Williams — Eudialyte and Eucolite from Arkansas. 459 



ordinarily accepted figures, a : c = 1 : 2 - 1117, while the value 

 for the Arkansas variety is considerably larger. 



The following faces have been observed and measured : 



c = 07? (0001), a = ocP2 (1120), 72 = + 7? (lOll), 

 d= —^7? (0112), « = — 27? (0221). 





Faces. 



Mean angle. 



Calculated. 



Variation. 



Von Koksch. 

 Calculated. 



c 



d, (0001): (0112) 



50° 43' 6" 



50° 43' 6" 



0' 24" 



50° 38' 



c 



7?, (0001) : (10ll) 



67 53 48 



67 45 24 



3 12 



67 42 



c 



n, (0001): (0221) 



78 25 



78 26 35 



10 30 



78 25 



c 



a, (0001): (1120) 



89 56* 



90 



15 



90 



d 



a, (1012): (11 20) 



47 49 30 



47 54 29 



4 30 



47 58 



d 



d, (1012): (0112) 



84 21 



84 11 2 



1 reading 



84 4 



The variation of these angles and of the axial ratio from 

 those of von Kokscharow would suggest some corresponding 

 variation in the chemical composition, and it is probable that 

 an analysis on which Mr. Hidden is engaged will bring out 

 this difference. 



An excellent example of that class of crystals in which the 

 positive rhombohedrons predominate is found in a small speci- 

 men — not more than 4 mm in its greatest diameter — loaned me 

 by Mr. C. S. Bement. This is shown in fig. 2. In this crystal 

 one face, -f-i? (1011), is so over-devel- / v ■ ■ c ^^ -T v^ 

 oped that all the rest of the faces seem £/ > .X\*'% 



dwarfed by it. At the first glance, and ^Tjtr W r ^-^>-&g|^ 

 in fact until the crystal is carefully meas- ^C '\\ "\? ~ :: %-/? 7 

 ured, this large face would be mistaken nL V V* / 

 for the base, and the fact that opposite %/. / '■/ 

 to it, the crystal is terminated by a six- ^■■-■ a ---\ ^u^ 



sided pyramid would seem to supoort this view. 



The crystal is, in reality, to be placed as shown in the cut, 

 and the faces, which make up the front of the crystal, are des- 

 ignated by the same letters as in the preceding figure. The 

 faces forming the back are for the most part composed of 

 known forms in oscillatory combination. Besides these well 

 known faces a large number of new ones appear, of which, 

 however, only a few are large and sharp enough to be worthy 

 of special mention. The most important of these forms are as 

 follows : 



— f, R (0.3. 3. 11), ifl(10l4), ^7? (1015) and -|7?5 (2363). 



The angles, measured as nearly as possible, were as follows : 



Angle. Measured. Calculated. 



07? : — A-T?, (000 1 ) : (0 • 3 • 3 • 1 1 ; 33° 54" 33° 41' 52" 



07?: i-7?, (0001): (1014=) 31 54(±30 y ) 31 26 12 



07?: IB, (0001): (1015) 25 59 33" 26 3 35 



The faces at the back of the crystal lying in the vertical 

 zone and beginning at the top are : 07$, 0<)01 ; — T 3 r 7?, 3 • • 3 • 11 ; 

 * Brogger, 1. c, measured this angle on Norwegian eucolite as 89° 58' 30". 



