THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



JANUARY 1865. 



I. Researches on the Mineralogy of South America. 

 By David Forbes, F.R.S., tyc* 



I. 



TylSMUTH. — This metal occurs in the native state, as well as 

 "*^ in combination with oxygen, sulphur, and tellurium, in a 

 small vein in the lower Silurian clay-slates of the mountain 

 Illampuf in Bolivia. The mine of San Baldomero opened upon 

 fchis vein is situated but little under the line of perpetual snow, 

 and has an elevation of between 14,000 and 15,000 feet above 

 the level of the sea. The vein cuts through the lower Silurian 

 strata, here tilted up at a very high angle (nearly vertical), and 

 has to some extent altered these strata : in immediate contact 

 with the vein, a sahlband of from a few lines to some inches in 

 width is seen in the rock on each side of the vein, the thickness 

 of the sahlband being dependent on the relative strength of the 

 vein itself; this is much darker in colour than the less altered 

 rock, and has evidently been altered in chemical composition by 

 the action of the mineral matter of the vein, most probably by 

 the sulphur and arsenic present ; at a greater distance the slates 

 are only hardened or, as it were, baked by heat, and not more 

 altered than that the abundant fossils in them are well preserved 

 — as various species of Homalonotus, Or this, Area, Tentaculites , 



* Communicated by the Author. 



t This mountain is in England generally, but erroneously, called Sorata, 

 owing to its being frequently called by the Spaniards " La nevada de 

 Sorata " from the town of that name situated at its base. Its altitude has 

 been determined to be 24,812 English feet above the level of the Pacific 

 Ocean ; and consequently it is the highest of all the peaks of the Andes 

 of South America. 



Phil. Mag. S. 4. Vol. 29. No. 193. Jan. 1865. B 



