260 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Although even in the preceding crystals, which are shown in the 

 drawings as simple crystals, twins are more abundant than single 

 individuals, the solitary ones are drawn to lessen the labor of pre- 

 paring the illustrations. The twins are penetration and contact 

 twins and are thus similar to preceding illustrations of twinned 

 groups, each individual of the group being like the one selected for 

 illustration. Certain specimens from the Lookout mine show trans- 

 parent contact twins of pale amber color up to 1 cm. in diameter, 

 incrusting quartzite. These are of peculiar aspect and were inter- 

 preted with some difficulty. The habit is well shown in Figure 69. 

 One of these gave the following measurements : 



Measurements of cerusite, Lookout mine, Figure 69, normal position 



Quality, description 



Very good 



Excellent 



Very good 



Good 



Excellent 



Excellent, striated- 



Good.... 



Excellent 



Striated 



Good 



....do 



Poor, multiple 



Excellent ... 



....do 



Measured 



89 59 

 00 



58 32 



28 34 



31 



00 



00 



00 



00 



00 



69 01 



58 40 



58 24 



90 00 



90 00 



00 



90 00 



90 00 



13 26 



19 56 



35 55 



53 57 

 70 40 

 74 40 



54 32 

 34 49 

 19 57 



Calculated 



90 00 

 00 



58 37 



28 39 



00 



00 



00 



00 



00 



00 



58 37 



58 37 



58 37 



90 00 



90 00 



00 



90 00 



90 00 



13 33 



19 52 



35 52 



55 20 



70 55 



74 32 



54 14 



34 46 



19 08 



The twinning plane is established by the following angular relation- 

 ships. v — of crystal 1 to v of crystal 2 will define the twinning 

 plane. Since the pinacoids are not well shown by the drawing the 

 plane common to both crystals may be established by the two faces 

 of z*(130) at the fore part of the figure. Thus 



v of r v of r =161 o 23'-90° = 71° 23' 



v of twinning plane = 71° 23' -v of crystal 1 = 58° 42' 



The twinning plane is thus the prism ra(110), calculated p for which 

 is 58° 37'. 



WASHINGTON COUNTY 



In the Heath district many prospects show silver-lead ore con- 

 taining cerusite. Specimens in the National Museum show massive 

 cerusite from the Keystone mine (Cat. No. 60940, U.S.N.M.) in 

 quartz with manganese oxide. Another specimen from the same 

 mine contains earthy cerusite with anglesite and limonite coating 

 very coarse-grained galena (Cat. No. 30198, U.S.N.M.). 



