108 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



the constituents favors the conclusion that they were introduced together 

 from without and are not products of decomposition. There is also very- 

 little magnetite, much pyrite, very little pyroxene, the latter in long 

 isolated and uncorroded blades. The latter characters ally the rock 

 more with holyokeite. The brown-dusted hyalopilitic ground is also in- 

 dicative of the same alliance, but the rock is generally distinctly different 

 from the first described holyokeite, though sometimes almost grading 

 into it and very peculiar. While the normal holyokeite has the usual 

 ophitic ground like the common diabase with the ferruginous constit- 

 uents (except pyrite) absent, this is a peculiar variant of the hyalopilitic 

 ground. It simulates a ganglionic structure. Central feldspathic areas — 

 sometimes regular phenocrysts, sometimes irregular angular areas — send 

 out abundant shining white needles which sometimes branch or seem- 

 ingly anastomose into pretty tufts like frost flowers, and the interspaces 

 are filled by micropegmatitic albite. There is also a remarkably complete 

 although irregular blending of this cryptocrystaliine ground with fibrous 

 quartz, so that sometimes minute black crosses, indicative of the latter, 

 can be seen everywhere in the field. The blue fibrous quartz thus asso- 

 ciates most curiously on the one side with the feldspathic ground and 

 on the other with the glass. In one slide many perfect quartz crystals 

 just visible with the lens were scattered porphyritically in this ground- 

 mass. 



The rock has thus affinities in several directions. It is plainly a trap 

 in its prevailing characteristics, and is a differential from the main quartz 

 diabase in the direction of the holyokeite, and a slightly greater abstrac- 

 tion of iron would have made it a purely feldspathic trap like the orig- 

 inal hotyokeite. I have no doubt that this is the explanation of the 

 latter rock. We may call the present case a quartz and glass-bearing 

 holyokeite. 



On the other hand, the chalcedonic quartz is often deposited so abun- 

 dantly in the mass that it forms a transition to the quartz-calcite vein- 

 stones to be described in the next section but one. 



Indeed, I do not know of a more perfect instance of a transition from 

 a purely igneous through igneoaqueous to purely aqueous solution than 

 is here presented.* 



.4 COMPOSITE DIKE 



On plate 31 is shown a peculiar composite dike about four inches 

 wide, which is exposed for several feet in the ledge. This dike is about 



♦Holyokeite is also found at Greenfield as a cement of the glass and sand fragments which 

 have been carried up into the sheet by explosions from below, and is there also a differential 

 result of the action of the water on the normal diabase. 



