92 J. E. Pogue — Cerussite Twin. 



right angles to it, is the composition plane ; and (2) the ^-law, 

 whereby the two individuals are united by the prism r — oo 3 

 J1305. The first law is more common than the second, 

 because the m-prism is potentially stronger than the r-prism, 

 as shown by its presence in the important zone cp m* and its 

 more frequent occurrence on crystals. In a tabulationf made 

 in 1904 of all the combinations thus far noted on cerussite, the 

 m-law was represented on 90, and the ^-law on 27, out of a 

 total of 398 crystals listed. 



The present specimen is twinned after the r-law ; the angle 

 between the two individuals, as shown both in the basal and 

 gnomonic projection, is 57° 18'. Twins of this type, first noted 

 in 1870 by KoksharowJ from the Altai Mountains in Siberia, 

 are now known from about 18 foreign localities and 4 sources 

 in North America. The latter include, in addition to the 

 locality afforded by the present example, the Begona Mine, 

 Cerro de San Pedro, San Luis Potosi, Mexico ;§ the Red Cloud 

 Mine, Yuma County, Arizona! (yielding specimens similar in 

 habit, though poorer in forms than the one here described) ; 

 and Granby, Missouri.^ 



Division of Mineralogy and Petrology, 



United States National Museum, 

 Washington, D. C. 



* For a clear exposition of the theory of twinning, illustrated by cerussite, 

 consult Goldschmidt, Neues Jahrb. fur Min., Beilage-Band, vol. xv, 1902, 

 pp. 562-593. The r-law of twinning in cerussite is discussed on p. 582. 



fHubrecht, Zeitschr. fur Kryst., vol. xl, pp. 179-185, 1904-05. 



{Mat. Min. Russlands, vol. vi, p. Ill, 1870. 



§ Hunt and Van Horn, this Journal, vol. xxxii, p. 45, 1911 ; Zeitschr. fur 

 Kryst., vol. xlix, p. 357, 1911. 



I Pirsson, this Journal, vol. xlii, p. 405, 1891. 



■f[ Pirsson and Wells, this Journal, vol. xlviii, p. 219, 1894. 



Williams (Johns Hopkins Univ. Circ, No. 87, April, 1891, p. 74) is cited 

 by Hunt and Van Horn (loc. cit.) as well as by Dana (System, 1892, p. 1030) 

 as describing cerussite twinned after the r-law from the Mountain View 

 Mine, Carroll County, Maryland. The writer has examined the original 

 publication and finds that of two twins described, the second follows the 

 m-law, and the first, while figured, is not sufficiently characterized to afford 

 definite recognition of its twinning plane, though the writer judges it also 

 to be m. 



