THE AMPHIBOLITKS 427 



minerals and a tendency to split in planes parallel to the minute mineral 

 constituents, 90 per cent of which is hornblende. Half way down the 

 slopes on either side of the highest point the rock assumes a finely fibrous 

 texture. This seems to be a local condition, and does not extend through 

 any great thickness in the central portion of the formation. Again, along 

 the fault on the southwest side of the mountain the amphibolite takes 

 on a fibrous and, at times, a lamellar structure. This also is quite local, 

 it being induced by the crushing and shearing consequent on the fault- 

 ing. The main body of the formation is moderately coarse in grain, 

 showing cleavage planes 6 or 7 millimeters in length and 3 or 4 in width. 

 It is not uncommon to find in cross-sections of individual hornblende 

 crystals a marked bending or curvature, best shown in sections along 

 the longer axis. The cleavage conforms very closely to one direction, so 

 that the rock splits very easily into plates with an apparent schistose 

 structure. Both pyrite and magnetite grains have been detected with 

 the unaided eye, but in most cases they are in very small amount. 



The remaining exposures of amphibolite do not differ in any respect 

 from the typical representative of the coarse grained type, already de- 

 scribed. In both exposures there is clear evidence of marked crushing 

 and shearing, particularly in the exposure on the brook as shown on 

 the map. Unfortunately these areas are very largely covered beneath 

 gravel and waste from the slopes, so that it is impossible to determine 

 the extent and relationship of these two small but important exposures. 

 The details of the contact of the amphibolite with the serpentine, as 

 shown in the Tucker quarry to the north of the brook section, will be 

 discussed later. It is sufficient to state here that this contact is believed 

 to furnish sufficiently clear and unmistakable proof of the intrusive 

 origin of the rock from which the serpentine has been derived. 



SERPENTINE 



Macroscopic characters and\mineralogy. — The serpentine is remarkably 

 uniform in texture throughout the entire area. It is rather fine grained, 

 grayish green to dark oily green in color and splintery or hackly in 

 fracture. On freshly broken surfaces it is not uncommou to discern a 

 tendency to assume a lamellar or more often a fibrous texture. The 

 fibrous tendency is invariably associated with zones of shearing and 

 possible thrust. In the upper part of the serpentine belt this texture 

 is most evident along a zone passing through the open cuts of the 

 New England Mining Company. When examined under a hand glass 

 it will be seen that much of the fibrous content of the rock is in 

 reality confined to minute seams and shearing planes and stretched out 



LVIII— Boll. Geol. Soc. Am., Vol. 16, 1904 



