REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



177 



superstructure, but from which the crater and accumulation of tufa have disappeared. 

 It is therefore probable that Juan Fernandez, the other islands composing the group, 

 and the reefs which surround them, belonged formerly to a volcano whose lighter 

 products have been disaggregated and carried away by mechanical agencies. These 

 islands being situated at a relatively short distance off an essentially volcanic region, it 

 is quite possible that the former eruptions of Juan Fernandez were related to those of 

 Chili. It has been ascertained that, when the latter country was devastated by great 

 earthquakes, phenomena connected with those on the Chilian coast were observed in 

 Juan Fernandez Islands. In the year 1855 thick columns of vapour, rising from the 

 sea, were observed at the distance of an English mile from the western island, and the 

 close proximity of a volcanic centre seems therefore to be implied. 



Cumberland Bay, Juan Fernandez. 



Amongst the rocks collected at Juan Fernandez by the Challenger Expedition in 

 1875, we have not, however, found any specimens which might belong to very recent 

 eruptions ; no tufas, no volcanic ashes are to be found, and everything seems to prove 

 that they have been washed away by the waves and the atmospheric denuding agencies. 

 The 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 about 

 to describe. 



The rocks which form the central mass of the island appear in the specimens as 

 dolerites or as common basalts. They have a tolerably fresh appearance, their colour 

 is bluish grey, the fracture is even, the grain is compact, very few vesicles are seen. 



(PHYS. CHEM. CHALL. EXP. — PART VII. — 1889.) 23 



