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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



probosciclea* , with numbers of other still more indistinct fossil im- 

 pressions. 



I have not a doubt that these beds, on careful search, would yield 

 a rich harvest to the palaeontologist ; but I had not time to devote 

 to more thau a very rapid survey of the country f. 



The thickness of the Silurian strata seen in this section cannot be 

 less than 10,000 feet : opposite San Felipe a good section is seen on 

 the nearly perpendicular face of the mountain called " Perolani," 

 which, by measurement, is 6000 feet above the valley ; and, as the 

 strata in the centre of this mountain are nearly horizontal, the thick- 

 ness of strata in that place cannot differ much from the total height of 

 the mountain above the level of the valley ; and it is not too much to 

 add 4000 feet for the strata visible both above and below these beds. 



The Silurian strata have been disturbed by the following igneous 

 outbreaks, in succession. Commencing with the most ancient, — 



1. Intrusion of the auriferous granite, along with its associated 



auriferous, and probably other metallic, veins : in parts it 

 has metamorphosed very considerable areas of the Silurian 

 beds. 



2. The porphyritic eruptions "of Hillabaya, Potosi, Oruro, &c. 



3. Protrusion of the metalliferous diorites. 



4. Still later trappean dykes, which, I am informed, occur at 



several localities ; but I have not personally come across 

 them in Bolivia. 



5. Volcanic eruptions near the Lake of Titicaca, &c, breaking 



out at the borders of this formation, but which, as far as 

 I am aware, do not anywhere disturb the main chain of 

 the Silurian Andes of Bolivia. On inquiry, I found that 

 this district was exempt from the earthquakes which are so 

 prevalent and destructive both in this and the adjacent 

 Republics. 



The metallic veins J which occur in these Silurian strata contain 

 the following minerals : — 



Metallic Gold. Fluor-spar. 



* I have to thank Mr. Kroeber, the Director of the San Felipe Mining and 

 Smelting Company, for his hospitality, and the assistance afforded me in my 

 search for fossils in this region. 



t Mr. Kroeber informs me of some anthracite-deposits, more hke fissures filled 

 up than true beds, and, according to his description, like some in Shropshire. 

 This, however, requires confirmation. 



\ I refer here to such veins as are not (so far as can be determined by super- 

 ficial examination) in connexion with the eruption of the metalliferous diorites 

 previously described as of Post-oolitic age. 



Tin-enamel. 

 Chloride of Silver. 

 „ of Lead. 



Oxide of Iron (magnetic). 



of Tin. 



Silver. 

 Bismuth. 

 Antimony. 



Blende. 

 Gralena. 



Magnetic-pyrites. 



Selenide of Lead. 



„ of Cobalt and Lead ? 

 Sulphuret of Antimony. 

 „ of Molybdenum. 

 „ of Silver (Silver-glance). 



