8o PERCY QUENSEL 



soda-trachyte from Maui and Anahulu, on the first island in connection with a 

 'picritic basalt' (U. S. Geol. Survey, Prof. Paper 88, 191 5, p. 26 — 28). This corre- 

 sponds exactly to the rock assemblage of Masafuera. 



Petrographic description of the Hawaiian basalts from the mentioned localities 

 is, also in other respects, found to agree with both megascopic and microscopic 

 features of the basalts of Juan Fernandez. Washington describes an aphyric basalt 

 from Kohala as follows: "The type is a light grey, almost aphanitic lava, except 

 that some rare, very small feldspar phenocrysts may be present, and a few pheno- 

 crysts of olivine are seen in most specimens . . . The texture is rough and trachytic, 

 so that the rock would probably be considered an andesite or trachyte in the 

 field. Most specimens are dense and very fine-grained or aphanitic, but vesicular 

 forms may occur" (1. c, p. 485). This description might as well refer to the light 

 grey basalts of Masafuera at elevations between i 100 and i 400 m, which I also, 

 before an analysis was made, tentatively denoted as a trachy-andesite (12, p. 282). 



Although certain lavas of Hawaii evidently present similarities with the basalts 

 from Juan Fernandez, the general assemblage has, however, a different character. 

 According to Washington "olivine-free labradorite basalts constitutes the most 

 abundant type, followed in abundance by andesine basalt and then by oligoclase 

 andesite" (Am. J. of Sc. 6, 1923, p. 355). The high percentage of andesine basalt 

 and andesite denote a magmatic sequence diftering from that of the non-alkaline 

 lavas of more southern latitudes of the Pacific. It may, therefore, be advisible for 

 the present to comply with Lacroix when he says: "Les roches de cette ile 

 (Hawaii) constitueraient une division speciale, ayant une originalite propre" 

 (14, p. 'jG). 



If we take into consideration the assemblage of olivine and feldspar basalts 

 and soda-trachytic lavas on Masafuera as a characteristic feature for this island 

 it seems evident that, as Richardson already has assumed, the rocks of the 

 Galapagos Archipelago display the closest similarities. According to the analyses, 

 published by Richardson, both the basalts and the soda-trachyte are in chemical 

 composition very similar to equivalent rocks of Masafuera. Also soda-trachyte is 

 of the same scarce occurrence in the Galapagos Islands as on Masafuera, the 

 only sample being collected by Darwin on the Beagle voyage of 1835. Richardson 

 says: "Juan Fernandez are the only islands on which both types of basalt are 

 found in addition to soda trachyte similar to that occurring in the Galapagos 

 archipelago" (27, p. 64). The low content of potassium is in common for the 

 basalts from both island groups. 



On the other hand we must evidently exclude any petrographical relationship 

 between Juan Fernandez and San F'elix — San Ambrosio where the lavas have, as 

 far as is known, a more pronounced alkaline composition, classified by Washington 

 as soda-trachytes and nepheline basanites (1. c, p. 382). Richardson says: "The 

 Juan Fernandez are petrologically closer to the Galapagos than are any other 

 islands . . . Both are situated comparatively near the American coast of the Pacific, 

 but their similarity is not shared by San Felix and San Ambrosio islands, or 

 any other islands on that side of the Pacific" (27, p. 64). Lacroix comes to the 

 same conclusion. He finds the closest connection with San Felix — San Ambrosio 



