0. C. Farrington — Azurite from Arizona. 305 



In the position adopted by Schrauf the vertical axis is given 

 double the length of that in our position. Taking, therefore, 

 one-half the value which he gives to <?, his axial ratio is : 



d: b: 6 = -85012 : 1 : -88054. /? = 81° 36' 



It will be seen that these ratios differ but little, the values for 

 /3 being almost identical, while those for a and c agree to the 

 third decimal place. The author's value for a is supported by 

 several very accurate measurements of the prism m^m, which 

 in every case showed a close approximation to the angle 81° 8' 

 instead of 80° 42' as given by Schrauf. Whether this varia- 

 tion is to be regarded as a fundamental difference in the pris- 

 matic angle of the crystals from the separate localities or, on 

 the other hand, as so small as to be within the limits of error 

 in observation, I cannot say. More data are needed for decid- 

 ing the question. The most satisfactory measurements that 

 could be obtained for judging of the correctness of the value 

 assigned to c, were those of c A |, 001 ^ 021, smdp^p, 021 /\ 021. 

 The measured and calculated angles compare as follows : 







Calculated. 

 Farrington. Schrauf. 



Measured. 

 No. 1. No. 2. 



c *p, 



001 ,v 021 



60° 33' 60° 24' 



60° 29' 60° 30' 



P"P, 



021 /v 021 



58° 56' 59° 12' 



59° 1' 59° 6' 



From these it would seem that the true value of 6 is about a 

 mean between that given by Schrauf and by the author. 

 Here, again, more accurate measurements are needed. 



In Table I the measurements of all the prominent zones in 

 these crystals are given and, for comparison, their values as 

 calculated from the author's axial ratio and from that of 

 Schrauf. Where measurements of other crystals were of suffi- 

 cient accuracy to make them trustworthy, these have been put 

 in the column headed "Other measurements;" but, though a 

 large number were studied in the hope of obtaining such, only 

 a few were found. Tables II and III give the calculated and 

 measured values of the angle of each face upon the pinacoids 

 <?, 001, and a, 100. Owing to the imperfect development of 

 these pinacoids in the two best crystals, the measurements 

 given do not have equal significance with those of the preced- 

 ing table. They are, however, the angles most commonly met 

 with in the Arizona crystals. The clinopinacoid b, 010, was in 

 no case observed. These tables show, as a rule, close agree- 

 ment of the calculated and measured angular values, and "like- 

 wise but small differences in the calculated values given by 

 Schrauf and by the author. The chief variations that arise in 

 the latter, correspond to the differences in the prismatic angle 

 which have been previously mentioned. It is to be hoped 



