from the Rosita Hills, Colorado. 473 



i mm . An examination with a hand lens shows that the crystals 

 are for the most part irregular granular aggregates of some 

 mineral possessing a distinct cleavage. This is especially clear 

 on fractured surfaces by the irregular positions of the cleavage 

 planes. In some of the thicker crystals, broken through the 

 center, is seen a clear glassy kernel of a colorless mineral, and 

 its position immediately suggests that it represents the original 

 substance of the crystal, of which the granular aggregate about 

 it is a pseudomorphic alteration product. 



Some of these white crystals, carefully detached, but sup- 

 posed to include the points of some of the quartz crystals upon 

 which they were deposited, were given to Mr. Eakins for 

 analysis. He found : 



Molec. ratio. 



A1 2 3 38-91 -381 3-42 



KO 4-03 



Na 2 4-32 j" • 113 lm01 



S0 3 35-91 -446 4'00 



H 2 ... 13-03 -724 6-49 



CaO 0-35 



Si0 9 ... 2-82 



MgO _ tr. 



99-37 



Tests showed that the silica came chiefly from included 

 quartz grains. The remaining constituents, aside from the 

 small amount of lime, are those of alunite, and by the usual 

 calculation it is seen that they are present in the required ratio 

 for that mineral, with a slight excess of water and of alumina, 

 which can be referred, with a portion of the silica, to admixed 

 kaolin, an observed associate of the crystals. 



The unexpected result of the analysis was followed by a 

 microscopical examination of thin sections prepared parallel to 

 the basal plane of several crystals, and normal to that plane in 

 a crystal with a glassy kernel. The fresh core possesses a 

 cleavage parallel to the basal plane of the crystal, and its 

 optical properties seem to be throughout those of alunite 

 The outline of the kernel is irregular. About the fresh alunite 

 kernel is an aggregate of irregular particles, arranged without 

 reference either to the outer crystal form or to the inner core. 

 Those grains which are decidedly elongated polarize brightly, 

 have sharp lines of cleavage parallel to the length, and the 

 direction of major elasticity is always parallel to the cleavage 

 lines. Many sections are not so elongated, and these polarize 

 less brightly, some giving only gray tones, while a few are 

 almost isotropic. The latter show a positive interference cross 

 in convergent polarized light. 



