The formation consists of granite, with veins of very pure quartz; but the plains are covered with large patches of fine dust, consisting chiefly of silica, containing potash and mica, with small quantities of the débris of the rocks associated with the soil, which Admiral FitzRoy suggests may have been the ashes ejected, at some remote period, from the volcano of Arequipa. Near the sea-shore, and about half a mile south-east of Islay, there is a very curious result of the constant action of the weaves, in two immense cavities hollowed out of the rock, called the _Tinajones_ (jars). They are circular holes about thirty yards across, and of great depth, separated from the sea by a wall of cliffs not more than four yards wide, the lower part of which is undermined, and forms a passage by which the waves rush into the great _tinajon_, or bowl, with a mighty roar; and, dashing themselves against the rocky sides, throw back clouds of white spray. The only vegetation near the coast consists of lowly little _Mesembryanthema_, scattered about at long intervals, and an occasional stonecrop (_Sedum_). During our stay at Islay we enjoyed the hospitality of Mr. Wilthew, H.B.M. Consul, and his wife, to whom we were indebted for much thoughtful kindness. The rest of the inhabitants consist of Peruvian officials, agents of commercial houses in Arequipa, and a few shopkeepers and artisans, besides the muleteers and other birds of passage, and the porters and boatmen of mixed Indian and negro extraction. The supplies for the market come almost entirely from the rich valley of Tambo, some leagues down the coast. On March 6th, our mules and horses having arrived, we started for Arequipa in the morning, a distance of ninety miles, and, crossing the country near Islay, entered a gorge in the mountains, which winds up to the great desert above, at the commencement of which there is a grove of dusty olive-trees. This dismal ravine, with arid scarped mountains rising up on either side, here and there a tall gaunt cactus, and everywhere a dense cloud of white dust, leads up to a little post-house built of canes, called the "Tambo de Guerreros," eighteen miles from Islay.