135 



frequently contain nearly or quite as much mica as hornblende, 

 and closely resemble portions of the adjoining micaceous gran- 

 ite. They are really hornblendo-micaceous or quaternary 

 granites, and bear but little likeness to the hornblendic granites 

 of the Huronian formation. (2) Prof. Edward Hitchcock, on 

 his revised map of 1844, represented a small patch of " granite" 

 (micaceous granite) enclosed in the Ludlow and Belchertown 

 area of syenite. (3) Hornblendic gneiss, hornblende slate, 

 and even pure hornblende rock, though not characteristic Mont- 

 alban rocks, certainly occur in this formation; and, since the 

 granites have resulted from the metamorphism of the stratified 

 rocks, I can see no reason why the Montalban granites may 

 not sometimes be hornblendic. The hornblendo-micaceous 

 granite of Smithfield, R.I., has already been noticed, ayite, 

 p. 128. 



The endogenous or vein granites are usually coarser than the 

 preceding and more distinctly crystalline ; never exhibiting 

 traces of schistosity, or genuine stratification. They sometimes 

 possess, however, the banded appearance due to their mode of 

 formation, which may be readily mistaken for stratification, 

 since the veins are frequently of a lenticular form, usually occur 

 in gneisses of similar composition, and conform in trend and 

 dip with the bedding of the enclosing rock. These- granites 

 are, as Dr. T. Sterry Hunt has shown to be the case with en- 

 dogenous Montalban granites generally, characterized by the 

 abundance and variety of the accessory minerals which they 

 contain, though appearing to be poorer in this respect than the 

 endogenous granites of some other regions. The endogenous 

 granite is usually associated with gneiss, though not infre- 

 quently occurring in mica-slate. In Eastern Massachusetts it 

 is most abundant in the gneiss of Middlesex County, especially 

 through the north-western half of this range. For obvious 

 reasons the endogenous granite has not been indicated on the 

 map. 



Gneiss. —This is the most abundant rock in Massachusetts, 

 forming nearly one-half the State. It occurs of all textures 



