130 PROCEEDINGS OF THE .AJMERICAN ACADEMY. 



width is here from 2 to 5 inches and it is marked toward the granite 

 by a greater segregation of dark constituents, chiefly rather slender 

 aegirite or riebeckite crystals. These are arranged to some extent 

 parallel to the margin. The same sort of segregation marks also 

 the inner margin. Between these, the rock in texture and composi- 

 tion resembles closely the granite except that the riebeckite and 

 aegirite are considerably more abundant and the mineral grains are all 

 of larger size. The proportion of aegirite to riebeckite is also greater 

 than in the granite. On one side of the pipe the dark margin is 

 less prominent, the whole zone having practically shrunk to an irreg- 

 ular streak of granitic material in which the dark constituents are very 

 abundant, particularly aegirite. 



As a rule the next zone is an irregular one, 1 to 2 inches wide, of fine 

 graphic-granite, containing a few prisms of riebeckite and aegirite. It 

 is the least clearly defined of the zones and passes almost imperceptibly 

 into the main part of the pegmatite. In places the graphic-granite 

 seems to be almost or quite wanting and its place is occupied either 

 by the coarse pegmatite or by a fine-grained mixture of microperthite, 

 quartz, aegirite and a little riebeckite. The main portion consists 

 essentially of a pegmatite or coarse-grained, aegirite granite, carrying 

 some riebeckite, abundant accessory zircon, fluorite, ilmenite, and 

 smaller amounts of other minerals including parisite. The feldspars 

 are an albite-microcline-microperthite of a pale greenish color except 

 where reddish from iron stains. Toward the center where they are 

 the largest, they measure as much as 2 by 2.5 cm. They contain 

 aegirite and riebeckite microlites and black inclusions, also the minute 

 cavities common to all the feldspar of the granite and pegmatites. 

 The quartz, in amount the next mineral present, is without crystal 

 form. Its grains are, except near the center, on the average smaller 

 than the feldspar and of a grayish to white semi-translucent variety. 

 Toward the center the quartz is coarser and more abundant, and at 

 intervals along the length of the pipe, it forms masses two inches or 

 more in thickness and several inches long. In fact the quartz forms 

 a more or less well-defined central core. The aegirite, besides occurring 

 in the form of many small, light-green grains and prisms, forms also 

 abundant, large prismatic individuals of a more or less composite 

 character. These frequently measure 1 cm. in width and 2 or 3 cm. in 

 length, are dark green in color, and form a very striking characteristic 

 of the polished specimens. Riebeckite forms occasional large crystals 

 always more or less completely enclosed by a parallel growth of aegirite. 

 Riebeckite is also usually intergrown in the aegirite crystals and com- 

 monly forms a sort of a central core in many of the crystals. These 



