128 PROCEEDINGS OF THE AMERICAN ACADEMY. 



coarser material. The largest vein lies some ten yards north of the 

 contact mentioned and has been traced for at least 75 yards in a nearly 

 E. W. direction. The maximum width of this vein is about 12 

 inches. Along its strike, at least to the east, in which direction it is 

 best exposed, it tapers to two or three inches in width. Its contacts 

 are well defined, but are in the nature of a sudden coarsening of the 

 granite. Its structure is symmetrical and indicates successive stages 

 of formation. First come bands of coarse material 2 to 5 inches in 

 width. This consists essentially of microperthite, albite, quartz, and 

 riebeckite. The riebeckite is very conspicuous, suggesting tourmaline 

 at first sight, and is the most abundant mineral present. While many 

 of the riebeckite prisms are of small size, the average will measure from 

 1 to 1 i cm. thick and perhaps 5 cm. long. Many crystals are much 

 larger, measuring 1 to 1^ cm. thick and 10 to 12 cm. long. There is a 

 strong suggestion that there was originally an orientation of these 

 normal to the walls, although they now lie at random and occasionally 

 parallel to them. Although they frequently include feldspar grains, 

 sometimes arranged symmetrically to their own structure, and are 

 deeply indented (or corroded) about the margins, they nevertheless 

 commonly show a fairly good crystal cross-section consisting of w, 110 

 and h, 010. They are commonly fractured and the fractures are filled 

 with fine-grained quartz and feldspar. This band is succeeded by a 

 narrow gray streak of much finer grained (1 to 2 mm.) which in turn 

 is followed by a narrow (1-2 cm.) streak of a light brown color, of 

 somewhat coarser grain than the preceding. In the gray streak the 

 riebeckite crystals lie irregularly or, if an)i;hing, show a tendency to 

 stand normal to the contact. In the central streak there is a marked 

 orientation parallel to the contact. The microscope shows that this 

 portion of the vein was probably undergoing movement during the 

 process of crystallization. The larger grains are much disturbed in 

 structure, broken, crushed, recrystallized particularly about the margins, 

 and are surrounded by very fine material consisting largely of albite or 

 quartz grains with shreds and particles of riebeckite. Fluorite, ilmenite 

 (or magnetite) and zircon are accessories. There has been considerable 

 kaolinization, and ferruginous decomposition products and carbonates 

 are often present. Except for the tendency of the riebeckite to develop 

 rough prismatic outlines in the prism zone, the minerals are xeno- 

 m Orphic. Aegirite is entirely wanting. It may be noted that the 

 enclosing granite here shows very little aegirite. 



These pegmatitic developments are entirely different in character 

 from those to be described later and certainly have a somewhat differ- 

 ent origin. While no positive conclusion has yet been reached regard- 



