Crystallization of Parisite. 



547 



Riebeckite. — The riebeckite occurs in the form of elongated 

 prismatic individuals, possessing a black color and a lustrous 

 cleavage, scattered through the coarser-grained main portion of 

 the pegmatite pipes. The associated minerals are quartz, 

 microcline-albite-microperthite and aegirite with accessory zir- 



Fig. 5. 



Fig. 5. Miero-photograph of a basal section of a small microcline crystal 

 in the fine-grained portion of pegmatite in the Fallon quarry. Shows twinning 

 after the albite law only. One set of lamellae are extinguished at an angle 

 of 16° with the trace of 010. 



Crossed nicols. Magnification about 250 diameters. 



con, fluorite and ilmenite. It varies in size from quite small 

 individuals up to crystals one or two centimeters thick and ten 

 or twelve long ; individuals a centimeter thick and five or six long 

 being quite common. The crystals show a tendency toward a 

 crystal outline consisting of the unit prism occasionally trun- 

 cated by the clinopinacoid. The immediate boundaries are, 

 however, always more or less intergrown with the surrounding 

 minerals. Terminal planes are not observed. Measurements 

 of the prismatic cleavage made on two individuals yielded 

 identical values of 55° 05', an angle considerably larger than 

 that of common hornblende, which is 55° 49'. The mineral is 



