278 PERU. 



fluctuations of seasons, be covered with several feet of earth. The 

 same reasons will account for the duration of the Inca villages that 

 cover many of the hills, and which a copious shower would entirely 

 wash away. The granite on its eastern side was coarse-grained, pre- 

 senting more of the ordinary appearance of that rock. 



Immediately eastward of the granite district commence the trap 

 rocks, consisting for the most part of porphyry. Dr. Pickering traced 

 the line of junction for some miles, the hills on one side being of 

 granite, on the other porphyry. The eastern limit of the trap region 

 is supposed to be distant some twenty miles from the western. The 

 porphyry resembles the Swedish, and that in the vicinity of Boston. 

 Many porphyry pebbles, supposed to be of this formation, were found 

 on the beach at Callao, having, it is to be presumed, been carried 

 there by the action of the water-courses. 



Next comes the plateau of the Cordilleras, which is formed of sedi- 

 mentary rocks ; this includes the silver mines, and the highest peaks, 

 and is apparently of the same age as the coast. Much of the rock is 

 argillaceous. At Banos an argillaceous limestone was used for burn- 

 ing, and quantities of gypsum, used for manure, was brought from the 

 vicinity of Casa Cancha, some twenty miles to the north. Conglo- 

 merates prevailed over a great portion of the crest the party traversed. 

 The included pebbles were observed to be of regular shape, smooth 

 and polished as if sea-washed. All the party remarked the smooth- 

 ness of the pebbles in the torrents of the Cordilleras, which had a 

 strong resemblance to those on the sea-beach. From the information 

 relative to the mines in the Cerro de Pasco, it will have been perceived 

 that blue limestone, slate, and sandstone, exist in that vicinity ; and at 

 the silver mines at Alpamarca, a compact bluish rock was observed, 

 probably the limestone ; it was not, however, ascertained whether it 

 was argillaceous or a pure limestone. Dr. Pickering remarks, that it 

 contained numerous hard seams of opaque calcareous spar, with some- 

 what the lustre of " satin spar." Sandstone with small pebbles was 

 not uncommon. 



The bare spots of the higher peaks did not present the variety of 

 colour of the Chilian Andes, but had a uniform dark slaty hue. 

 Many incrustations were seen forming on the rocks and plants : this 

 was found to be gypsum. 



Previous to our departure, I felt desirous of having an excursion 

 made to the ruins of Pachacamac ; and having heard that the landing 

 was easy and good, on the inside of the island, I sent the tender 

 Flying-Fish thither, with Dr. Pickering and Lieutenant Underwood. 



