318 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



granites of this section, as it is more clearly affected bj- the last foldings, 

 and it cuts the Brimfield schist and the Hardwick granite-gneiss. The 

 latter is thus materially older than the other granites, and more nearly 

 contemporaneous with the Brimfield schist, in which I suppose it to have 

 been intruded l^efore the final folding of the series, and with which I have 

 therefore described it. 



BIOTITE-GRANITE, OR GRAISTITITE. 



Within the central granite area I have found but one dike of a purely 

 biotite- granite, an exceptionally fresh, coarse, subporphyritic rock with 

 white orthoclase, much plagioclase, and amber quartz. It occurs 820 feet 

 west of the outlet of Burnell's pond in Chesterfield. 



The granite described above — that extending west fi'om the toualite 

 through Florence, and that around Williamsljurg — is often in hand speci- 

 mens a purely biotitic rock; but it generally contains at least a small 

 amount of muscovite. This muscovite has always in the freshest rock the 

 character of an original component, and is so regularly present that I have 

 classed these rocks as two-mica-granites. 



CONTACT METAMORPHISM OF THE GRANITITE AND SCHISTS. 



The granitite is a highly feldspathic rock, and it has had great influ- 

 ence upon the rocks bordering it on either side. 



The rusty fibrolite-schists become garnetiferous gneisses, porphyritic 

 with a great number of rounded masses of clear, fresh, transparent ortho- 

 clase, which often furnish good moonstones, and were formerly quoted as 

 adularia from Brimfield and Sturbridge. Tliey appear also in the dark 

 Hardwick granite, where the porphyritic granitite approaches it, and they 

 continue to ap})ear in the fibrolite-gneiss far south of the most southern point 

 to which the granitite can be traced, across Brimfield and Monsou, as if 

 they marked its subterranean continuation. They are often crushed at the 

 border into a fine, sugary mosaic, and this cataclase structure is at times 

 continued clear to the center. 



THE MIDDLEFIELD PORPHYRITIC GRANITITE. 



The great dike of granite in Middlefield, about 6 miles long, is widely 

 separated from all other outcrops, and is unlike all the other masses of 



