O, II. Williams — Celestite from West Virginia, 185 



(Observed.) (Auerbedh.) 



0*0 (Oil): (Oil) 103 33' I'M B' 1 l 



d~d (102): (tOS) mi 26' 101° 11' 



d*c (102): (Oon 139 5t' 1 10' 35'30* 



X~o (U4):(0ll) 166° 60'— 166° 12' lC.v I7'n 



X*X 1): (144) 150" 20'- 153° 10' 161° 36' J 



The variations in the angles measured against the face y are 

 due to its extremely rounded character and to the flatly convex 

 prominences with which it is invariably covered (figs, 8, 9, 10 



and 11). Quite as reliable measurements were made on the 

 largest crystals with a hand goniometer, which gave for the 

 angle (144):(l44) values between 150£° and 152£°, with an 

 average closely approaching 151J . This is sufficient to fix the 

 symbol of the predominating form, since the corresponding 

 angle for the pyramid Pi (155) is 157° 7' ; and for P* (133), 

 142° 47 . 



The irregularities of growth which have interfered with the 

 evenness of the planes on the celestite crystals, do not appear 

 to have materially affected their internal homogeneity. This 

 was shown by an optical examination. A section cut parallel 

 to the macropinacoid, a, showed a normal interference figure in 

 converged polarized light, by means of which the optical angle 

 was measured in a Thoulet solution of specific gravity, 2*922. 



This gave for lithium light, 2H a = 49° 04', 

 and for sodium light, 2H a =49° 28'. 



From Goldschmidt's curve* the index of refraction for this 

 solution was found to be for lithium light 1*666, and for sodium 

 light li>75; and, assuming the mean index of refraction of 

 celestite, ft, to be for lithium light 1621, and for sodium light 

 1*624,+ we obtain as the true optical angle : 



Li 2V a = 49°18' 



Xa 2V a = 49° 54' 



The celestite crystals were subjected to a chemical examina- 

 tion by Mr. W. F. Ilillebrand of the IT. S. Geological Survey, 

 who reports that they are composed of strontian sulphate, in 

 which he was able to detect only a faint (spectroscopic) trace 

 of BaO and 0*12 per cent of CaO. The adjacent limestone 

 was also tested for strontian which was found so abundantly in 

 the form of the sulphate as to indicate that the rock was 

 strongly impregnated with celestite substance for some dis- 

 tance. 



Celestite crystals of a pyramidal habit are not common, but 

 they have, nevertheless, been described from several localities. 



* Neues Jahrbuc-h fur Min., etc. Beil. Bd. i, PL vii. 1881. 

 f Cf. Arzruni: Zeitsclir. fur Kryst., vol. i, p. 179. 1877. Babcock : Neues 

 Jahrbuch fur Min., etc., 1879, p. 838. 



