362 



University of California Publications. 



[Geology 



over the region that if completely developed would be occupied 

 by the faces P and P, where they form the salient angle, and 

 over part or all of the area of I, I (110). The re-entrant angle 

 between the bases is therefore uniformly presented towards the 

 observer and the basal planes are very decidedly the most lustrous 

 faces on the crystals. The faces o and j) are very largely 

 developed, being sometimes longer than M and M and the crj^stal 

 appears prismatic parallel o, o, p, p. 



These dominant faces are modified by the following: /(130), 

 z (130). v (120), c (111), x (101), 8 (TT2), © (131), u (221). 

 The form x shows an unusual lack of prominence, occurring as a 

 very narrow strip. Of the others, u (221) is a rather uncommon 

 form for albite, and c (111) and (120) are very rare. The 

 former (c) is reported by Jeremejew 16 from the Lake Baikal 

 region; the latter, -q (120) by Klockmann from the Riesengebirge 

 granitic rocks. 17 As far as known to the writer © (131) has not 

 been previously reported as occurring on albite. The form c is 

 very narrow but bright; y, u and © are generally progressively 

 broader in the order named, the last mentioned being sometimes 

 over y 2 millimeter wide. The relations of these planes are shown 

 in figures 5 and 5a, plate 38, which were drawn from a crystal 

 about 5 millimeters high. 



The following measurements were obtained, the calculations 

 being based on Brezina's elements for pure albite: 



Measured. Calculated. 

 <t> p <t> P 



P(001) 



81° 



56' 



26° 



50' 



81° 



51' 



27° 



01' 



,¥(010) 







set 









00 



90 



00 



'(110) 



60 



27 



90 



00 



60 



25 



90 



00 



T(1T0) 



119 



54 



se 



fc 



119 



52 



90 



00 



,,(110) 



138 



47 



90 



00 



138 



42 



90 



00 



f(130) 



30 



16 



90 



00 



30 



23 



90 



00 



0(130) 



149 



47 



90 



00 



149 



44 



90 



00 



«(T01) 



80 



50 



25 



58 



80 



54 



25 



48 



r(lTl) 



108 



28 



57 



35 



108 



18 



57 



27 



o(ITl) 



T35 



26 



34 



07 



T35 



21 



34 



11 



5(TT2) 



177 



00 



11 



42 



176 



17 



11 



38 



0(131) 



T63 



19 



59 



13 



T63 



30 



59 



13 



lt(22l) 



T25 



30 



60 



50 



T25 



26 



60 



50 



i«Z«i. fur Kryst., 32 (1900), on albite, pp. 494-495. 



" Zeit. d. deutschen geol. Gesell., 34 (1882), on albite, pp. 416-426. This 

 form is characterized as doubtful by Dana, System of Mineralogy (1892), 

 p. 328; and as "nicht ganz sieher" by Hintze in his Handbuch der 

 Mineralogie, 2 (1897), p. 1447. 



