chap. xiv. Section of the Portillo Chain. 497 



of dark-grayish, harsh rocks, intermediate in character 

 between trachyte and basalt, containing glassy feldspar, 

 olivine, and a little mica, and sometimes amygdaloidal 

 with zeolite : the basis is either quite compact, or crenu- 

 lated with air-vesicles arranged in laminse. The 

 streams are separated from each other by beds of frag- 

 mentary brown scorise, firmly cemented together, and 

 including a few well-rounded pebbles of lava. From 

 their general appearance, I suspect that these lava- 

 streams flowed at an ancient period under the pressure 

 of the sea, when the Atlantic covered the Pampas and 

 washed the eastern foot of the Cordillera. 1 On the 

 opposite and northern side of the valley there is another 

 line of lava-cliffs at a corresponding height; the valley 

 between being of considerable breadth, and as nearly as 

 I could estimate 1,500 feet in depth. This field of 

 lava is confined on both sides by the mountains of 

 mica-schist, and slopes down rapidly but irregularly to 

 the edge of the Pampas, where, having a thickness of 

 about 200 feet, it terminates against a little range of 

 clay-stone porphyry. The valley in this lower part 

 expands into a bay-like, gentle slope, bordered by the 

 cliffs of lava, which must certainly once have extended 

 across this wide expanse. The inclination of the 

 streams from Los Arenales to the mouth of the valley is 

 so great, that at the time (though ignorant of M. Elie 

 de Beaumont's researches on the extremely small slope 

 over which lava can flow, and yet retain a compact 

 structure and considerable thickness) I concluded that 

 they must subsequently to their flowing have been 



1 This conclusion might, perhaps, even have been anticipated, 

 from the general rarity of volcanic action, except near the sea or 

 large bodies of water. Comformably with this rule, at the present 

 day, there are no active volcanos on this easternsideof the Cordillera ; 

 nor are severe earthquakes experienced here. 



