148 GEOLOGY OF OLD HAMPSHIEB COUNTY, MASS. 



serpentine is also a process not requiring special consideration at this place, 

 as it has been demonstrated in a preceding chapter that the serpentine still 

 contains traces of unchanged enstatite, or of the other original mineral in 

 the case of the corresponding serpentine. The bastite formation in the 

 black rock is a part of the same process. 



3. The tremolitisation. — The alterations by hydration mentioned above 

 were subsequent to the change of the limestone at the Westfield quarrv, 

 for several feet inward on its eastern margin, into a quite pure, matted and 

 radiated tremolite, and subsequent to tlie partial change of the whole thick- 

 ness of the limestone bed into the same tremolite, which is later than the 

 formation of the enstatite, since its needles end against the enstatite crystals. 



The band of actinolite which cuts across the main quaiTy, and the 

 broad actinolite selvage on the west wall farther south, in the same way cut 

 across both limestone and enstatite, and are plainly of later formation, 

 nearly contemporaneous with the tremolite, and formed, like it, by the 

 action of heated siliceous solutions, here ferruginous and there not, which 

 have been infiltrated from the schistose walls. The same alteration appears 

 at many places farther north in the limestone. On the Alderman place in 

 Becket there has just been uncovered a wall of pre-Cambrian limestone, 

 where the limestone, for about the same distance in, is changed in the 

 same way into a mass of matted tremolite fibers. 



4. The shearinfj. — The peculiar bed at the quarry, and the one which 

 promises to be of the most economic importance, is the central band of the 

 foliated serpentine marble, which seems to me plainly formed by the shear- 

 ing of a rock like the black spotted marble forming now the eastern band. 

 The former enstatite has been wholly changed to serpentine, often to an oil- 

 green precious serpentine, but every stage of the change can be seen, from 

 that in which the black enstatite is crushed into black bands between layers 

 of limestone, only a few bronzy cleavage surfaces remaining, to that in 

 which the long, narrow bands and spots of the rich green serpentine, with 

 their lighter centers, ai"e the last remnants of the black serpentinized ensta- 

 tites with their gray centers. 



5. The formation of the enstafitc. — The bed is thus traced back to a con- 

 dition when it consisted of about a hundi-ed feet of a white crystalline 

 limestone mottled with enstatite crystals, and an adjoining bed to the west, 

 60 feet thick, of a massive rock consisting almost wholly of the same coarse 



