288 p E R u - 
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 Baiios 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 per- 
ceived 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 somewhat 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. 
Pachacamac is one of the most interesting spots on this part of the 
coast, although it is said it will not compare with many others in 
various parts of the country, especially at Cusco. 
They left Callao on the afternoon of the 28th of June, and were at 
anchor about midnight abreast of the place. At daylight the surf 
was found so heavy as to render it dangerous to land in the whale- 
boat. By the perseverance of the officers, a raft was formed of the 
India-rubber mattresses and oars; two balsas were also provided. 
Lieutenant Underwood made the first attempt, and paddled himself 
into the rollers, the first one of which threw him and the balsas 
end over end. Shortly after, the raft was seen bottom up, the oar 
broken, and the fragments sticking up in various directions, but he 
was missing. He soon, however, made his appearance at some dis- 
