150 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the pocket, and Figure 16, Plate 3, one of an approximately basal 

 section of a small microcline crystal in a thin section from some of the 

 fine-grained portion of zone C. In these figures one set of lamellae 

 are seen extinguished. Although there is some kaolinization, the cry- 

 stals are as a whole quite fresh. 



The microcline of the microperthite throughout the massive parts of 

 the pegmatites, as well as in the enclosing granite, is also quite fresh. 

 The relative amounts of microcline and albite appear to be fairly con- 

 stant, the latter being slightly greater in amount particularly about the 

 margins, and hence appears as the host. The microcline strips as seen in 

 010 sections appear as narrow bands with somewhat irregular outlines. 

 They have the usual orientation, steeply inclined to the vertical axis, usu- 

 ally extend with fair continuity and variable width nearly across the 

 width of the crystal, pinching out at or near the border. In basal sections 

 they are commonly quite irregular in outline although they preserve 

 in a general way a course across the crystal parallel to the ortho-axis. 

 The twinning in the microcline, when present, is nearly always after the 

 albite law only, "gitter" structure being rare. In many crystals the 

 microcline strips show twinning throughout, in others a portion only of 

 the strips show the twinning, while again a part of one strip will be 

 twinned but not the remainder. There is a close similarity between 

 the microperthite of the Quincy granite and its pegmatites and that of 

 certain dike-rocks (aegirite-gorudite) from Norway. Brogger, Becke and 

 Kloos 8 in writing of these dike-rocks from the neighborhood of Lan- 

 gesundfiord, Laurvik, Fredriksvarn, Lovo, etc., note that the predomi- 

 nant microperthite is a microcline — plagioclase intergrowth in which 

 the microcline has the relations that orthoclase usually does. The 

 microcline departs from the usual habit of granitic microcline in that 

 the lack of the gitter-structure is altogether characteristic, the mineral 

 being often untwinned, or again twinned after the albite law only. 

 Microcline showing this particular habit of twinning has been termed 

 moir(^-microcline by Brogger to distinguish it from the more common 

 "gitter-microcline." The general shape of the perthite lamellae and 

 their manner of arrangement appears also to be much the same as in 

 the Quincy rocks. 



The albite throughout seems to have a composition not more basic 

 than Ab 95 An 5. Some measurements on the smaller separate individu- 

 als indicate that some of it may be nearer an albite oligoclase. The al- 

 bite of the microperthite is characterized by its freshness and relative 

 freedom from included black particles and cavities. As already noted 



8 Zs. f. Kr., 16, 54 et seq. (1890). 



