134 PROCEEDINGS OF THE AMERICAN ACADEMY. 



similar to the Ballou pipe. Figure 1 is a sketch of this section and 

 with its legend will serve to give a general idea of the character 

 and arrangement of its parts. It should be noted that the demarka- 

 tion between the zones is not usually as sharp as those in the figure. 

 Figure 13, Plate 1, a photograph, gives also an idea of its structure 

 and appearance. 



The dark marginal zone (A in the figure) is here somewhat finer in 

 grain than the granite and much darker in color. There is a distinct 

 arrangement of its minerals parallel to the margins which are also 

 characterized by a greater abundance of dark silicates with which are 

 numerous small grains of black oxides and chalcopyrite. The zone is 

 particularly dark, fine grained, and sharply defined about the upper 

 side of the pipe and in one place is sharply offset. Except in its finer 

 grain, the parallel arrangement of its minerals and the greater propor- 

 tion of dark minerals, particularly aegirite, which occurs to the ex- 

 clusion of the riebeckite in some of the slides examined, the rock of 

 this zone appears in thin section under the microscope to be much like 

 the granite ; the textural relations of the minerals are the same and 

 their composition appears to be identical. 



The zone of graphic-granite (B of Figure 1) seems to be practically 

 continuous just inside the dark band above described and with a width 

 of from 2 to 4 inches passes almost imperceptibly into the main zone 

 of coarse granite or pegmatite within. 



In the graphic-granite, individual cleavage surfaces of the feldspar 

 will perhaps average two to three cm. on a side. Their long direction 

 stands generally perpendicular to the margin. In this occurs, some- 

 times quite plentifully, riebeckite crystals somewhat intergrown with 

 aegirite. The feldspar is a microcline-albite-microperthite. As de- 

 termined by a Rosival measurement made on two extra-large thin 

 sections it makes up approximately 60 per cent of the intergrowth. 

 The albite appears to be slightly in excess of the microcline and always 

 acts as the host. It strongly predominates about the margins of the 

 crystals and quite generally forms practically the outside of the per- 

 thite crystals. The feldspar contains the same microliths of aegirite 

 and riebeckite, the black specks and cavities, as do those of the granite. 

 The contacts of the feldspars (largely the albite member) and the 

 quartz are mutually indented and show what has been described by 

 Pirsson^ as the " interdented " texture. Minute rounded grains of 

 quartz are common just within the feldspar along the contacts, and 

 suggest chains of tiny islands in appearance. The quartz contains 



Am. J. Sci., 23, 272, 1907. 



