422 Palache — JYotes on Tellurides from Colorado. 



individuals, in contact along the plane 101, are very unlike in 

 development. The crystal figured in normal position is 

 bounded by numerous positive pyramids and domes while the 

 twinned individual is much larger and a much simpler combi- 

 nation. The reentrant augle between the two basal planes 

 with the intervening narrow portion of the twinning plane 

 101 made the twinned character of the crystal quite evident. 

 In a subordinate development this mode of twinning is quite 

 frequent, the twinned individual appearing as a narrow trans- 

 verse ridge on the surface of the m plane of a larger simple 

 crystal such as shown in fig. 2. 



A less simple phase of the twinning is represented in fig. 4 

 in orthographic projection on 010, where apparently no plane 

 boundary exists between the two individuals. The planes 

 n_, u_, ±, and _e_, constituting the upper part of that figure were 

 only recognized as in twin position by the complex symbols 

 which they yielded in the projection, whose study, in the light 

 of the simple twin already observed, showed their true nature. 

 It is possible, however, that this is a contact twin like No. 3 ; 

 for the remaining forms shown in the upper part of fig. 4, 

 s, r, v, etc., are not affected in their position by the twinning ; 

 but in the absence of the planes bounding the opposite end of 

 the crystal the course of the twinning boundary cannot be 

 accurately defined. 



A chemical analysis was made upon about '5 grams of the 

 sylvanite crystals, which were carefully picked over under the 

 lens until apparently entirely freed from adhering gangue. 

 The analysis shows, however, that considerable of the siliceous 

 gangue was still present. The composition is that of a normal 

 sylvanite, Au,AgTe r 



Sylvanite. Cal. to 100$. Mol. ratio. 



Insol. 1-02 



Au 26-09 26-25 '1334) 



Ag... 12-49 12-57 -1164 j 



Te 60-82 61-18 -4894 = l '96 



Fe 1-19 



Total 101-61 100-00 



2. Crystallographic Identity of Goldschmidtite with Sylvanite. 



In 1899, Hobbs* described goldschmidtite, a new silver- 

 gold telluride, intermediate in composition between sylvanite 

 and calaverite. The analysis was made on a very small amount 

 of material and seemed therefore open to question, but the 

 crystallographic results appeared to place the mineral distinctly 



* This Journal, vii, 357, 1899. 



