460 Williams — Eudialyte and Eucolite from Arkansas. 



-IR_, 1012 (large); -±JR, 101m; (point at back) £i?, lOln; 

 J7?, 1011 (large but dull) ; \R, 1015 and 0i?, 0001 (bottom). 



The right and left inclined zones at the back are made up of 

 the following faces ; 7?, 0111 and 1101 ; the prisms a/^, 1120 

 and 2110; (point at back) -%R, 1102 and 0112. The small 

 zones lying back of a ( ccP2, 1210 and 1210) are made up of a 

 recurrence of the prisms with the scalenohedrons — £ Ro, 2363 

 and 6323 (determinable only through zone relations.) 



In order to make sure that the crystal was properly placed 

 and the faces correctly determined, it was imbedded in a short 

 piece of glass tubing just large enough to hold it, and which 

 was filled with Canada balsam. This was then placed on an 

 object glass and a thin glass cover placed over it.* The out- 

 side of the glass tube was covered with black paint, in order to 

 cut off any side reflections, and the preparation was then exam- 

 ined under the polariscope. Owing to the thickness and irregu- 

 larity of the crystal, the black cross was not very plain, but it 

 was evident on revolving the stage of the polariscope that there 

 was no extinction such as was observed when the crystal was 

 placed in any other position. 



The specific gravity is comparatively low and lies between 

 2-804 and 2-833" at 15° C. These values were obtained from 

 those crystals which were measured, and the lowest was that 

 of the crystal figured in No. 2. The determination was made 

 by means of a cadminm-borotungstate solution, in which the 

 crystals were placed and which was then brought to such a 

 density that the mineral remained suspended, neither rising 

 nor sinking. The specific gravity of the solution was then 

 determined by means of a 12 cc. pycnometer. 



Thin sections, cut at right angles to the vertical axis, show 

 in parallel light a pink color, and between crossed nicols remain 

 perfectly dark during a complete revolution of the stage. In 

 convergent polarized light such sections show a wide black 

 cross which sometimes opens a little owing to slight optical 

 anomalies, but no colored rings appear. Both the double 

 refraction and the index of refraction are weak. The index is 

 lower than that of Canada balsam and the surface of the section 

 appears smooth. The character of the double refraction is 

 positive. By sinking the polarizer and the converging lens, 

 the irregular cleavage, which lies approximately parallel to 

 \R (1014), becomes visible. 



The mineral is comparatively free from inclusions for a 

 crystal which was formed as late in the period of solidification 

 of the rock as this. Magnetite and segyrite or acmite are the 



* Since makiDg this experiment I have received Professor C. Klein's exhaustive 

 paper on this method of examining: crystals without cutting them. JSitzungsber. 

 d. k. Akad., Berlin, xviii, p. 347, 1890. 



