Vol. 5] 



Louderback. — Benitoite. 



361 



To get a more definite idea of the chemical nature of the 

 amphibole occurring in the druses carrying the perched natrolites, 

 a separation was attempted. The material is essentially of the 

 type just described as related to crossite, but contained a small 

 amount of actinolitic and other amphibole material in spots and 

 zones on the crystals, and also a very small amount of natrolite 

 that could not he entirely separated. Professor Blasdale's analysis 

 follows : 



Amphibole of Druse Crossite, North Berkeley (Smith) 



SiO, 



52.94 



55.02 



ALO :i 



3.76 



4.75 



Fe 2 O s 





10.91 



FeO* 



13.40 



9.45 



MnO 



1.44 



trace 



¥gO 



11.54 



9.30 



CaO 



5.45 



2.38 



Na 2 



5.11 



7.62 



K 2 



0.43 



0.27 



H,0 at 110° 



1.31 





Ign 



3.72 



f 





98.67 



99.70 



*A11 Fe assumed to be ferrous. 



Albite. 



Albite has not been found in direct association with benitoite 

 or neptunite. It is common in minute veins in some of the sur- 

 rounding rock, where it is generally granular and determinable 

 only by means of the microscope. But in places it is developed in 

 druses in the zone of mineralization and appears as crystals 5 to 

 10 mm. long growing from the blue-green amphibole wall. These 

 crystals are generally translucent grayish or greenish in hue, due 

 to included amphibole fibres. 



Most commonly the crystals are twinned according to the albite 

 law. Simple twins are the rule, but sometimes there are several 

 very thin polysynthetic lamellae intercalated between the two 

 main halves of the twin. The habit is defined by the dominance of 

 M, M (010) ; /, I (110) ; T, T (110) ; o, o_ (111) ; p, p (Til) and 

 P, J 3 (001). The forms are named in order of size. M is the 

 largest though never so large as to produce a distinctly tabular 

 habit, T and o about the same, and P distinctly smallest. The 

 general mode of growth is such that the crystals are attached 



