Eakle.] 



Colemanite from Southern California. 



33 



are attached to the matrix by one cud of the vertical axis, 

 leaving one end well terminated by front and rear forms. 

 This circumstance rendered but one mounting on the gonio- 

 meter usually necessary in order to measure all of the forms 

 on a crystal, and measurements consequently could he rapidly 

 made. Complete measurements were made of thirty crystals, 

 and many more were examined for additional forms 



Elements. — The axial ratio and angle P generally accepted 

 for colemanite are those determined by Jackson. He derived 

 his elements from the measurements of one crystal, but subse- 

 quently verified his calculations by the examination of more 

 crystals. By the two-circle method of measurement all of the 

 readings can enter into the computation of the axial lengths; 

 consequently a ratio obtained by using a large number of 

 readings from the best faces, is presumably more exact than 

 when calculated from a few interfacial angles. As shown 

 later in the detailed description of the work, the axial ratio 

 and angle /J, obtained for colemanite by the writer were as 

 follows : 



a: 6: 0=0.7768:1:0.5430 ; /8 = 110° t. 

 For comparison the elements calculated by others are as follows : 



Bodewig and vom Rath, a-.b-.c = 0.7759: 1:0.5416 ; /? = 110°16fK 



Forms. — The number of forms observed was forty-seven, of 

 which thirteen were new. The forms are arranged in three 

 columns below, those in the last column being new. The let- 

 tering is the same as that given by Dana in his "System of 

 Mineralogy." Goldschmidt's and Miller's symbols are given for 

 each form. 



The commonest forms are c, b, a, m, t, a, h, ft, y, v, <l, 

 and o. Less common, yet of quite frequent occurrence, ai'e Jc, 

 A, i, <j, w, e, \jr, x, U. The remainder of the known forms are 

 rare: Q occurred four times, y three times, r, h, V, twice, and 

 e, q, B, ©, each once. W is common on one type of the crystals, 

 but occurred only twice in the others. 



Jackson, 

 Hiortdahl, 



a: b :c = 0.7755: 1:0.5415 ; /? = H0°13' 

 <t:b:c = 0.7747: 1 :0.5418 ; /8=110°13' 



