B. K. Emerson — Northfieldite. 213 



This is especially well exposed in the type localities of the 

 two variants of the rock, which are described below, that 

 of Crag Mountain in Northfield and Mt. Orient in Pelham. 

 The great ridge of Brush and Crag Mountains in North- 

 field is elevated above the surrounding country because it is 

 made up of this coarse resistant quartz rock. Its southern 

 apex, Crag Mountain, with its steep southern scarp is a 

 prominent landmark from the valley in Montague or North- 

 field. 



The rock has the aspect of a greisen, a coarse pegmatite 

 without feldspar. It is a coarse vein-quartz in flat bands 

 about an inch thick with distant films of shining white 

 muscovite. This appears in mountain masses and for miles 

 makes the major part of the ridge mentioned above. At the 

 end of the blind road going onto the north end of the mountain 

 it is pseudo-conglomeratic. 



Slides cut from the rock at this place were made up of a 

 mass of coarse, limpid, interlocking, unstrained quartz grains 

 which often contain rounded blebs of quartz differently 

 orientated from the host, and probably indicating rapid crystal- 

 lization. It contains a few blades of muscovite and small 

 triangular tourmalines, zircon, and negative quartz cavities, 

 and large motionless bubbles. 



At the south the great mass tapers suddenly to an end in 

 the sharp hill a mile north of the junction of Jack and Keyup 

 brooks. Here it contains some magnetite and biotite. Large 

 veins of later normal pegmatite appear in the rock along the 

 road south of Crag Mountain. The junction of the northfieldite 

 with the Pelham gneiss below is well exposed where it crosses 

 this road north of a house named Sky Farm. Both have 

 a conformable schistosity and easterly dip where they join. 



A second great bed of the 6ame border rock appears further 

 north extending into Winchester, and is well exposed along the 

 Ashuelot road skirting Perchog brook on the north and along 

 the next road north where it abuts against the great fault.* 

 The same rock appears predominantly in the hill a mile north 

 of Tullyville where it occupies a circular area a mile across. 

 Another area over three miles long and in places 50 rods wide 

 runs up past Mallard Hill in the east part of "Warwick. These 

 seem to be the domes of other similar batholiths which if more 

 deeply eroded would expose a center of the normal Pelham 

 granite. 



The Pegmatite Schist. 



The great Crag Mountain band is seen to form a border to 

 the Pelham granite for a long distance and to be divisible into 



* See map, Mem. xxix, U. S. Geo. Sur. 



