S. L. Penfield — Bertrandite from Colorado. 53 



Other minerals occurring at the locality are phenacite, ortho- 

 clase, muscovite and fluorite. 



The crystals are little rectangular blades 5 mm long, 2 mm wide 

 and 0'2-0'4 mm thick. The largest faces, 5 X 2 mm , correspond to 

 the basal plane of Bertrand lengthened out in the direction of 

 the brachy-axis, a, and marked by slight striations parallel to 

 the shorter diameter or macro-axis, b . Opposite this flat basal 

 plane the crystals have a curved surface composed of the 

 basal plane and brachydomes in oscillatory combination. The 

 curved surface either joins the basal plane directly, forming a 

 sharp, thin edge along the whole length of the crystal, or a 

 narrow brachypinacoid is present between them. This curious 

 development gives to the crystals a hemimorphic aspect which is 

 very characteristic and not accidental ; for all of the eight or ten 

 crystals on the specimen were of this same character. The 

 general shape of the crystals is that of a thin slice cut from the 

 side of a cylinder parallel to its axis. The crystals are attached 

 at one end and are terminated at the free end by a macro- 

 pinacoid. The observed planes are therefore the three pina- 

 coids, one of the basal planes being rounded by oscillatory 

 combinations parallel to the brachy-axis. The faces have a 

 good luster, that of the basal plane being pearly, the others 

 vitreous. They are not well suited for measurement. There 

 was one V-shaped twin in the specimen, the twinning plane 

 being the brachy-dome 031 (3-£) of Bertrand. The flat basal 

 planes formed the outside limbs of the V and made an angle of 

 61° 52' with one another, the curved surfaces formed the re- 

 entrant angle. Similar twins are described by Bertrand with 

 re-entrant angle of about 60°. Two cleavages were identified, 

 prismatic and basal, both highly perfect. The measured angles 

 are as follows : 



Calculated : Bertrand. 

 c ~b 001 ,s 010 approx., 89° 54' 90° 



m a m 110 a HO both cleavage, 59° 34' 58° 40' 



m^raliO^llO " " 120° 36' 121° 20' 



a atoIOOa 110 m cleavage, 150° 50' 149° 40' 



c aC 001 ^001 twin, 118° 8' 120° 50' 



These values differ quite widely from the calculated values 

 of Bertrand, but if we regard the cleavage angle m^m = 

 59° 34' as good (it was certainly free from disturbing influences 

 such as striations) and couple with it the angle of the twin 

 ca<?=118° 8', We obtain the axial ratio for orthorhombic axes 



a : I : « = 0-5953 : 1- : 0-5723. 



The important measurements of Des Cloizeaux and Scha- 

 rizer, with the values calculated from the above axes, are : 



