24 PBOCEEDINGS OF THE NATIONAL MUSEUM. vol. 66. 



from the pyroxene's alteration. The diopside around the borders of 

 the mass is thus clearly pseudomorphous after original augite, being 

 in fact a reaction product between previously formed basic aug'tes and 

 the acidic magma. The pyroxene of the main part of this rock is, 

 however, clear green diopside which shows no evidence of derivation 

 from original augite. It occurs as coarse graphic or plumose inter- 

 growths with albite. While this dendritic diopside is believed to be 

 an original consolidation product, it is possible that it also may be 

 secondary after an original intergrowth of augite, although few augite 

 intergrowths of this sort have been seen in the specimens studied. 

 The diopsides of this plumose form are associated with skeletal 

 octahedra of titaniferous magnetite and crystals of titanite, the latter 

 somtimes replacing the former. 



The feldspar, like that of the normal diabase pegmatite, is in fairly 

 large prismatic crystals showing fine albite and pericline twin lamel- 

 lae. The indices of refraction in all directions are definitely below 

 that of the balsam and the feldspar is now entirely albite, all of 

 which is muddy from dustlike inclusions and is also in considerable 

 part sericitized by the growth of numerous variously orientated mica- 

 ceous flakes, probably of paragonite. These nucleal crystals are sur- 

 rounded by broad haloes of micropegmatite which make up the 

 greater part of the rock (see photomicrograph, pi. 9, lower). The 

 feldspar of the micropegmatite is also albite, in large part continuous 

 with the feldspar of the crystal which it surrounds. It is all mud- 

 died and much of it shows a microclinelike grating structure from 

 the combination of albite and pericline twinning. Apatite is abun- 

 dant in small crystals, especially in the micropegmatite at the coales- 

 cence of two or more separate areas. Texturally this rock differs 

 from the normal plagioclasic pegmatite chiefly in the absence of the 

 long diallagic augites, in the presence of the dendritic diopsides, and 

 in the much greater abundance of micropegmatite. A sample of 

 rock having essentially the texture and composition of the central 

 portion of the specimen illustrated in plate 6, but from a much larger 

 mass, was analyzed. A thin section of the analyzed specimen is illus- 

 trated in the photomicrograph, plate 7, upper. This contained a 

 smaller number of the dendritic diopside intergrowths and no visible 

 iron ore. The larger feldspars were albite with albite and pericline 

 twinnning, and the albite of the micropegmatite was in considerable 

 part twinned to give the microcline grating. All of the feldspar was 

 muddy and some of that of the groundmass had a higher index of 

 refraction very slightly above that of the Canada balsam. The analy- 

 sis, ratios, and norm of this rock are as follows: 



