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PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



tions produced by the action of the volcanic fumes and acid vapours 

 on these rocks, and the formation of various salts of iron, lime, &c. 

 To the south of Tacora I noticed an evolution of smoke, as if from a 

 similar solfatara; and my fellow-traveller, M. Priesach, informed 

 me that, when he passed in October 1859, a volcano situated a little 

 to the right of Sajama, and apparently one of the three cones named 

 Las Tetillas on the new map of Bolivia by Mujica and Ondarza, was 

 observed by him in eruption, vomiting forth immense volumes of 

 smoke, and apparently also lava, not from the cone itself, but from a 

 lateral orifice situated at the base of the cone. The volcano of 

 Tutapaca is also situated in this direction, and is still in activity ; 

 and near this place M. Modesto Bazadre informs me he visited a 

 valley containing several hundred of little volcanic cones emitting 

 boiling water, and in many respects resembling the Geysers of 

 Iceland ; like these latter, the cones around the orifice of ejection are 

 formed by the deposit from the water itself. 



Although we find the volcanos of this part of South America pre- 

 senting themselves as lofty cones, rising high above the surrounding 

 plateau, we do not observe in general that crater-form of summit so 

 usual in mountains of this class in other parts of the world : we 

 certainly find, as in the Misti (or Volcano of Ariquipa), some well- 

 developed small craters ; but these seem rather to have served as so 

 many safety-valves to the volcanic boiler, and to have played but a 

 very subordinate part in furnishing the great amount of lava and 

 other volcanic matter here met with, which appears in greater part, 

 if not entirely, to have made its way up through the great lateral 

 fissures or openings (similar in many cases to dykes) which appear 

 to have poured forth sheets of lava, covering vast areas of the sur- 

 rounding country. This class of eruptions appears peculiarly cha- 

 racteristic of the Pacific side of South America, where they seem to 

 attain a magnitude unknown in any other part of the world. The 

 southern part of Bolivia shows such lateral eruptions, covering the 

 ground with trachytic lava for more than 300 miles continuously ; and 

 in the northern part, as seen in Sections Nos. 1 and 2, the same occurs, 

 — some of these eruptions appearing to proceed from such lateral 

 dykes or fissures, at the lowest estimate not less than fifty miles in 

 length, if not much more. 



The volcanic rocks here described are strikingly distinct from those 

 which I met with during my examination of the volcanic islands of 

 {lie Pacific Ocean and Polynesia: these latter are generally of very 

 dark colours, are of a veiy basic chemical nature, and characterized 

 by the abundance of augite and olivine and the absence of quartz ; 

 whereas here in Peru and Bolivia the rocks are invariably of lighter 

 colour, generally even white, are of a much more acid or siliceous 

 chemical nature, contain abundance of quartz, and only in some 

 instances was olivine at all met with. 



Before concluding this notice of the volcanic rocks, I may direct 

 attention to a point connected with the crystallization of the same, 

 and which, I bebeve, has not been previously noticed : I allude to the 

 occurrence in the trachytic and trachydoleritic rocks of perfect 



