116 GEOLOGY OF THE N ARE AGAN SETT BASIN. 



Near Easton the glacial drift from the granitite area carries a variety 

 of the rock of bluish-white color with large crystals of hornblende and 

 prismatic crystals of segirine. At a few points the rock becomes fine 

 grained and is in contact with undetermined rocks, presumably pre- 

 Carboniferous elastics. Such contacts may be observed north of Mansfield 

 and near Hanover Four Corners. In Montello, north of Brockton, the gran- 

 itite with a pink feldspar was seen to contain small veins of prehnite. 



Along the eastern border of the basin, in Plympton, there also is a fine- 

 o-rained granitite. Southward, in the northern part of the area represented 

 on the Fall River sheet, red granitite is again seen in bowlders, and the 

 ordinary phase of the hornblendic granitite is well exposed in the quarries 

 at Fall River, though here the rock has been locally sheared and in zones 

 is quite gneissoid. 



In conclusion, it can not be said that the facts at hand warrant a more 

 definite statement than that these granitites were erupted at some period 

 between the Carboniferous and the middle Cambrian. The granitic rocks 

 near Providence appear, as shown on page 105, to be older. 



PLYMPTON FBLSITES. 



In Plympton and extending toward Halifax is a small area of reddish 

 and greenish felsites, the exact limits of which are not known. At one 

 point on the eastern border of the Carboniferous area the felsite in the form 

 of a dike several feet wide may be seen cutting the granitite. North of 

 this locality is the main occurrence, in the form of a flow, the bands of 

 which dip gently westward toward the Carboniferous basin. As no Carbon- 

 iferous strata are seen in connection with the igneous rocks at this point, 

 their age is not definitely known. The imperfect exposures seem at least 

 to indicate that the felsite broke through the granitite, and the absence of 

 Carboniferous rocks at the locality, taken in connection with the fact that 

 the supposed boundary of the basin passes through this locality, may be 

 interpreted to mean that the felsite flowed out upon the surface of the 

 granitite at the beginning of Carboniferous deposition. This conclusion 

 is supported by facts set forth on page 155 concerning the felsites of North 

 Attleboro. 



