370 



University of California Publications. 



[Geology 



riebeekite and crossite-rhodusite, tl-.c— 10°-21°, optic character 



positive), an identification of the mineral without analysis is 



valueless. 



The constitution of crocidolite is : 



NjSi0 3 

 3(FeMg)Si0 3 



Fe,Si 3 9 Fe" : Mg = 4:1 minimum. 

 ± H 2 Si0 3 



i.e., the formula of a rhodusite where almost all Mg is replaced 

 by Fe" ; except for water, it is the identical constitution of the 

 riebeckites of Colorado and of Cape Ann, Massachusetts. 



The blue asbestos-like amphibole in the syenite of Spanish 

 Peak, Plumas County, California, is identical (optically) with 

 the blue amphibole from Eosita Hill described by Cross. Pleo- 

 chroism: a = Prussian blue, b = lighter blue, C = greenish 

 blue. Absorption very strong: a>b>C; the colors are very 

 intense. Angle of extinction (in the opposite side of that of 

 katophorite) a:c = +21° (C:C==-69°). The axial plane seems 

 to be longitudinal; on account of the absorption and dispersion 

 (pO) one cannot determine the optic character, y — P very 

 small, almost zero; P — " = 0.00-1 ca. 



It is very important to note that some sections from this sye- 

 nite and some from Eosita Hill (a section in the collection of Dr. 

 Palache) show such orientation on (001) and (010) that we can 

 suppose the axial plane to be perpendicular to the plane of sym- 

 metry: &:c, t — b. This fact indicates that crocidolite under 

 goes the same variation in its properties as glaucophane, and in 

 the same way.* 



Fig. 2. — Crocidolite (Cross' amphibole) with axisjl plane normal to the plane 

 of symmetry, a, basal section, b, clinopinacodal section. 



* In Vienna I discussed the chief points of this paper with Professor 

 F. Becke, and Dr. C. Hlawatsch. The latter showed me an arfvedsonite-like 

 amphibole with transverse axial plane (a:c; t — b), which he was studying. 

 In the same rock there occur arfvedsonite and ossanite (n.s. Hlawatsch), 

 like the crocidolite from Eosita Hill and Spanish Peak, with transverse 

 axial plane. 



