44: GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



Zircon is quite regularly disseminated in colorless to pale-yellow crys- 

 tals one-fifth to one-tenth the size of the other constituents and regularly 

 crystallized in stout prisms, some almost cubical. The forms P, 2 P 2, co P cx) 

 could be seen in one crystal. It is included in all the other constituents. 



Magnetite is absent. 



2. Biotite-gneiss from Bassett's quarry, Northfield. 



A tine quarry gneiss, light-gray. On the foliation faces distant, squar- 

 ish, thin plates of hornblende make the rock appear as if ink-spattered. The 

 feldspar appears as glassy as the quartz, which is common in all these gneisses, 

 though it shows traces of change into muscovite vmder the microscope. 



There are present orthoclase, microcline, and albite; a fragment of the 

 latter gave extinction +15"^ on go Pdb , and the triclinic feldspar in all these 

 gneisses give commonly an extinction of +4° on either side of the twinning 

 sutures on P. Fine minute zircons are present, but no titanite. 



3. Hornblende-gneiss from southwest Shutesbury, opposite W. Thresher's, 

 adjoining trap dike. It is a sandy-granular rock of very fine and even 

 grain, and of very dark-gray color. It is a rock quite common in the Monson 

 gneiss, and found also in the Becket gneiss, in the northeast of Tolland. It 

 become^ much more abundant in the eastern area, in its southern exten- 

 sion into Connecticut, where it is Percival's C 3Mn its eastern portion. 



Microscopical character : The background is made up of little quartz, 

 little albite (extinction 6° on either side twinning lines), and much lim^Did 

 orthoclase, Avithout cleavage, and determined only by its positive biaxial 

 character. 



The abundant hornblende molds and incloses the other constituents; 

 it shows peculiar basal cleavage in fine, close, straight lines. Its absorp- 

 tion and pleochroism are exceedingly strong )C>b>a. c=deep blue; t»=dee2) 

 olive; a=bright yellow; much deep-green biotite and large light-red garnet, 

 many plates of tremolite, much black and red ore, and a single group of 

 leucoxene grains. 



4. Biotite-gneiss from east foot of Mount Hygeia, upper quany. A 

 white gneiss, making heavy beds above the normal gneiss of Pelham, dif- 

 fering from it by the small amount of black biotite in distant scales and 

 the abundance of small red garnets. 



The quartz contains no rutile needles, and is in romaded grains that 



' Rept. Geol. Conn., p. 222. 



