102 B. K. EMERSON — PLUMOSE DIABASE AND PALAGONITE 



These inclusions become elongate and lobate and produce a kind of 

 micropegmatitic structure, or are in bands parallel to the outer bound- 

 aries, or beautifully plumose, completely filling and skeletonizing the 

 crystal (see plate 27, figure 3, lower part). The crystals are squarish, but 

 often send out long lobes, as if formed very rapidly, and indeed a rapid 

 growth is indicated by the abundant inclusions. They are often in large 

 crossed forms like the cross-section of a staurolite or double arrow-head 

 twin of gypsum (see plate 28, figure 6). 



The pyroxene is often very inconspicuous, and over considerable areas 

 is found wholly in poikilitic arrangement inside the feldspars. Rarely 

 a long twinned blade connects this type with the short plumose variety. 

 Many large sections are almost wholly made up of cross-sections of the 

 large fresh feldspars crowded full of these inclusions, often in beautiful 

 dendritic arrangement and on so large a scale that it can be studied with 

 a lens. The magnetite is also in unusual forms — in large perfect octa- 

 hedra, the faces marked by strong cleavage lines or beautifully skeleton- 

 ized with inclosures of the colorless ground or of glass. When, at the 

 surface of blocks etched by the marsh waters, the inclusions have been 

 removed, and the magnetite rusted to a rutile color while retaining its 

 brilliant luster, it looks like a sagenitic network of three dimensions 

 (see plate 27, figure 3). 



A surface from the large and best glass-bearing mass is covered by a 

 fine striated slickensides made up of dark green antigorite with its prisms 

 set at slight angle to the surface. The color of the mineral in a slide is 

 green to brown. It is optically negative, with small optical angle.* 



Gabbroid diabase. — This most abundant variety of diabase in the schlie- 

 ren is so coarse that all the constituents are visible to the eye. It is 

 often entirely fresh, so that the lathe-shaped feldspar is- wholly trans- 

 parent, and the coal black color is given by the magnetite and black 

 pyroxene. The magnetite is often in large, sharp octahedra with stri- 

 ated faces. The pyroxene is in short blades, showing the beginning of 

 the feathery form described" above. Glass appears in small spots, hard 

 to detect because of the freshness and dark color of the rock, and this 

 glass appears even where the grain sinks to the fineness of the common 

 trap. Only the granular calcite and the deep blue quartz appear promi- 

 nently in this variety. Here alone the glass has opaque, black central 

 portions from the great amount of the iron. This variety is especially 

 abundant in the great schlieren along the electric railroad. It may grow 

 even coarser and the pyroxene occur in blades an inch long with the 



*In the Massachusetts state collection no. xvi is a specimen of the coarse porphyritic trap with 

 long plumose pyroxenes, which manifestly came from the reservoir region. It contains many 

 quartz inclosures and cavities with dog-tooth spar crystals. The specimen is labeled " Greenstone 

 passing into syenite (boulder), West Springfield." 



