ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEZ ISLANDS 45. 



Masatierra. 



(Lat. 33° 37i' S.. Long. 78° 50' W.) 



The two collections of specimens from Masatierra by Skottsberg and my- 

 self have partly been taken from different localities. The central parts of the 

 island are however sparsely represented. Somewhat more complete series of rock 

 samples originate from the heights around Bahia Cumberland (Ensenada de San 

 Juan Bautista), Puerto Frances and Portezuelo as well as from Bahia del Padre. 

 From the adjacent small island Santa Clara there is only one specimen. It is 

 not easy to single out the local distribution of the different lava beds on account 

 of the inconnected localities from which specimens have been collected. Some 

 characteristic features may however be found which indicate that certain types 

 of lavas are restricted to localised areas. To some extent one may then draw 

 conclusions regarding the sequence of the volcanic eruptions. 



The predominating rocks of Masatierra are olivine basalts, differing only in 

 their content of olivine. Lava beds from around Puerto Frances are exceptionally 

 rich in this mineral, the content of which can reach 40 vol.% of the rock (Fig. 6). Such 

 rocks, with an extreme content of olivine, I named picrite basalts in my earlier 

 paper (12, p. 265). Lacroix originally named such basalts 'picrite feldspatique' 

 but later discarded this name, substituting for it the name oceanite', under which 

 name he includes the basalts of the Juan Fernandez Islands with an exceptionally 

 high content of olivine (14, p. 65). Since the name picrite basalt, as originally 

 defined in my former paper, has later been adopted ^ I will retain this name for 

 the rocks in question, with the name oceanite as synonym. 



From the extremely olivine-rich basalts around Puerto Frances there is every 

 transition over intermediate types to feldspar basalts without any olivine at all 

 or with only a very insignificant amount thereof. Such rocks, however, have a 

 relatively limited distribution on Masatierra. In general one may say that types 

 with a very high content of olivine are restricted to lower elevations, whereas 

 higher up more normal olivine basalts predominate. 



A second feature of dissimilarity in the basalts is found in their texture. 

 Many of the lava beds show a coarse-grained ophitic texture and may be classed 

 as dolerites, and have been so named by Hagerman and myself in our previous 

 papers. Such rocks are usual around Puerto Frances and Bahia Cumberland as 

 well as at Vaqueria and Tres Puntas. Specimens from all these localities are 

 in outward appearance very much alike. In general these rocks, in contrast to 

 other lavas, are singularly fresh. Only the olivine often shows a dark brown 

 rim, indicating an incipient alteration to iddingsite {c(. 12, p. 260). 



The doleritic basalts seem only to occur at lower horizons, where the lava 

 beds generally attain their greatest thickness. All the specimens thereof at hand 

 are from between sea level and 200 m. A sample in Skottsberg's collection from 

 Bahia Cumberland (Fig. 7) and another from Vaqueria are both from 1 50 m above sea 



' The name oceanite was first proposed by Lacroix in 1923 (in Mineralogie de Madagascar, 



Vol. Ill, p. 49). The name is given "k cause de leur abondance dans les iles du Pac-hque". 



^ Holmes, Q. J. Geol. Soc, 172, 1916, p, 231; Washington, Am. J. of Sc. V, 1923. p. 471- 



