72 



PERCY QUENSEL 





Fig'. 30. Picrite basalt (masafuerite). Dike rock. Nat. size. Loberia vieja, Masafuera. 



Hagerman has recorded in his paper in tabular form his conception of the 

 horizontal distribution of the different lavas of Masafuera. This is reproduced on p. 

 71 with some slight corrections on the base of renewed examination. 



My former supposition that the basaltic lavas only occur up to an elevation 

 of about I 000 m, from there on being succeeded by more alkaline rocks of 

 soda-trachytic composition, is no longer in agreement with more recent observations, 

 based on Skottsberg's new collections. Any thought of gravitative differentiation 

 to explain the sequence of the volcanic rocks, which I tentatixely proposed in 

 my former publication, must in the light of later observations be discarded now. 



The numerous basaltic dikes, traversing the whole island in a West-East 

 direction, are worth special notice as representing rocks exceptionally rich in 

 olivine. In this respect they exceed the most olivine-rich picrite basalts from 

 Puerto Frances on Masatierra. BOWEN has commented on these rocks as follows: 

 "One other rock may be mentioned in this connection. It is a picrite basalt from 

 Juan Fernandez, a dike, not a lava, but quenched so as to reveal the fact of its 

 origin. In it is shown the highest amount of normative olivine (53 %) of any rock 

 termed basalt by the author describing it. Great crystals of olivine lie in 

 an aphanitic ground composed mainly of plagioclase and augite (Fig. 30 — 32). 

 Some of the olivine basalts of this island group are, locally at least, about 

 as rich in olivine as this dike, but they ha\e not been analyzed. Their high 

 olivine content is invariably due to an increased amount of phenocrysts of 

 olivine about i cm in diameter. Plainly these crystals were not in solution 

 in the dike or flow material at the time of its intrusion or extrusion. This 



