GABBROID DIABASE 103 



feather-like character described above, but it does not then contain the 

 glass in great quantity. It occurs also, but without glass (though much 

 that we have called delessite may be devitrified glass), far north of the 

 locality we are describing, at the surface of the ledge at Titans pier, on 

 the Connecticut river. This type often forms also a transition to the 

 last, in that the pyroxenes are often partly melted. A central unaltered 

 nucleus is surrounded by a broad band of granular recrystallized pyrox- 

 ene. Outside this is sometimes a partial band of a deep green chloritic 

 mineral, and outside this a broad area of dark glass, doubtless derived 

 from the entire melting of much of the pyroxene. In other cases the 

 nucleus is surrounded by a very broad black resorption band, largely 

 made up of granular magnetite. 



These last varieties are generally perfectly fresh and are located far 

 out on either side of the central band of impurities, and yet a trace of 

 the bluish quartz appears here to show that the mineralizing fluid was 

 carried to the outer border, but did not act to produce decomposition of 

 the minerals formed. It promoted a widespread differentiation of the 

 magma into a very hydrous glass, which, calculated in the anhydrous 

 state, is more than a third iron. As the magnesia went with the iron into 

 the glass the acid differentiate had almost the composition of an albite. 

 We have thus to discuss (1) the basic glass or palagonite, (2) the spheru- 

 litic inclosures in it, and (3) the acid halo surrounding the glass and 

 forming a felsitic ground in all the coarser varieties, and also appearing 

 as small aphanitic dikes. 



DESCRIPTION OF THE PALAGONITE 



The glass is in irregular portions, the longer from 1 to 30 millimeters 

 in length, occurring often 20 to 30 in a square inch, and with a lens and 

 the microscope many more swarms of minute patches and globules of 

 the red brown glass appear. They are lobate and have formed in place. 

 They are generally velvet black with highest glassy to resinous luster, 

 more like common asphaltum than like obsidian. At times the larger 

 grains are centrally of a slightly lighter shade of color, or two shades are 

 concentrically interbanded as in agate. It is very soft and brittle ; its 

 hardness is 3, and its specific gravity is 1.91. It is deep red brown by 

 transmitted light with brownish white streak. It is red brown in thin- 

 section, and sometimes a central portion is so deep colored as to be 

 opaque. It is very rapidly dissolved in weak cold hydrochloric acid, 

 and when a piece of the rock is put in strong acid all the grains of the 

 glass soon show a cracked surface and a golden brown color like a dark 

 resin ; but the acid soon dissolves all the iron and leaves behind a white 

 curd-like mass of silica which fills with cracks like drying starch. Often- 



