GRANITE 225 



silicates were present in the rocks originally could not have been abundant 

 because of the present small amounts of secondary minerals that could 

 have been derived from them. 



The most abundant constituent of the rock is feldspar, which is chiefly 

 orthoclase and microcline, and always some sodic plagioclase (albite- 

 oligoclase) in varying amount, almost equaling in a few thin sections the 

 potash varieties. Much of the orthoclase is intergrown with albite as 

 microperthite, but graphic intergrowths with quartz are rare. Quartz is 

 next to feldspar in importance and is of the usual kind in granites. The 

 common accessory minerals are the same as those observed in the syenite 

 which comprise magnetite, apatite, and zircon. Others sometimes noted 

 are needle-like inclusions of rutile in some of the quartz, garnet in irreg- 

 ular masses, and crystals frequently partially or completely altered to 

 chlorite and epidote. 



Noeite 



general discussion of characteristics and distribution 



Gabbros have thus far been observed by the writers only at several 

 localities in the Blue Eidge region of Virginia in association with the 

 pyroxene syenite. In each instance the rock is hypersthenic, and is there- 

 fore a norite. So far as our field investigations have extended, two 

 varieties of the norite have been studied, the most abundant one of which 

 is a hornblende norite (analysis III, page 229), the other a variety abnor- 

 mally rich in apatite and ilmenite (analysis I, page 229) and closely re- 

 sembling in this respect the gabbro-nelsonite 38 (roselandose) of the high 

 titanium-phosphorus rocks of the Amherst- Nelson counties comagmatic 

 area near the southeastern foot of the Blue Bidge. Both varieties are 

 rich in hypersthene and diallage (see analysis II, page 229, and map, page 

 195). Analyses of these two varieties of gabbro (I and III) are given in 

 the table of analyses on page 229, with which are compared analyses of 

 gpbbro and gabbro-nelsonite of the Amherst-Nelson counties area 39 (II. 



38 Nelsonite, a new rock type, is the name that was given by Watson to a group of 

 high titanium-phosphorus-bearing rocks of igneous origin, occurring in dikelike bodies 

 of varying size and irregular shape in the Amherst-Nelson counties region, Virginia, and 

 to a less extent farther southwest on the northwest slope of the Blue Ridge in Roanoke 

 County. The name gabbro-nelsonite. a new rock type, was proposed by Watson and 

 Taber for a holocrystalline igneous rock having a mineralogical composition interme- 

 diate between normal gabbro and nelsonite proper. The nelsonites proper belong to 

 class V of perfemanes, but occupy new rang and subrang positions in the quantitative 

 system, and accordingly are new rock types to which appropriate magmatic names have 

 been given. See T. L. Watson : Mineral resources of Virginia, 1907, p. 300 ; Watson 

 and Taber : Bull. 430, U. S. Geol. Survey, part i, 1909, pp. 200-213 ; University of Vir- 

 ginia publications, Bull. Phil. Soc, scientific section, vol. 1, 1913, No. 14, pp. 331-333 ; 

 Bull. III-A, Virginia Geol. Survey, 1913, pp. 100-155. 



30 Watson and Taber : Bull. III-A, Virginia Geol. Survey, 1913, pp. 138-145. 



XVII— Boll. Geol. Soc. Am., Vol. 27, 1915 



