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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



mines were native copper, atacamite, malachite, silicate of copper, 

 black and red oxides of copper, purple and yellow pyrites, covelline, 

 sulphate of copper, &c. These minerals frequently contained native 

 gold in specks disseminated through them, but were stated to be 

 unusually poor in silver. 



The metallic veins which occur near La Portada, in Section No. 2, 

 and above Tarata, Section No. 2, present features in every way 

 similar to the above described. 



In Section No. 2 is seen probably the best illustration of the 

 arrangement and extent of the strata composing this formation. 

 From this it will be seen that, after passing over a series of very 

 highly inclined and thin-bedded Liassic shales dipping to the west- 

 ward, we meet with the dioritic rocks of Chuntacollo and Guanuni 

 breaking through them and altering them at the point of contact. 

 Above these the same shales, bearing north and south with a dip of 

 30° to westward, are again met with, and continue, with occasional 

 interstratification of porphyries, claystones, and porphyry-conglo- 

 merates, through Palca, Los Troncos, El Ingenio, Quebrada de la 

 Angostura, up to Questa Blanca, where they have a north or south 

 strike, and dip 50° to westward ; here they have some beds of white 

 trachytic tuff superposed upon them, as previously mentioned ; and 

 near this place we find an anticlinal, causing them now to dip to the 

 eastward, which dip they retain up to the summit of the nearly 

 15,000-feet high Pass of Huaylillos. Shortly after passing Questa 

 Blanca these rocks are very much altered, become flinty and siliceous, 

 and continue so for a considerable distance, bearing N.N.W., with a 

 dip of 50° cast : in these rocks several old workings are seen on some 

 strings of copper. The strike of the beds was at these mines found 

 to be still N.N.W., with a dip of 20° eastward. The change in 

 mineral nature here noticed is evidently due, as explained in a former 

 section of this memoir, to the vicinity of dioritic rocks, and conse- 

 quent metaniorphic action produced by the intrusion, which also has 

 developed the copper-veins before mentioned, and those near La 

 Portada. The diorite is, as seen in the section, visible at one spot, 

 and probably is much more extensive than would appear from the 

 small eruption crossed in the line of section. From La Portada to 

 the summit of Huaylillos the rocks are nearly all porphyritic con- 

 glomerates, frequently much altered and siliceous ; and on the slope 

 to the Rio de Azufre several beds of true interstratified porphyries 

 are seen before coming to the great volcanic ridge of Chipicani. 



Crossing these volcanic rocks, we next meet the strata pertaining 

 to this formation at the River Cano, where they present themselves 

 as beds of purple porphyry-conglomerate, dipping to the westward, 

 and broken through by the lateral fissures or dykes of trachyte 

 seen in the section both here and further eastward, at the Rio Mauri, 

 at which place they cover uneonformably the porphyry-conglome- 

 rates, as seen on both sides of the steep ravine through, which this 

 river passes. The beds here were thick porphyry- conglomerates of a 

 purple colour and composed of smaller pebbles of porphyry over- 

 lying beds of porphyry and porphyry-tuffs, which in turn are sue- 



