Eakle.] 



Colemanite from Southern California. 



45 



In monoclinic crystals, c is also equal to the coordinate q o; 



the clino-axis d, however, is equal to — , therefore the form- 

 sin ^ 



ulae for monoclinic crystals, in terms of poqo or p oq o, when 

 h = 1 , become 



^ ° sin /x ' ;/„ sin fx P« /'» sin /a 



From these equations, a = 0.77G8 and c - 0.5430; giving 

 the axial ratio for eolemanite ii : b : c = 0.7768 : 1 : 0.5430. 



Record of the measurements. — As an illustration of the actual 

 measurements and method of deriving the symbols for the 

 forms, the work on Crystal 23 is given. Each crystal was 

 sketched before measuring in order to show the relative size of 

 the faces, and the faces were provisionally lettered. The kind 

 of reflection was designated as good, fair, or poor (g. f. p.) . 

 Starting with the angle on the side pinacoid (010) = 139° -44', 

 and plotting the ('-angles in a circle in the direction of the hands 

 of a watch, it is easily determined which forms are positive and 

 which negative. The h for the instrument was 57° 54', and the 

 vo for the crystal was 139° 44. 



From the columns p]/ and qq the symbols p q are at once 

 apparent, since p' = 0.7443 and q' = 0.5430. The record is 

 simply a sample page of the note book, and it illustrates the 

 simplicity and shortness of the work required to determine 

 any form, as well as the neatness and compactness of the whole 

 method. 



