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PEOCEEOTNGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



present would alone produce had been neutralized or concealed by 

 tbe admixture of the other salts. 



In the midst of these plains at Ancara, I noticed some recent con- 

 glomerate-beds, of a brown colcur, composed of small and very angu- 

 lar pebbles, and more like a breccia than a true conglomerate. They 

 were of very small extent, and had an apparent strike nearly N. and 

 S., with a low dip to eastward. 



The central division of this diluvial formation is distinguished at 

 first sight from either of the others by the redness and sandy nature 

 of its soil, showing at once its derivation from the Permian or Tri- 

 assic sandstones and marls ; occasional patches are covered by vol- 

 canic detritus where the sandstone hills have been disturbed by the 

 intrusion of the trachytie rocks, well illustrated in Section No. 1. 

 The plains thus formed are well watered and frequently marshy, 

 and are cut up by numerous rivers, at least in the northern part 

 of the district here described* ; we do not find the surface-water- 

 saline, as is invariably the case in the western division ; but occasion- 

 ally, as for example at Santiago and at San Andres (both on Section 

 No. 2), we meet with brine-springs, which furnish the inhabitants 

 with an abundant and cheap supply of culinary salt of excellent 

 quality, by simply allowing the water to evaporate in the open 

 air from the heat of the sun. These brine-springs are most pro- 

 bably due to salt-beds situated at greater depths in the sandstones 

 of the formation itself, and not to be attributed to saline deposits of 

 more recent origin. 



The third or eastern division of this plateau is, in its turn, so dif- 

 ferent in character from either of the preceding, that it is at once 

 recognized when encountered. 



In Sections Nos. 1 and 2 (PL II.), this formation is seen as a 

 great plain abutting to the east against the Silurian rocks of the 

 highest range of the Andes, to the debris of which it owes its origin ; 

 whilst to the westward it is confined by the rang-e of low hills of 

 Devonian strata which separate it from the central division of this 

 diluvial formation. The intermediate basin, occupying the space be- 

 tween these Silurian mountains and Devonian hills, is filled up to the 

 level of the plain by an immense accumulation of clays and gravels, 

 with larger pebbles and boulders of Silurian and granitic rocks, — the 

 former being represented by grauwackes, indurated sandstones, clay- 

 slates, and shales, which latter occasionally contain fossils of Silurian 

 age. 



Where, as in the valley of the river of La Paz (which from its 

 abruptness might almost be termed a ravine), a section of this basin 

 is disclosed (Section No. 2), its surprising magnitude is seen, as in 

 this place. The thickness, reckoning from the level of the plain 



* I may here mention that in a spring at Comanche, the water of which 

 appeared to feel slightly warm on immersing the hand, I found numbers of a 

 small univalve shell ; and on submitting them to the inspection of Professor 

 Philippi. of the University of Santiago, in Chile, he considered them identical 

 with his Paludina Atacamensis, which he discovered in a tolerably hot spring at 

 Tilopozo, in the northern part of the Desert of Atacama. 



