496 THOMAS T. READ 



tion products of pinite and other secondary minerals having 

 replaced it to a considerable extent. In most cases, however, 

 what Dana describes as the first stage in the alteration of iolite 

 has taken place — namely, the division of the iolite prisms into 

 plates parallel to the base, and the development of a pearly 

 luster on the plates. This pearly luster is especially prominent, 

 and at first glance would deceive one into believing that the 

 mineral is muscovite. The hardness and brittleness of the plates 

 quickly dispel that illusion, however. It is interesting to note 

 that, usually, a nodule displays this luster equally all over its 

 surface at the same angle of illumination, showing that the cr3^s- 

 tallographic axes coincide throughout the mass, as though it 

 were originally a single crystal. 



In the hand specimen the nodule appears practically homo- 

 geneous, but in thin section, in addition to the iolite and altera- 

 tion products, grains of quartz appear. How much quartz is 

 present cannot be readily determined, for where the interference 

 figures cannot be obtained quartz and iolite are indistinguishable, 

 except by microchemical tests. I do not think the amount is 

 large, however. The iolite shows its characteristic pleochroic 

 halos, and often contains inclusions whose nature could not be 

 determined. 



The ground-mass of the schist is ordinary muscovite-biotite 

 schist, the two micas being about equal in amount. It seems 

 extremely probable that the muscovite has resulted from the 

 alteration of grains of iolite scattered throughout the rock mass, 

 as it is slightly more brittle than the ordinary muscovite. 



The origin of the iolite, and, more especially, the origin of 

 the nodules, is a question that could not be answered without 

 further study of the region and also of the rock. 



Three miles northeast of Pearl, in Wyoming, and apparently 

 diametrically across the pegmatite area from the iolite schist, is 

 another interesting belt containing veins of argentiferous lead 

 and zinc ores. It is a schist resembling in a general way that in 

 which the iolite occurs, and it, too, is full of nodules. These 

 are irregularly prismatic in shape, differing from the iolite 

 nodules in that respect. Further, they consist entirely of quartz 



