434 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



cemented by an olive-green glass, containing a few crystals of jjlagioclase 

 and scattered spherulites, penetrating among the sand grains and to the 

 very center of sand areas, which would otherwise have been called sand- 

 stone fragments. The whole thus formed has been again shattered, and is 

 now cemented by a hot-water deposit of albite, calcite, diopside, and 

 segirine-augite. Beautiful large hexagonal plates of hematite bristle over 

 the trails of sand grains, and in all the other constituents except the basal 

 trap fragments. Sometimes cavities of later formation are filled by radiat- 

 ing chalcedonic growths, with centers of calcite and aukerite and copper 

 pyrite. 



The water-deposited plagioclase (PI. VIII c, fig. 2, p. 430, the colorless 

 center) has the appearance and the optical character of the small but per- 

 fect albites (PI. Vlllr, fig. 1, and fig. 24, C, p. 422) which line the steam 

 holes in many places in this bed, and often rest upon the earlier diabantite. 

 These I have proved by optical and specific-gra^at}- tests to be albite.' 

 It has also a curious resemblance to the albite of the "albitic" schists and 

 amphibolites, and the whole mixture has some resemblance to a crystalline 

 scliist. 



The segirite-like mineral (PI. Vlllf, fig. 2, the dark grains) is in 

 shapeless grains and shows a strong prismatic cleavage like that of augite. 

 It is intergrown with the feldspar, calcite, and diopside in such a way as to 

 show that they were all deposited together. The absorption in this min- 

 eral is very strong : a = deep blue-green, 6 = violet to olive-brown, some- 

 times with shade of green, c = brownish yellow. A single twin with an 

 extinction of 38° on either side of the suture was found, and the maximum 

 of the blue-green absorption was also at 38° on either side of the sutm-e, 

 and this blue absorption represented the greatest elasticity. The mineral 

 has thus the negative" sign and the strong absorption of segirite and the 

 optical figure in the position of augite. It is therefore allied to the eegirine- 

 augite of Rosenbusch, but the absorption parallel to a is clear blue-green 

 and not grass-gi-een. Large patches of the mineral are changed to a yellow- 

 green sei'pentinous mineral, which under crossed nicols is almost black, but 

 with scattered points of light. 



•Miiieralogie.ll Lexicon, under "Albite": Bull. U. S. Geol. Survey No. 126, 1895. 



= By an oversight the mineral is said to have the positive sign in the article cited, and the absorp- 

 tion color is given as blue. This is only true in some sections between a and b, which blend the blue, 

 green, and violet. 



