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PPvOCEEBIXGS OF THE GEOLOGICAL SOCIETY. 



[NoV. 21, 



will require a chemical examination to determine its nature satis- 

 factorily. 



These dioritic rocks show themselves as a series of more or less 

 detached or isolated patches of rock, protruding themselves through 

 strata of various ages older than the Cretaceous period, which last- 

 mentioned system appears to be of still later geological age. 



On the Pacific, or western side of the high Andes, from Peru to 

 Puerto Montt in Southern Chile, a distance of some forty degrees of 

 latitude, we find at intervals such diorites breaking through the 

 other rocks ; and lines drawn through the points at which they 

 make their appearance show that there are two parallel systems of 

 eruption running not far from north and south, and probably at a 

 distance of about 100 miles from one another. 



In the part of South America forming the subject of the present 

 memoir, the most western of these lines commences from a little to 

 the east of Paposo, in the Desert of Atacama, passes through the 

 metalliferous district of El Cobre, runs along the cliffs at Cobija, and 

 touches the coast at Gatica a few leagues to the north of Cobija ; 

 then passing through Tocopilla, Algodon Bay, and the Ansuelo Rocks 

 near Iquiquc, again enters the mainland, and, after showing itself at 

 several points before coming to Arica, is seen in Section No. 2 at 

 Chuntacollo and Guanuni, and still further north in Section No. 1, 

 between Tarocaehe and Tarata ; and, from what I can learn, it shows 

 itself still further north in Peru, and appears to run right through 

 South America. 



The eastern line of eruption, after breaking through the Lias- 

 rocks between La Encantada and Sandon in the Desert of Atacama, 

 shows itself at several points before coming to Tilopozo, from which 

 place I have specimens brought me by Dr. Philippi ; entering the 

 central part of Bolivia, unexplored as yet by any geologist, it shoAvs 

 itself at the Cerro de las Esmeraldas south of Corocoro, and the 

 Hill of Comanche to the north of that place ; and, from a specimen 

 sent me, it must appear in the neighbourhood of Tio Guanaco, at 

 the southern extremity of the Lake of Titieaca, beyond which I have 

 at present no data for following it further north. 



The eruption of these diorites appears to have been generally 

 accompanied by the evolution of much acid vapours, probably sul- 

 phurous, to judge from the effects produced on the rocks in imme- 

 diate contact with the diorites ; as, wherever they break through 

 sedimentary strata, these latter are much changed in appearance 

 and chemical composition. 



Thus we find the Lias-shales and porphyritic clay and mudstones 

 converted into a pure white matter resembling china-clay, by the ' 

 abstraction of the lime previously contained in them ; and when 

 this change has proceeded a step further, and, besides, the lime has 

 also removed much of the alumina present, we find these rocks con- 

 verted into siliceous or hornstonc-like compounds, which have by 

 several observers been regarded as rocks entirely distinct from those 

 from which they have originally been derived: thus the quartz- 

 porphyries of M. Domeyko and M. Pissis are of this latter character ; 



