and 

 with other phenomena revealed in this study. Where this gran- 

 ite adjoins rusty schist, it has a dark gray color, is more or less 

 rusty on weathered surfaces, and is of medium fine, granular tex- 

 ture. 



The quartz and the feldspar form an intimate granular mix- 

 ture, in which the biotite is quite uniformly distributed in fine 

 scales. Muscovite occurs in varying quantity, but is not charac- 

 teristic of the granite, as biotite is. Fine magnetite and little, 

 brown, wedge-shaped crystals of titanite occur in this granite 

 along with some small particles of secondary epidote. Along the 

 immediate border the granite sometimes contains black tourma- 

 line. Tourmaline is, however, more frequently seen in the schist. 



Away from the schist the granite is lighter in color, more mus- 

 covitic and less biotitic, contains less of the other minerals — is, 

 in fact, more nearly a simple, medium fine, crystalline mixture 

 of feldspar, quartz and mica. In mapping the granite this vari- 

 ation is a good index of the nearness or remoteness of the schist 

 border. 



In places the granite, in the southern part of this area, is por- 

 phyritic, the feldspar phenocrysts sometimes measuring an inch 

 by one quarter, and showing the Carlsbad twins. The feldspar 

 of the groundmass is partly triclinic. The granite of the north- 



