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



liarity is that the cavities seem to have been filled with the mixture 

 described above, and then the sand has shrunk away from the rilling for 

 a considerable distance along one or more sides, leaving a film of the 

 black sand grains attached, and then a more limpid feldspar has grown in 

 the narrow cavities thus formed. 



Contact material. — A slide cut within the porous outer portion of the 

 trap from the contact of the bomb-like masses of trap with the glass 

 breccia, showed only a very feldspathic and vesicular diabase. 



Specimens cut from the fused border between the two showed a rock 

 with the aspect of an augite-andesite (see plate 7, figure 5). The well 

 shaped feldspars of two generations and the equally well shaped olivines 

 were enclosed in an opaque red brown base, which in thinnest places 

 revealed its hyalopilitic structure. Its outer surface had at times a 

 rounded and lobed fused surface, and just under the surface a single row 

 of rounded and lobed steam holes filled with silica, all indicating a 

 superficial rem el ting. 



IAthophysds. — In one large specimen from near the base of the bed north 

 of the quarry at Greenfield the breccia was full of well formed lithophysse 

 a half inch to an inch and a half in diameter. The cavities were half 

 filled with curdled masses of a lighter rock. Indeed the collapsed steam 

 holes in the pure glass Where the wrinkled walls show a fluidal structure 

 suggest lithophysse (see plate 8, figures 4 and 5). 



MERIDEN "ASH BED:' 



Basal bed. — The diabase at the bottom of the normal and unmoved 

 basal bed south of the blasting is a very fresh, fine grained, compact trap, 

 in which rarely a fine microscopic steam hole occurs. It has a few small 

 cavities filled with delessite. It is very feldspathic and the two genera- 

 tions of feldspar graduate into each other, are much feathered out at the 

 ends, and make a regular network, in the meshes of which lie the augite 

 grains. The peculiarity of this trap consists in the fact (which is true 

 of the base of the overflows in many cases) that the interstices between 

 the feldspar rods are very often filled with beautiful arborescent growths 

 of magnetite, consisting of long lines of octahedra branching at right 

 angles to the central stem. This seems due to gravity, or possibly to 

 incipient magmatic differentiation. 



Hyalopilitic diabase. — (See plate 7, figure 5.) In a specimen taken from 

 the north edge of the continuous portion of the basal mass, two feet above 

 the base, where it graduates into the glass breccia, a light yellowish gray 

 compact part seemed to be an indurated sandstone, but proved to be a 

 margaritic diabase. The whole ground surrounding the porphyritic feld- 

 spars has solidified as a matted mass of curved and beaded feldspar 



