Hunt and Van Horn — Cerussite Twins. 45 



Art. VI. — Cerussite Twins from the Begofia Mine, Cerro de 

 San Pedro, San Luis Potosi, Mexico; by Walter F. Hunt 

 and Frank R. Van Horn. 



Introduction. 



In March, 1910, Sefior Felipe Borrego, foreman of the 

 Guadalupe Mine, Cerro de San Pedro, Mexico, sent several 

 specimens of minerals to Case School of Applied Science for 

 identification. Among the specimens were 8 arrow-shaped 

 twins which the junior author recognized as cerussite of rather 

 uncommon development and type, on which account they were 

 taken to the University of Michigan, where their crystallo- 

 graphic properties were studied by the senior author. 



Geology. 



The twins were found at the Begofia Mine in the Cerro de 

 San Pedro district. Details concerning their occurrence were 

 furnished by Sefior Borrego, as well as by Mr. R. B. Cochran, 

 Director of the Cia. Minera El Barreno y Anexas, San Luis 

 Potosi. The San Pedro mining district is situated about 20 

 kilometers east of the city of San Luis Potosi, and the mines 

 have been worked more or less since 1575. It has been stated 

 that 413 million dollars were taken out of this region between 

 1656 and 1778. The village of Cerro de San Pedro is located 

 at the foot of the hills at an altitude of 2180 meters above sea 

 level. The district contains but little water and timber, and 

 the mines are situated on low hills which are from 200 to 400 

 meters from base to summit. The mineralized district is 

 practically confined to an area which extends about 2000 

 meters in a north and south direction, and 800 meters east and 

 west. This area covers parts of three hills known as San 

 Pedro, El Populo and La Raposa, which consist of limestone 

 into which has intruded a mass of andesite locally known as 

 "porfido." The strike of the "upper contact" of andesite and 

 limestone is usually north and south, with a dip of 55° to the 

 west, while the "lower contact" strikes in the same direction 

 but dips westward about 70° degrees. There is but one mine, 

 the Cocinera, on the lower contact. All the others, of which 

 the Begofia is one, are situated on the upper contact. The 

 main ore occurrences are between the limestone and andesite, 

 although many large and rich deposits have been found in the 

 limestone hanging wall up to a distance of 300 meters from 

 the contact. The plane of the upper contact is very irregular, 

 making troughs and rolls along both strike and dip. In these 



