70 B. K. EMERSON — DIABASE PITCHSTONE AND MUD ENCLOSURES. 



run through the granular calcite like micropoikilitic quartz in a feldspar. 

 It produces entirely the impression that the calcite has crystallized in 

 the midst of the glass as the feldspar has. 



It is further striking that the red sand is quite full of long "lathes," 

 which have the shape and appearance of the plagioclase rods of the glass, 

 but which now show the aggregate polarization and consist, probably, of 

 calcite pseudomorph after plagioclase. 



GREENFIELD BED. 



Glass breccia. — Under the microscope a fragment of the greenish tuff- 

 like mass, taken 20 feet from the base of the Greenfield bed, was composed 

 as follows : 



The first thing that attracted attention was the fine red sand, each grain 

 being covered with iron rust. Where this was in thick masses it was still 

 red in the interior, but on the exterior was black from the recrystalliza- 

 tion of the iron rust by the caustic effect of the melted lava, in which it 

 had been disseminated in threads and sheets. In the interstices between 

 these dark sand portions many minute angular grains of diabase, like 

 that found in the basal bed, were scattered. These had been broken up 

 by an early explosion and carried up from the base with the sand. The 

 whole had been cemented by an olive green glass, containing a few crys- 

 tals of plagioclase and scattered spherulites, penetrating among the sand 

 grains and to the very center of sand areas, which would otherwise have 

 been called sandstone fragments. The whole thus formed has been again 

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

 diopside and segerine-augite. Beautiful large hexagonal plates of hema- 

 tite 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 radiating chalcedonic growths, with centers of calcite and 

 imkerite and copper pyrite. 



The water-deposited plagioclase (plate 7, figure 2) has the appear- 

 ance and the optical character of the small but perfect albites (plate 7, 

 figure 1) which line the steam holes in many places in this bed and 

 often rest upon the earlier delessite. These I have proved by optical and 

 specific gravity 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 schist. 



The segyrite-like mineral (plate 7, figure^) is in shapeless grains and 

 shows a strong prismatic cleavage. It is intergrown with the feldspar, 

 calcite and diopside in such a way as to show they were all deposited 

 together. 



* Mineral Lexicon : Subject, Albite. Bull. No. 126, U. S. Geological Survey. 



