CALIFORNIA ACADEMY OF SCIENCES. 5 



granular, gray colemanite, the latter somewhat impure from finely 

 disseminated argillaceous particles. One cavity was first lined 

 with small, brilliant, colorless crystals of quartz, of the common 

 form, which were subsequently partially covered by numerous 

 crystals of colemanite. One magnificent geode is 0.1 m. in di- 

 ameter, and entirely lined with large, colorless, brilliant, and 

 highly complex crystals of colemanite. Considerable compact, 

 grayish quartz is finely disseminated through the walls of these 

 cavities or segregated in small irregular masses or thin layers, 

 always underlying the colemanite crystals. The mineral is said 

 by its discoverer to be intimately associated with snowy white, 

 massive, extremely fine scaly pandermite. Small patches of this 

 latter borate (?) adhere to the larger specimens, but give no clue 

 to the exact relation, with respect to mode of occurrence, that 

 may exist between them. A small quantity of an actively effer- 

 vescing insoluble carbonate (probably calcite) also adhered to 

 one of the geodes. 



The crystal-system was established by an optical investigation, 

 and Mr. Evans' assumption found to be correct. Extinction be- 

 tween crossed Nicols in parallel polarized light, took place exactly 

 at right angles to, and parallel to, the plane of perfect cleavage 

 oo 3? oo (010) when crystals were viewed through n (Fig. 1, 

 PI. I.); when viewed through cleavage laminae, the direction of 

 extinction makes an angle of something over 6^ with the vertical 

 axis (assuming as has been done in this paper, g = (001) P), 

 lying in the acute angle of the morphological axes. 



System: monoclinic. 



Axes-ratio: 



a: b": c = 0.774813: 1: 0.540998 



/i = 69° 50' 45" 



determined from the fundamental angles 



oo P : oo P = (110): (110) = 107° 56' 9" 

 oo P : P = (110): (001) = 106° 10' 43" 

 P : 2 P oo = (001): 201) = 111° 35' 39" 



all taken from one crystal and from faces giving excellent reflec- 

 tions. 



Cleavage: perfect parallel to oo P oo and distinct parallel to P. 



