G. C. Martin — Dunite in Western Massachusetts. 247 



amount of serpentine depends in a large degree upon the 

 amount of recrystallization and upon the relative positions 

 assumed by these crystals. The magnetite is largely contem- 

 poraneous with the serpentine, It increases in amount with it, 

 and shows a tendency to segregate into large grains, strings, and 

 plates, some of several square inches in area and about tj-inch thick. 



Other minerals are : hematite, with the magnetite ; opal, 

 occurring in cracks ; chlorite, in irregular angular masses scat- 

 tered throughout the dunite and serpentine ; a carbonate, in 

 the serpentine near the northeast corner of the main mass at 

 Loc. 115 ; and a bronze non-magnetic mineral which gave the 

 bead test for nickel and is probably niccolite, although with 

 the quantities used no decided test for arsenic was obtained. 

 The latter mineral is scattered in grains throughout the serpen- 

 tine. The rocks of the area consist then of olivine and the 

 serpentine which has been derived solely from its hydration. 

 The original rock was a mass of olivine, with some members 

 of the spinel group as minor constituents, — a strict Dunite; 

 and it has hydrated in places to a I) unite- serpentine. 



Field- Relations. — The olivine and serpentine are irregularly 

 distributed, but the latter is in general more abundant around 

 the border, especially at the northern end. Slickensides ; a well 

 marked foliation of the serpentine ; and veins of fibrous ser- 

 pentine traverse the area from north to south, dipping steeply 

 to the east : but the foliation deviates from this general direc- 

 tion at the northern end, striking parallel to the border and 

 dipping more moderately to the south. These characters are 

 evidently due to shearing, — either that which accompanied the 

 expansion during hydration, or that which attended some of 

 the mountain-making movements by which the region has 

 been affected. The slightly altered dunite which covers the 

 greater part of the area has absolutely no structure but a strong 

 irregular jointing. 



The surrounding rocks are of a varied and complex char- 

 acter. The entire mountain was mapped by Wolff (Mono. 

 XXIII, U. S. Geol. Survey) as u Vermont Formation (con- 

 glomerate, quartzite, and white gneiss)." The general struc- 

 ture is given as an overturned anticline with easterly dip and a 

 strike varying somewhat from north. The presence of a dike 

 of amphibolite is indicated in a position about half a mile 

 north of the main mass of dunite. 



The studies upon which this paper is based have shown that 

 there are in the immediate vicinity of the dunite, beds of 

 impure crystalline limestone, amphibolite, and gneiss. The 

 limestone is apparently independent of the dunite, and it will 

 be considered in a future detailed discussion of the entire 

 mountain. The amphibolite is very generally distributed ; is 

 usually at least, interbedded with the gneiss ; and the two are 



