458 Williams — Eudialyte and Eucolite from Arkansas. 



obtain some very good specimens of this rare mineral, several 

 of which were well adapted for crystallographic measurement. 

 Through the kindness of Messrs. "W. E. Hidden of New York 

 and C. S. Bement of Philadelphia, I was enabled to measure 

 two other crystals from this locality, which were especially 

 interesting. 



The crystals that I have seen from this region range from 

 3 to 18 mm in diameter and are, for the most part, thick tabular 

 parallel to the base. They are transparent to semi-transparent 

 and in color vary from rose-red to deep crimson. Cleavage 

 parallel to the base is indistinct, and the crystals appear to be 

 traversed by irregular cracks in all directions. The cleavage 

 parallel to \R and i?, as noted in the Greenland eudialyte, 

 has disappeared almost entirely from these crystals. The 

 surface of the crystal is in some cases covered by a yellowish 

 coating of altered material ; this, however, does not appear to 

 diminish its brilliancy, but when it occurs on the base increases 

 the luster to mother of pearl. 



The crystals may be divided according to their form into 

 two classes : first, those in which the negative rhombohedrous 

 predominate, and second, those in which the positive ones are 

 the larger. In general, the crystals are terminated above and 

 below by hexagonal basal planes, but these occasionally become 

 triangular or disappear entirely. The most satisfactory crystal 

 for the measurement of angles was one of only about 3 mm in 

 its greatest diameter and of half that thickness. This crystal 

 belongs to that group in which the negative rhombohedrons 

 predominate, and is shown in fig. 1. The measurements from 



-, _ this, combined with those from several 



AvK L. S r^r^y% k other crystals, are given below, and after 

 L' A * '\k ^r^y each measured angle the extreme variation 



^^ " a ~^~J ^/ from the mean is appended. 



T - 3 ' The axial ratio, as calculated from meas- 



urements, made on three very good crystals, of the angle 

 between the base and the largest rhombohedron, — -Ji?, is 

 found to be a : g = 1 : 2*1174. As this is deduced from a 

 mean of not less than ten angles, none of which varied more 

 than 25 seconds from the mean angle, 50° 43' 6", it is evident 

 that it cannot be far out of the way for the Arkansas variety 

 of eudialyte. Brogger in his splendid work, " Die Mineralien 

 der Syenitpegmatitgange der Siidnorwegischen Augite- und 

 Nephelin -syenite,"* considers the latest measurements of von 

 Kokscharow f the most correct for eudialyte. Yon Kokscha- 

 row gives a: c = 1 : 2*1129, which differs but little from the 



* W. C. Brogger, Zeitschrift fur Krvst. und Mineral, xvi, p. 498, 1890. 

 f Yon Kokscharow, Verhandl. der kais. russ. min. Gesellschaft zu St. Peters- 

 burg, II, xiv, 205, 1879. 



