40 



I'KRrV (UF.NSKL 



the microscope tlie fiiiulaineiUal mass, in wiiich the plagioclase crystals are embedded^ 

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

 which ma\- be compared with tiiose of b\townite. The sections of the mineral 

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

 This matter which looks slightlx' gre\-, when seen by ordinary light, remains 

 obscured l)etween crossed nicols . . . The olivine, of which large sections are seen, 

 is uniform!)- changed into a red hematite; these sections, however, still show 

 extinctions like those of the unaltered olivine" (8, [). 176). The characteristic 

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

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

 I"'or the rest his description mostly refers to the colour and texture of differ- 

 ent samples of tlolerite and basalt. 



In 1886 L. l).\K.\i'SK\ published a short report on detached rounded lumps 

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

 campana' but named "(dockenstein" by the author (9). R. PoilLMANN 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. ScHULZE 

 and K. I'( »lll.M.\NN, participants in Fedkrico Joiiow's expedition to the islands 

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

 'Ivstudios 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 

 (ieologie der Juan I^'ernandezinseln' (12). In 1916 — 17 Skottsberg again visited the 

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

 described by '1\ H.\(;krmax under the title 'Beitrage zur Geologie der Juan 

 I'ernande/. Inseln' ( I 3). 



Since Skottsberg's visit in 1917, no further exploration in the field has been 

 |)ublishe(l concerning the geology of the islands in question. And, with the exception 

 ot tour recent analyses of lavas from Masatierra from collections made by members 

 of Dumont d Frvillcs A'oyage au I'ole Sud et dans rOceanie' in 1838, commented 

 on 1)\- L.\(k(»i\ (14, p. 64), no further observations have, as far as known, been 

 j)ublishcd on the gecjlogy or petrography of the Juan Fernandez Islands. On the 

 other hand (|uestions relating to their lithological connection with other intra- 

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

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



Main Geological Features. 



1 he two islands Masatierra and Masafuera j)resent very different aspects with 

 regard to their bulk conti-uration. Masatierra exhibits a rugged appearance of 

 isolated jagged cliffs lI"io. 21. The highest peak, VA Vunque, is 927 m. On the 

 clear-cut sliore boundc-d escarpments one can distinguish many hundred lava 

 beds overlying one another. In thickness they can vary from .some few metres 

 up to 20 m or more. 



