' Pikes Peak, Colorado. 



somewhat less than half of the complete crystal. The ac 

 panying figure represents the smaller crystal in about the i 

 ral size ; the other one measures nearly 7 cm 

 in longest diameter, and has the same faces 

 developed in a similar manner. In neither 

 crystal do any faces of the vertical zone ap- 

 pear, thus producing a flat lenticular habit- 

 us. The forms appearing have been identi- 

 fied as R, -\ {-\R\ -1 (-R), and f-2 (f P2), and although>ll 

 faces are too rough to admit of exact measurements with the 

 reflection-goniometer, the size of the faces and their simple de- 

 velopment renders sufficient accurate results with the hand 

 instrument possible. The angles obtained, as means of several 



!(',>, 



The figures of ih<- third column arc the calculate,! angles given 

 for phenacite by Dana* or Seligmannf, or else our own calcu- 

 lations based on the theoretical values given by them. The 

 agreement between the angles measured on these crystals and 

 the theoretical ones is sufficiently close to justify the signs 

 given to the faces of the figure. In the development of the 

 different forms, R and -| are always prominent, while the 

 faces of f-2 are variable. One face of f-2 on crystal b is 2-5 cm 

 broad, although usually each face of -f is broader than both 

 faces of f-2 ; -1 is subordinate, and the faces are 

 quite rough ; f-2 appears with its full complement of faces. 

 The roughness of the faces is in part caused by stria' which 

 on -£ and f-2 run parallel to the terminal edge of R replaced 

 by those faces. On R the markings are less distinct. These 

 •tially regular depressions seem like natural etch- 

 [ bring out the rhombohedral symmetry of the min- 

 eral very plainly. 



The cry.-' .mution of these crystals as phe- 



nacite is continued by all the pi .-s, as far as 



observed, and by the chemical composition. There is an 

 imperfect cleavage parallel to i-2 (ooP2). Both crystals are 

 clear and colorless, resembling quartz, and the hardi ess is 

 nearly or quite 8. The specific gravity of the crystal figured, 

 though containing some impurities, is 2*967 at 23° C. 



no'T.i 



