66 \'V\iC\ (JUKNSKL 



berg's intorniation, boulders of tliis character were ver}' numerous in the talus 

 under a steep escar|)nient and can here hardlx' ha\e originated from higher eleva- 

 tions tlian about 400 ni. 



In outwarti a[)|)earance this rock shows an aspect different to the compact 

 soda tiachxte i)et\\een I 000 and i loom, which on account of its dense texture 

 at first sii;ht i^ives the impression of a (piart/itic rock. The new specimen has 

 on the other hand a roui^h. loose grained structure. Minute inter^jjranular cavities 

 in the rock are seen under the microscope to be tilled with limonitic matter. But 

 allowing for tiiis structural dissimilarity the rocks from the talus below Tierras 

 Hlancas seem to l)e closel}' related to the soda-traclu'te as described above (Fig. 21). 

 Taking into consideration the seemingl}- horizontal j)()sition of the lava beds of 

 -Masafuera. the conclusion must be that la\as of soda-trachytic composition have 

 been emittetl at different times and that they have alternated with lavas of more 

 normal basaltic comj)()siti()n (see j). 71). 



.S|)ecimens of the rocks from between i 100 and i 400 m j)resent again a 

 different asj^ect. I'^ive sam|)les from I lOO, I 200, I 300 and I 420 m all have 

 \er\- much the same a})pearance. The}' are all phyric lavas of an ash-grey colour, 

 i-'eldspar and, more rarel}-, olivine occur as ])henocrysts in a groundmass of 

 basic plagioclase, augite and abundant magnetite. In a somewhat vesicular specimen 

 from I 100 m olivine is also present as sporadic grains in the groundmass. 



The groundmass has in most of the specimens a trachytic texture; the narrow 

 laths of feldsjjar circuiting the phenocrysts in a more or less well-defined 

 manner (I'^ig. 22). 



In m\- previous paj)er I named the rocks of this nature trachyandesite. 

 Ilagerman sa\-s, referring to much the same specimens in Skottsberg's collection: 

 "Die Klassifi/ierung hiehergehdriger Gesteine ist etwas unsicher. Geniigende 

 Grimde sie als Trach\-andesite zu bezeichnen, liegen jedoch nicht vor" (13, p. 30). 

 I iagerman names the rock andesite in his j)aper. 



Tliese rocks evidently have a rather wide distribution on ^lasafuera, j^robably 

 lorming the whole comi)lex of la\a beds between i 100 and i 420 m. To certify 

 their j)etrographic ])osition an analysis has now been made of a typical specimen 

 ( Tabk- I, No. 5). The calculated norm and Xiggli values definitely classify the 

 rock as a tel(lsj)ar basalt, in many respects of similar composition as some of 

 tile analxsed basalts from Masatierra (Table I, \o. i — 4). A difference of interest 

 is the still lower content of KoO; the Xiggli value X' is now 0.08 against O.I 5 — 0.20 

 in the basalts fiom Masatierra. 



I hese phyric feldspar basalts from ^lasafuera in general aspect also very 

 much resemble the apii\ric fel(lsj)ar basalts from between 400 — 600 m elevation 

 on Masatierra. ( )ccasionall\- at lower levels they also on Masafuera may be aphyric 

 in texture il'ig. 23). dhey seem also in several respects to correspond to certain 

 ph\ric and aph\ric basalts from the I lawaiian Islands, as described by Washinotox 

 (Petrology of the I lawaiian Islands. I. Kohala and Mauna Kea. Am. J. of Sc. V, 

 '9-3' P- 4''^"*- ^ '> this (juestion I will return under a concluding heading dealing 

 with the chemical and petrological connections between the rock of the Juan 

 I-'ernandez Islands and other volcanic islands of the eastern Pacific. 



