8 



CALIFORNIA ACADEMY OF SCIENCES. 



The following angles will make this clear: 



g : vert. ax. = 69° 54f 

 i : vert. ax. = 69 25 



Furthermore in the reversed position, v becomes — 2 3? 2, 

 h becomes — P oo, Y becomes — 5 P oo, and the following fur- 

 ther coincidences may be noted: 



Numerous other coincidences could be traced, but I have 

 already adduced more than enough to show the necessity for 

 some caution in interpreting a crystal, whenever the forms of the 

 zone y : d are well developed. The danger of error in this re- 

 spect is considerably diminished by the fact that the faces in the 

 zone y : d are often striated parallel to the edge y : d. When 

 this striation is not present, one can of course always avoid error 

 by determining the direction of extinction in a cleavage lamina 



