580 EARL V. SHANNON 



crystals of pyroxene and plagioclase in a subordinate fine-grained 

 ground-mass composed largely of needle-like crystals of plagioclase 

 with interstitial augite in anhedral patches now largely altered to 

 a greenish-brown chlorite which is probably diabantite.^ The large 

 pyroxene phenocrysts are rather pale greenish-brown in color, with 

 well-developed cleavage. They are unaltered and are in part 

 transparent and in part turbid from the presence of an opaque 

 brownish pigment. They are not commonly simple but are 

 bladed aggregates made up of a number of irregular interlocking 

 crystals not quite in parallel position so that they do not all 

 extinguish simultaneously. In composition this pyroxene is 

 ordinary augite. The plagioclase phenocrysts are characterized 

 by rather narrow twinning lamellae and are near-basic andesine 

 in composition. The most striking microscopic feature of the 

 rock is the presence of broad patches of a graphic intergrowth of 

 plagioclase and augite in fixed orientation, giving remarkable 

 pegmatitic textures entirely analogous to the quartz-feldspar 

 aggregates of graphic granite. In places a phenocryst of plagioclase 

 terminates in a branching fernlike intergrowth of plagioclase and 

 augite. 



A question naturally arises as to the origin of such coarse forms 

 in an intrusive flow less than a hundred feet in thickness. Ordi- 

 narily such surface flows cool too rapidly to permit the growth of 

 large crystal individuals. The mode of occurrence of this coarse 

 phase indicates that the large crystals of plagioclase and augite 

 were formed after the lava had reached its present position and 

 precludes the supposition that it was intruded into the extrusive 

 sheet after the sheet had been buried by later sedimentation. 

 Emerson^ has described similar coarse forms from this trap sheet 

 further north associated with mudstone, pitchstone, etc., formed 

 by the molten lava coming in contact with water and mud. 

 Strangely enough, he attributes the coarseness of grain of the 

 plumose diabase to sudden cooling or quenching of the molten 



'Earl V. Shannon, "Diabantite, Stilpnomelane, and Chalcodite from Westfield, 

 Massachusetts," Froc. U.S. Nat. Mus. In print. 



^B. K. Emerson, ''Plumose Diabase and Palognite from the Holyoke Trap 

 Sheet," Bull. Geol. Soc. Amer., XVI (1905), 91. 



