212 PROCEEDINGS OF THE AMERICAN ACADEMY. 



deep-seated differential alteration of the granite mass. In the same 

 way there are streaks and patches of much lighter color than the 

 average. This phase shows many aegirite microhths in the feldspar 

 but little riebeckite. In some of the quarries (Faulkner's, North 

 Common Hill) a rather poorly defined zone of very light granite was 

 observed on either side of a small quartz vein, and there appeared to 

 be some connection between the \'ein and the wliitening of the adja- 

 cent rock. 



The pink and reddish granites (see later) are the result, for the most 

 part, of surface oxidation of the iron content of the hornblende, etc. 

 Along shear zones and in the neighborhood of trap dikes the granite 

 has often a dark, greenish color due to the presence of streaks, 

 disseminated scales and masses of some dull green, secondary mineral, 

 probably chloritic, but whose composition has not been more closely 

 investigated. 



Microscopic. — The minerals observed in thin sections, are: an 

 albite-microcline microperthite, quartz, soda-iron amphiboles, in part 

 riebeckitic, in part cataphoritic, aegirite and sometimes a little of 

 some closely related pyroxene; accessory minerals — aenigmatite, 

 astrophyllite, zircon, titanite, iron oxide minerals, fluorite and very 

 minor amounts of various alteration products. 



The feldspar content of the rock is normally almost entirely a micro- 

 perthite. Its grains are roughly equidimensional in cross-section; 

 their elongation is in the direction of the edge 001/010. The outlines 

 are always xenomorphic, except that occasionally in contact with the 

 other minerals, particularly quartz, they show some development of 

 crystal planes. It, nevertheless, clearly dominates the texture of 

 the rock. The tMO members of the microperthite are very finely inter- 

 grown following the well known law for such intergrowths. The 

 relative amounts of the two feldspars present in different grains is 

 probaljly pretty uniform, although considerable apparent variations 

 may be noted in the sections of different crystals and even in the same 

 one. From point to point in a crystal there is undoubtedly consider- 

 able variation. The plagioclase member, as judged by its optical 

 properties, is very near albite and this is borne out by the chemical 

 evidence derived from the rock analyses. Its most probable composi- 

 tion appears to be Abgg Ano to Abgs Ans. It is commonly, though 

 not always, very finely twinned after the albite law. Locally in 

 crystal sections where it predominates over the microcline, the twinning 

 lamellae are broader and more uniform. A commonly noticed feature 

 of the albite is its predominance about the ends and margin of the 



