486 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



The augite is also perfectly fresh, although the abundant inclusions, 

 combined with the irregular cleavage, make it only imperfectly transparent. 

 Separate crystals are bounded by perfect planes. An acute pyramid or 

 dome appears with especial frequency. They are finely twinned, with the 

 interposition of several very fine laminae at the center, and nearly colorless. 



In one portion of the field is a group of the largest feldspar crystals 

 (described above) loosely interlaced, and in the interstices the augite has 

 developed so as to fill the spaces and produce exactly the basaltic structui'e. 

 Only where the meshes were not closed do the augites project outward with 

 terminal faces. If the crystallization had continued undisturbed it would 

 have produced a rock having exactly the texture of a Tertiary basalt. As 

 it is, it is plain that the delicate featherwork of feldspar plates was floated 

 along in the magma and its crystallization arrested, and one can see that 

 the formation of the basaltic structure does not necessarily depend upon 

 the crystallization, first, of the feldspar in a network of bars, and then, second, 

 of the augite in the interstices thus left, but rather upon the different 

 methods of the crystallization of the two minerals simultaneously; the 

 augite, having started a crystal in contact with a feldspar blade, tended to 

 increase this to a large, stout crystal, while the feldsjiar, rapidl}' shooting out 

 new blades, inclosed and bounded the augite on all sides, hindering the for- 

 mation of crystal faces. 



Hematite is quite abundant in the rock, blood-red, with botryoidal 

 fibrous structure, showing the black cross abundantly, and aggregate polari- 

 zation, and especially surrounding the inclusions of quartz and microcline. 

 It also occurs, with ])lood-red color and fibrous structure, pseudomorph after 

 olivine. The olivine form is sharp and clear, 2 P dib (021) = 99° 15' (cal- 

 culated 99° 06'), and this sharp dome is combined with the prism. When 

 traces of the original mineral remain they extinguish longitudinally. 

 Irregular cracks run across the mineral as in olivine and bound separate 

 fibrous masses, the fibers being at right angles to the fissures. I conclude 

 that an olivine very rich in iron — a fayalite — has changed into hematite, 

 more or less mixed with serpentine. The pseudomorphs are always sur- 

 rounded by a whitish, semiopaque halo, probably from the silicic acid 

 expelled from the primary mineral. Other similar forms are olive-green 

 and polarize only in traces, and seem to be olivine changed to serpentine. 

 They have also the white, opaque halo. Very rarely a rounded grain, which 



