374 .T. K. KKMP GRANITES OF ATLANTIC COAST 



but they are si,u;nificaiit as bearing on the general production of micro- 

 perthites. A section i)arallel witli the basal pinacoid is illustrated in plate 

 39, figure 1. In this connection it is interesting to note that Descloiseaux 

 remarked the presence of inclusions of albite in a specimen of micro- 

 cline from the McCurdy quarry at Lyme, but he speaks of them as being 

 extremely minute.* 



One microcline crystal, about 6 inches on each axis, quite perfectly de- 

 veloped and doubl}^ terminated, although attached at the end of the 

 a-axis, was obtained on Masons island. 



The biotite of the pegmatites is a brownish green and is strongly 

 biaxial. One small vein of it and garnet was found on Masons island, 

 Mystic, as is illustrated in plate 40, figure 1. It is a rather brittle 

 variety. The quartz was the last of the chief components to crystallize 

 and forms a brittle, interstitial filling, not otherwise noteworthy. The 

 ilmenite occurs in tabular crystals varying in thickness up to 6 milli- 

 meters. The flattening is parallel with the base. One rhombohedron 

 was noted, but it was too rough for sharp determination. Apatite has 

 been discovered once, and that on Masons island. The crystal is bounded 

 by the hexagonal prisms of the first and second orders, but is not ter- 

 minated by faces. Tourmaline is rare in the pegmatites all along this 

 portion of the sound, although it is abundant farther west. Only once 

 was it noted, and that was at Millstone point. Molybdenite is very 

 abundant in a pegmatite that is exposed in a railway cut just east of 

 South Lyme. It forms large flakes, sometimes a square inch in area, 

 and is thickly set in the rock. In the neighboring quarry, on the other 

 side of the track, the exceptionally good monazite crystals were discov- 

 ered which have been described by W. D. Matthew.f They speciall}'- 

 favor the large leaves of biotite and occur closely packed in under the 

 latter. This association facilitates their discovery, for they are rare at 

 best. Matthew also figures one of the manebacher twins of microcline, 

 regarding it as orthoclase. 



The only other minerals which have been noted in the pegmatites are 

 magnetite and muscovite. The former exhibits the usual lamellar planes 

 of growth parallel to the octahedron, and the latter is in no way note- 

 worth}'. 



The proportions of the several minerals in the pegmatites var}'- con- 

 siderably, but in the normal specimens one might say that red micro- 

 cline is most abundant and makes up about 50 per cent of the whole. 

 Natron-orthoclase and albite follow with about 25 per cent, then quartz 

 with perhaps 20 per cent, leaving 5 per cent for all the rest. Quartz 



* American Journal of Science, October, 1880, p. :«.5. 



fW. D. Matthew: Mouazite and orthoclase tVom South Lyme, Connecticut. School of Mines 

 Quarterly, April, 1895, p. 231. 



