96 GEOLOGY OF OLD HAMPSHIKE COUNTY, MASS. 



opment of seipentine from the hornblende-schist bed, since this extensive 

 development of serpentine extends north and south from this fissure, and 

 the fault perhaps aided the work by bringing to the bed for great depths 

 an abundance of water, and may have further intervened by localizing 

 the earthquake forces, which may have shattered the rocks for a distance on 

 either side of the fissure, thus aiding the chemical activity of the water. 

 The sharp bend of the stream and its long course parallel to the direction 

 of the fault show that the fault early controlled the du-ection of the river, 

 and it probably did this because the softened rock was more easily eroded. 

 The next large area of serpentine — and these two areas are vastly larger 

 than the others — is in Middlefield, in the only other large transverse valley 

 in the State. Here, also, I have mapped a fault in the valley bottom, and 

 it seems probable that here also the fault may have had something to do 

 with the hydration of the hornblende to serpentine, as well as with the 

 position of the transvei'se valley. I have noted also a sharp bend, which 

 is almost a fault, at the Osborn quarry. The other serpentine and stea- 

 tite deposits are comparatively unimportant in size, or show trace of olivine 

 and enstatite. This relation did not attract my attention until the field 

 work was ended, or 'other similar coincidences might have been detected. 



PETROGRAPHICAL DESCRIPTION. 

 THE AMPHIBOLITES. 



1. Epidotic amphibolite. — Blandford; North Blandford road at watering 

 trough. Typical jet-black, fissile schist, the shining hornblende needles just 

 visible to the eye. Drusy surfaces of epidote and adularia upon fissures. 



Under the microscope the abundant hornblendes appear as broad 

 plates with strongest extinction and pleochroism; jc>tr>-a, c= blue-green, 

 I>=olive, a=yellow. Extinction, 22° 30'. Interspersed everywhere among 

 the hornblende needles are abundant grains of pistachio-green epidote. 

 There is a sparing groundmass of rounded, untwinned albite grains, show- 

 ing positive bisectrix. Magnetite is abundant, but no leucoxene. Rutile 

 occurs in the feldspar. 



3. Feldspathic amphibolite. — Blandford; Osbom's soapstone quarry, at 

 west junction of soapstone bed and granite. (See p. 87.) From a thin bed 

 of black, very heavy feldspathic amphibolite, 20-30°"" wide, with fringe of 

 coarse, black ti'ausverse biotite 10°"° wide adjoining granite, and therefore 



