PERUVIAN REGIONS. 143 



an hour; when the angle of light from the sun having changed, it 

 disappeared ; and only after sailing onward for hours, again came into 

 view. At noon, the Latitude was found to be " 12° 31' S." 



In the afternoon, we were so near, that the Andes assumed the 

 form of separate peaks, on several of which the snow was distin- 

 guishable. And at 10 p.m., the Relief anchored in the harbor of 

 Callao; situated in " Lat. 12° 02' S." 



When daylight came on the morning of the 13th, the Andes were 

 observed to be nearer the sea than at Valparaiso. We looked up a 

 valley, traceable by decussating mountain-crests, each succeeding one 

 rising higher in the distance ; in fact, the whole country seemed full 

 of mountains and mountain-ridges; yet the general ascent so uniform, 

 that the base of the Andes seemed to commence at the sea-shore. 



Geological structure. On landing, and in the course of our sub- 

 sequent excursions, the geological structure was found to agree with 

 that of Chili, even to a seeming continuity of rock-formations ; but 

 with the addition, of an outside strip of sedimentary roch. This strip 

 is chiefly submarine; but includes the projecting promontories, and 

 the linear series of outlying islets extending along the coast ; as San 

 Lorenzo, Fronton, and the Pachacamac Islets. The rock proved argil- 

 laceous; its disturbed stratification and the presence of an Ammonite, 

 marking a very early period ; earlier, probably, than the Cretaceous 

 or chalk formation, but perhaps contemporary with the Lias. An 

 allied species of Ammonite from Truxillo" afforded proof, that this 

 strip of sedimentary rock extends more than three degrees farther 

 North. 



The granitic formation w'ds ascertained to be about forty-five geogra- 

 phical miles wide ; extending from the coast to a little beyond Yaso. 

 The rock proved exceedingly variable in composition and aspect ; 

 being in many places coarse-grained, and readily recognized as 

 granite; but over extensive areas, jagged, full of seams, and alto- 

 gether argillaceous ; presenting often a semblance of stratification 

 that might deceive even a practised eye. Of contained minerals, 

 epidote was occasionally present; but I did not meet with garnets. 

 The unusual ruggedness of the surface of the country was evidently 

 in part due to the absence of rains ; decomposition taking place more 

 slowly, and many ridges of rock remaining bare, that in a rainy cli- 

 mate, would have been deeply coated with earth. 



The trachytic or traf) formation succeeded inland ; the hills for 



