SHORT PLUMOSE VARIETY 101 



and the prismatic cleavage can be traced through the whole glass. The 

 central part is mostly non-polarizing, but small unchanged patches still 

 polarize like the pyroxene above. The border of lighter color is a devit- 

 rified, less ferruginous glass, and polarizes independently. This is seen 

 to run out in a delicate fringe of glass filaments into the colorless ground , 

 and this fringe is more or less filled with grains of magnetite, as in a 

 common resorption rim. Outside this is a narrow band of the quartz - 

 plagioclase ground, full of long feldspar microlites in fluidal arrangement. 

 This is surrounded by a broad band of original calcite, partly in broad 

 well cleaved untwinned grains, partly of grains made up of a matted 

 fibrous mass, as if changed to aragonite. This contains several clots of 

 the brown fibrous devitrified glass, and is outwardty interlaced with the 

 coarse feldspar and pyroxene crystals. This variety lies next outside 

 the long plumose variety, where the strong currents swept the suddenly 

 formed blades out into a hotter part of the magma and the partial remelt- 

 ing occurred, with the formation of a glass blacker and still more ferru- 

 ginous than the normal palagonite described below. 



The type contains many large glass masses in which secondary altera- 

 tion has developed many structures which are concealed in the fresher 

 types. Their description is therefore deferred until after that of the 

 simpler varieties (see page 118). 



Glass-bearing porphyritic diabase. — This is the most interesting variety 

 of the trap. All the best specimens came from a great block near the 

 south end of the reservoir clearing, but all the peculiarities were found 

 on both sides of the central bands described above in the great schlieren 

 everywhere, but not all concentrated so abundantly in single hand speci- 

 mens. I do not think any one familiar with the Holyoke trap would 

 think for a moment that the rock could come from that sheet. The 

 black, perfectly fresh surface, with strong greasy luster, that suggest an 

 ultrabasic rock rather than an exceptionally quartzose one, the large 

 and abundant porphyritic and poikilitic feldspars, inconspicuous only 

 because of their perfect freshness and limpidity, and the abundant clots 

 of jet black glass with their spherulites and inclosures of calcite crystals 

 and sphserocrystals of ankerite and blue quartz, form a strong contrast 

 to the monotonous Holyoke diabase. The large porphyritic feldspars 

 are often almost an inch square. They are so fresh, glass} 7 ", and trans- 

 parent that they are little distinguished from the mass of the rock. They 

 have straight triclinic striation produced by twinning according to sev- 

 eral laws, which is often wanting over large areas and very unequally 

 spaced. One large crystal cut on (001) extinguished at 12 degrees, indi- 

 cating labradorite. They are poikilitic, with many rounded or elongate 

 inclusions of the dull black pyroxene and of glass and spherulites. 



