40 



Hunt and Van Horn — Cerussite Twins. 



troughs or offsets the ores are often richer. In several 

 instances, the mineralization has extended in narrow seams 

 completely through the limestone to the surface, and on El 

 Populo hill, the ore is mined on these seams in open cuts. 

 The ores at present worked are rather low-grade gold and sil- 

 ver ones with some lead. They are mostly oxidized, although 

 some sulphides are found. Gold occurs native, as also does sil- 

 ver. Other minerals which have been noticed are cerargyrite, 

 argentite, cerussite, pyromorphite, and galena. Some' zinc 

 minerals are said to occur but copper compounds are very rare. 

 Pyromorphite is very abundant, and sometimes occurs in large 

 botryoidal clusters over 6 inches in diameter. The cerussite 

 twins were found along with granular cerussite, gypsum, and 

 limonite in one of the troughs or offsets of the Begoila Mine. 

 They were taken from a cavity in the limestone about 1 meter 

 from the andesite contact. 



Fig. 1. 



>/\ 



lJ (-. 



p-{ 

 p-\ 



¥^^VC'''' -'A'"-- 



«-; .-' f-v---- 







^^-^JT* tt 





Grystallc 



>graphy. 



The twinning plane observed, although not new, is the rather 

 uncommon brachy prism /"{130}. The crystallography is of 

 interest inasmuch as a new locality for this type of twinning 

 has been discovered, and also as the crystals examined revealed 

 several forms, either not recorded on twins referred to this 

 law or mentioned only as of doubtful occurrence. Cerussite 

 twins referred to this law have been described by Kokscharow,* 

 by Williams from the Mountain Yiew Mine, near Union 

 Bridge, Carroll County, Maryland,! ar >d similar twins, some- 

 times abnormally developed by extension of the prism faces r, 

 were recorded by Pirsson from the lied Cloud Mine, Yuma 

 County, Arizona.:}: The contact twins from the Mexican local- 

 ity ranged from one half to one centimeter in length and rep- 

 resented but one type of development, namely forms resem- 

 bling arrow-heads, this development resulting in part from the 



* Materialien z. Min. Eussl., vi, 135, 1870. 



f Johns Hopkins Univ. Cir., No. 87, April, 1891. 



X This Journal, xlii, 405, 1891. 



