The eastern or inland side of Chiloe, with its many adjacent islets, consists of tertiary and boulder deposits, worn into irregular plains capped by gravel.Near Castro, and for ten miles southward, and on the islet of Lemuy, I found the surface of the ground to a height of between twenty and thirty feet above high-water mark, and in several places apparently up to fifty feet, thickly coated by much comminuted shells, chiefly of the Venus costellata and Mytilus Chiloensis; the species now most abundant on this line of coast.As the inhabitants carry immense numbers of these shells inland, the continuity of the bed at the same height was often the only means of recognising its natural origin.Near Castro, on each side of the creek and rivulet of the Gamboa, three distinct terraces are seen: the lowest was estimated at about one hundred and fifty feet in height, and the highest at about five hundred feet, with the country irregularly rising behind it; obscure traces, also, of these same terraces could be seen along other parts of the coast.There can be no doubt that their three escarpments record pauses in the elevation of the island.I may remark that several promontories have the word Huapi, which signifies in the Indian tongue, island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi, etc.; and these, according to Indian traditions, once existed as islands.In the same manner the term Pulo in Sumatra is appended to the names of promontories, traditionally said to have been islands (Marsden's "Sumatra" page 31.); in Sumatra, as in Chiloe, there are upraised recent shells.The Bay of Carelmapu, on the mainland north of Chiloe, according to Aguerros, was in 1643 a good harbour ("Descripcion Hist.de la Provincia de Chiloe" page 78.From the account given by the old Spanish writers, it would appear that several other harbours, between this point and Concepcion, were formerly much deeper than they now are.); it is now quite useless, except for boats.
VALDIVIA.
I did not observe here any distinct proofs of recent elevation; but in a bed of very soft sandstone, forming a fringe-like plain, about sixty feet in height, round the hills of mica-slate, there are shells of Mytilus, Crepidula, Solen, Novaculina, and Cytheraea, too imperfect to be specifically recognised.At Imperial, seventy miles north of Valdivia, Aguerros states that there are large beds of shells, at a considerable distance from the coast, which are burnt for lime.(Ibid page 25.) The island of Mocha, lying a little north of Imperial, was uplifted two feet, during the earthquake of 1835.("Voyages of 'Adventure' and 'Beagle'"volume 2 page 415.)
CONCEPCION.
I cannot add anything to the excellent account by Captain Fitzroy of the elevation of the land at this place, which accompanied the earthquake of 1835.(Ibid volume 2 page 412 et seq.In volume 5 page 601 of the "Geological Transactions" I have given an account of the remarkable volcanic phenomena, which accompanied this earthquake.These phenomena appear to me to prove that the action, by which large tracts of land are uplifted, and by which volcanic eruptions are produced, is in every respect identical.) I will only recall to the recollection of geologists, that the southern end of the island of St.Mary was uplifted eight feet, the central part nine, and the northern end ten feet; and the whole island more than the surrounding districts.Great beds of mussels, patellae, and chitons still adhering to the rocks were upraised above high-water mark; and some acres of a rocky flat, which was formerly always covered by the sea, was left standing dry, and exhaled an offensive smell, from the many attached and putrefying shells.It appears from the researches of Captain Fitzroy that both the island of St.Mary and Concepcion (which was uplifted only four or five feet) in the course of some weeks subsided, and lost part of their first elevation.I will only add as a lesson of caution, that round the sandy shores of the great Bay of Concepcion, it was most difficult, owing to the obliterating effects of the great accompanying wave, to recognise any distinct evidence of this considerable upheaval; one spot must be excepted, where there was a detached rock which before the earthquake had always been covered by the sea, but afterwards was left uncovered.