172 PROCEEDINGS OF THE ACADEMY OF [Part I 



magnetite and chromite. Microscopic study indicates that the 

 original rock was a saxonite composed of predominant enstatite or 

 bronzite. 



The plumasite dikes are very irregular in thickness and direction, 

 and are composed of white granular oligoclase on the margins; to- 

 wards the center corundum makes its appearance with silvery white 

 margarite; in places corundum and margarite may occur to the ex- 

 clusion of the oligoclase. The composition of the plumasite veins 

 varies considerably, in some places being entirely feldspathic, and 

 elsewhere quite rich in corundum. The oligoclase forms granular 

 aggregates, only one third of which exhibits repeated twinning, 

 although the bulk of the remainder possesses a higher index of re- 

 fraction than balsam. The corundum forms dark gray, grayish 

 green or bluish crystals, which are frequently sheathed by a thin 

 shell of primary margarite. At times secondary margarite pene- 

 trates the basal cleavage plates of the corundum, or forms margari- 

 tized individuals with remnants of corundum. 



The most interesting plumasite dike pierced a large sill of serpen- 

 tine flanked below by granulite which was exposed through a ver- 

 tical range of 150 feet. In the granuHte the dike was a normal 

 granitic pegmatite composed of quartz, orthoclase, and subordinate 

 plagioclase, with hardly any biotite, and no corundum. In the 

 serpentine above the dike had contracted in width, lost all its 

 quartz, but now carried corundum. 



Du Toit advanced the view that the plumasite owes its genesis 

 to the desilicating action of the serpentine walls upon the originally 

 acid magma constituting the dike. Evidence of reaction between 

 the invading acid magma and its ultrabasic walls is quite evident. 

 Between the plumasite and the serpentine is a contact zone from 

 a few feet to 10 feet in width. Adjacent to the plumasite the zone 

 consists of a belt of brown mica, variable in width, which passes 

 laterally into a proportionally wider belt of massive or foliated 

 talc. The mica is brownish, with weak to moderate pleochroism, 

 and an axial angle of 2° to 20° or 30°, and considered to consist of 

 both biotite and phlogopite.'^ Irregular or lenticular patches of 

 the mica occur as xenoliths in the plumasite. Isolated patches of 

 the talc may occur in the serpentine. In the reaction between 

 the pegmatitic solutions and the serpentine the serpentine received 

 an addition of silica, alumina, potash and fluorine to form the 



1^ More probably vermiculite. 



