104 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



times the whole interior of the grain except a thin exterior film, which 

 remains jet black, changes into a dull opaque greenish black amorphous 

 mass. Its hardness is then 2.5-3 with the streak light green. The con- 

 trast between the thin continuous border of the velvet black glass and 

 the dull greenish black slightly fibrous mass is marked and the transition 

 line is sharp.* The shining black glass border is wholly non-polarizing. 

 The dull black center shows in thin-section a shade of green and polar- 

 izes very dimly in several broad concentric bands. Each band is opaque 

 at its outer edge and gradually lightens up more and more to the inner 

 edge. The bands are faintly fibrous. When treated with acid the black 

 exterior unalterated glass becomes crackled, and because of internal 

 reflections looks brown, but. soon changes into a white-fissured mass, 

 while the weathered interior changes first to a light green and then more 

 slowly than the border becomes opaque white, but almost without crack- 

 ing. It melts into a scoria with the blowpipe. 



There is probably a slight chemical difference between the first formed 

 central portion of the glass and the exterior, in virtue of which the for- 

 mer is less stable, and it is possible that iron is more protoxide in the 

 green interior and more peroxide in the brown exterior. 



This glass is entirely different from that found at Meriden and Green- 

 field mentioned above. That occurs in large masses, with isolated micro- 

 scopic crystals of feldspar and augite, and its spherulites are microscopic 

 drops of glass made up of concentric layers and mingled in the confused 

 breccia and beautifully devitrified. It is very hard, not affected by acids 

 or alkalies, and is a liver-colored basic pitchstone, resembling the tachylite 

 of Ostheim, in Hessen. It decomposes very slowly under weathering. 

 This glass, on the contrary, is very soft and brittle, very easily dissolved by 

 acid and slowly by caustic soda, and decomposes very easily into brown 

 greasy masses, and is quickly removed from the cavities near the weath- 

 ered surfaces. It is thus quite exactly like the palagonite of von Wal- 

 tershausen, and I have used this name to distinguish it from the normal 

 tachylite from Meriden. 



BOTRYOIDAL GLASS WITH RADIATE FIBROUS DEVITRIFIED LAYERS 



In the short plumose diabase, with remelted feldspars (see page 100), 

 the glass is more complex. Many large glass masses occur, which are 

 generally centrally altered to a dull greenish black mass, with a thin rim 

 of the unaltered fresh glass remaining. These glass clots are often beau- 

 tifully botrj^oidal and show agate-like color banding in shades of black 

 (see plate 30). The botryoidal cavities are sometimes empty or have a 



*The dull glass appears in the fresh coarse rock at the big schlieren on the electric railroad 

 and in the central portion of the glass-bearing diabase at the reservoir. 



