Pirsson and Washington — Geology of JYew Hampshire. 449 



The feldspars consist of orthoclase with microperthite mter- 

 growths of soda microcline. In a section perpendicular to c 

 and nearly parallel to b (010), oriented by the good cleavage 

 of c (001) and rough parting parallel to m (110) and by the 

 arrangements of inclusions giving the direction of the vertical 

 axis and a measured angle for j3 of 64°, the main feldspar 

 extinguished at 7° from the trace of the base e, that of 

 the microperthite intergrowths at 11° in the obtuse angle /S'. 

 Thus the mail] crystal is of orthoclase, the intergrowths of 

 soda-microcline (anorthoclase). The amount of these inter- 

 growths is very large, indeed in most cases they appear to be 

 as great as, or even greater in volume than, that of the host 

 crystal. Sometimes these intergrowths show distinct multiple 

 twinning and the optical properties prove them to be of oligo- 

 clase-andesine (Ab 2 An,). Feldspars enclose all the other 

 minerals save quartz. They are in formless masses, at times 

 having poor tabular development, with Carlsbad twinning com- 

 mon. So microcline was observed, and the feldspars are 

 somewhat kaolinized. 



The hornblende is in anhedral masses, at times poorly 

 developed as short columns. Prismatic cleavage is good. It is 

 strongly pleochroic : c, olive-green to deep green ; B, olive- 

 brown ; a, pale brown; absorption medium strong, c>b^>a. 

 Angle of conc = 18°-20°. From these properties it is prob- 

 ably a mixture of the common hornblende and barkevikite 

 molecules. It rarely contains a paler green core, and includes 

 iron ore, zircon, apatite and biotite. In some places it is alter- 

 ing into a reddish substance, probably gothite. The rock at the 

 summit of Gunstock contains 13 '4 per cent of hornblende ; the 

 type analyzed has somewhat less, probably not over 8 per cent. 



'Biotite is quite subordinate in amount, while in other varie- 

 ties of the massif it becomes more abundant, increasing with 

 the amount of quartz, and in the marginal facies, which are 

 rich in the latter mineral, it entirely replaces hornblende. It 

 is of the type of common biotite, brown, strongly pleochroic, 

 with inclusions of iron ore, apatite and zircon and with pleo- 

 chroic halos. It is older than hornblende and automorphic 

 against it, and quite unaltered. Iron ore occurs in occasional 

 scattered grains. Zircon is present in crystals varying from exces- 

 sively minute microlites to some of good size, rather common 

 and sowed through all the later minerals. It is well crystallized 

 with m (110). and p (111) well developed. Apatite is also com- 

 mon in slender microlites and larger stout prisms. 



Quartz is seen in very small amount in the type specimen in 

 small xenomorphic masses, serving as a cement between the 

 other minerals. It is too rare to characterize the rock as quartz- 

 bearing. In a variety on the top of the hill beside the road 

 from Gilford to West Alton it is entirely lacking in two thin 



Am. Jour. Sci. — Fourth Series, Vol. XXII, No. 131. — November, 1906. 

 31 



