58 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



There are also here and there in the mass, and especially upon the 

 foliation faces, squarish spots of a black hornblende, generally from half an 

 inch to an inch across, but at times forming gi-eat lustrous masses larger 

 than one's fist, and in other places concentrating in distinct strata — often 

 accompanied by a trace of copper — to so great an extent as to form a black 

 homblendic gneiss, as in the western side of the Monson quarry and for a 

 long distance north and south. This bed does not, however, so differentiate 

 itself from the remainder of the gneiss that I thought it desirable to separate 

 it upon the map. 



Titanite is a very generally distributed constituent of the rock, in 

 honey- to straw-yellow crystals, flat, often well formed, and reaching a 

 size of from 2 to 5""'. 



Pistachio- to oil-green epidote in grains is aggregated with the biotite, 

 and especially with the hornblende, making a loose border to the squarish 

 plates of the latter. Garnet and magnetite occur in small grains. A pale- 

 green pvroxene appears rarely in large, stout prisms embedded in the rock. 



The narrow, white interrupted planes which express the foliation are 

 structure planes and not planes of separation of the rock, and the latter 

 planes are at times so closely approached as to divide the rock into thin 

 plates (about 4 inches thick), whereby it becomes "scaly" (the local 

 quarrymen's term) and useless as a building stone. In other places the 

 latter planes separate more widel}", furnishing thick banks of excellent 

 quarry stone. The blotching with hornblende, or with large roundish 

 masses of white feldspar, and the amouiit and parallel arrang-ement of the 

 biotite may vary in all these structural varieties, forming two types of 

 special importance. On the one side, by the great increase of the feldspar 

 nodules, a strongly marked " avigen-gneiss " is formed, which is the "sub- 

 porphyritic" gneiss of Percival,^ the "glandulous gneiss" of E. Hitchcock," 

 and which differs decidedly from the porphyritic gneissoid granite of 

 Worcester County, for that is a complete granite with porphyritic carlsbad 

 twins, while here the feldspar is in roundish masses with no approach to 

 crystallographic outline and not twinned. On the other hand, b^- the 

 sinking of all the constituents to the same size and by the more uniform 

 tirrangement of the biotite, a fine-grained granitoid rock results, like the 

 best at the jMonson quarry, though it is nowhere so completelv granitoid 

 as at the Middlefield and Becket quarries. 



' Geol. Conn. - Am. .Tour. Sci., 1st series, Vol. VI, 1823. p. 19. 



