40 PERCY QUEXSEL 



the microscope the fumlainental mass, in which the plagioclase crystals are embedded^ 

 has a dolerite structure. The felspar crystals show large extinction angles (38° — 41°) 

 which may be compared with those of bytownite. The sections of the mineral 

 are cracked and pervaded with zeolite matter, which forms an irregular network. 

 This matter which looks slightly grey, when seen by ordinary light, remains 

 obscured between crossed nicols . . . The olivine, of which large sections are seen, 

 is uniformly changed into a red iiematite; these sections, however, still show- 

 extinctions like those of tlie unaltered oli\ine" (8, p. 176). The characteristic 

 change of olivine to iddingsite in the basalts, which will be treated below in 

 some detail, is here evidently noted by Renard, though b\- him named hematite. 

 For the rest his descrij^tion mostly refers to the colour and texture of differ- 

 ent samples of dolerite and basalt. 



In 1886 L. DaRAP-SKV published a short report on detached rounded lumps 

 of magnesite from Bahia del Padre in Masatierra, locally known as "piedras de 

 campana" but named "Glockenstein" by the author (9). R. POllLMANX later (1893) 

 described this singular formation in rather more detail and discussed its origin (10). 



An account of some geological and petrographical observations by J. SciiULZE 

 and R. POiil.mann, participants in Fedekico Joiiow's expedition to the islands 

 in 1891, has been published by the latter as an introduction to Johow's monograph: 

 'Estudios sobre la Flora de las islas de Juan Fernandez' (11, p. i). 



As a member of the Swedish Magellanian Expedition of 1907 — 1909 I had 

 the opportunity, together with Professor Skott.SBERG, of visiting the islands in 

 1908 and later described the rock specimens then collected in a paper: 'Die 

 Geologie der Juan Fernandezinseln' (12). In 1916 — 17 Skottsberg again visited the 

 islands and collected specimens from some new localities. These were subsequently 

 described by T. Hagerman under the title 'Beitrage zur Geologie der Juan 

 Fernandez-Inseln' (13)- 



Since Skottsberg's visit in 19 17, no further exploration in the field has been 

 published concerning the geology of the islands in question. And, with the exception 

 of four recent analyses of lavas from Masatierra from collections made by members 

 of Dumont d'Urville's 'Voyage au Pole Sud et dans I'Oceanie' in 1838, commented 

 on by LacroIX (14, p. 64), no further observations have, as far as known, been 

 published on the geology or petrography of the Juan Fernandez Islands. On the 

 other hand questions relating to their lithological connection with other intra- 

 pacific islands and their geophysical position in relation to eastern-pacific volcanic 

 centres has been the subject of repeated discussions during later years. 



Main Geological Features. 



The two islands Masatierra and Masafuera present very different aspects with 

 regard to their bulk configuration. Masatierra exhibits a rugged appearance ot 

 isolated jagged cliffs (Fig. 2). The highest peak, El Yunque, is 927 m. On the 

 clear-cut shore-bounded escarpments one can distinguish man\' hundred lava 

 beds overlying one another. In thickness the}- can \ ar\' from some few metres 

 up to 20 m or more. 



