Report on the scientific results of the voyage of during the years 1873-76 : under the command of Captain George SNares, , Captain Frank Turle Thomson, . und there. See also Hahn,Jnsel Studien, p. 108. REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 177 superstructure, but from which the crater and accumulation of tufa have is therefore probable that Juan Fernandez, the other islands composing the group,and the reefs which surround them, belonged formerly to a volcano whose lighterproducts have been disaggregated and carried away by mechanical agenc
Report on the scientific results of the voyage of during the years 1873-76 : under the command of Captain George SNares, , Captain Frank Turle Thomson, . und there. See also Hahn,Jnsel Studien, p. 108. REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 177 superstructure, but from which the crater and accumulation of tufa have is therefore probable that Juan Fernandez, the other islands composing the group,and the reefs which surround them, belonged formerly to a volcano whose lighterproducts have been disaggregated and carried away by mechanical agencies. Theseislands being situated at a relatively short distance off an essentially volcanic region, itis quite possible that the former eruptions of Juan Fernandez were related to those ofChili. It has been ascertained that, when the latter country was devastated by greatearthquakes, phenomena connected with those on the Chilian coast were observed inJuan Fernandez Islands. In the year 1855 thick columns of vapour, rising from thesea, were observed at the distance of an English mile from the western island, and theclose proximity of a volcanic centre seems therefore to be Cumberland Bay, Juan Fernandez. Amongst the rocks collected at Juan Fernandez by the Challenger Expedition in1875, we have not, however, found any specimens which might belong to very recenteruptions; no tufas, no volcanic ashes are to be found, and everything seems to provethat they have been washed away by the waves and the atmospheric denuding rocks which have been submitted to examination all belong to the basaltic type,and it seems probable that the whole island is made up of those that we are aboutto describe. The rocks which form the central mass of the island appear in the specimens asdolerites or as common basalts. They have a tolerably fresh appearance, their colouris bluish grey, the fracture is even, the grain is compact, very few vesicles are seen. (PHYS. CHEM. CHALL. EXP.
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