Bowles — Pyromorjphite from British Columbia, Can. 41 



Fig. 1. 



The basal pinacoid, o jOOOlf, is very poorly developed. 

 Reflections could be obtained from it on only five of the crys- 

 tals studied. Od many of the crystals it was so rough and 

 uneven that it appeared to be merely a 

 fracture surface. 



The prism of the first order, m J1010}, 

 is the most prominent form on all crystals, 

 and is usually represented by well-reflect- 

 ing surfaces, from which satisfactory read- 

 ings may be obtained. These faces com- 

 monly exhibit minute longitudinal stria- 

 tums. 



A very important fact which has not, to 

 the writer's knowledge, been as yet ob- 

 served is to be noted in connection with 

 the prismatic faces. They do not exhibit an 

 absolute parallelism, but converge slightly 

 toward the upper end of the o axis. From 

 this_ it would appear that the symbol 

 ]1010f is only approximately correct, the 

 true prism faces being replaced by vicinal 

 planes which depart from the theoretical 

 position of the real prisms by a definite measurable angle. Only 

 in exceptional cases are true prism faces present, for almost 

 invariably they are replaced by these vicinal planes. The 

 readings for all the faces in the prismatic zones of twenty-one 

 crystals give a mean angle of 89° 33' between the normal and 

 the. vertical axis. The Miller symbol thus becomes j 135* 0* 

 135* 1\. The frequent recurrence of these faces, indicating 

 an approximately constant deviation from the theoretical value, 

 gives weight to the theory of S. M. Websky,* that vicinal 

 planes are not accidental, due to distortion of the crystal, but 

 that they follow some definite law which has its foundation in 

 the internal molecular arrangement. In the table of angles it 

 will be noted that the other forms show considerable variation 

 from the calculated values also, but it must be remembered 

 that the values for these forms were obtained from poorly 

 reflecting surfaces, while in the case of the prism faces well- 

 defined images were obtained. 



The prism of the second order, a ]1120|, was observed on 

 two crystals only, the faces being very narrow, and in some 

 instances curved. As shown by the table on page 42, the read- 

 ings are, however, sufficient to indicate that the faces are 

 undoubtedly prisms of the second order. 



The unit bipyramid, x jlOllj, is the most prominent of all 

 the pyramidal forms. The faces are in most cases very dim, 



*Zeitsclir. d. d. Geolog. Ges., xv, p. 677,. 1863. 



