ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEIZ ISLANDS 49 



a periferic alteration to iddingsite which is otherwise usual in the olivine of the 

 surrounding basaltic lavas. 



At higher levels the basalts have a more normal character. Large phenocrysts 

 of olivine, or olivine and feldspar, lie in a more or less glassy groundmass of 

 augite, olivine and labradorite with abundant small grains of magnetite, llmenite 

 in tabular form or in skeleton cry.stals is now often present. Most of these lavas 

 are vesicular, scoriaceous or slaggy. The cavities are in many cases rimmed or 

 filled with opal. 



The occurrence of basalts of this type is widespread up to the highest parts 

 of the island, and they are without doubt the dominant lavas of Masatierra. 



Somewhat divergent lava beds seem, however, to predominate at intermediate 

 horizons. All the specimens at hand of this type are holocrystalline rocks of an 

 ash-grey colour, aphanitic and aphyric in texture (Fig. 8). They generally show a 

 characteristic light zone of weathering and a tendency to develope a columnar struc- 

 ture (Fig. 9). The fine-grained mineral assemblage consists of augite, labradorite and 

 very abundant magnetite in small euhedral crystals, evenly dispersed throughout 

 the rock. llmenite is also generally present in skeleton crystals. Stray small 

 grains of olivine may in some cases be observed; in other specimens olivine is 

 completely absent. In vesicular lavas of this type the cavities are again more or 

 less filled with opal (Fig. 8 b). 



These lava beds, which represent the only specific feldspar basalts of Masa- 

 tierra, are found at heights between 400 and 500 m (Cordon Chifladores 400 m, 

 Portezuelo 500 m). At an elevation below 100 m at Punta Larga in the more 

 western part of the island the same type has been found, but then in the form of 

 a dike, which may signify a channel for the analogous lavas at higher levels. 



The very characteristic aspect of these rocks seems to indicate that they 

 represent a definite epoch of intrusion, intermediate between the doleritic basalts 

 and picrite basalts of the lower parts and the scoriaceous olivine basalts of the 

 higher horizons. An analysis has been made of a very similar rock from Masafuera, 

 which confirms its classification there as a feldspar basalt (see p. 66). 



Lacroix's four new analyses of basaltic lavas from Masatierra indicate that 

 the analysed rocks are similar in composition. It is regrettable that the specimens 

 all originate from Bahia Cumberland. Probably the members of the d'Urville 

 expedition only brought back samples from that locality and these were there- 

 fore the only specimens available in Paris for Lacroix's analyses. On the other 

 hand the insignificant dip of the lava flows on Masatierra (i5' — 20°) may infer 

 that the analysed rocks can be taken as representative in chemical composition 

 for the basal basalts of the island. Microscopic determinations of corresponding 

 specimens from the other localities point in the same direction. 



With the exception of the ultra-femic picrite basalts from Puerto Frances 

 and the inclusions therein of dunitic rocks, and the light grey feldspar basalts 

 of intermediate horizons, we may conclude that the dominant rocks of Masatierra 

 consist of rather normal olivine basalts deviating principally in texture and in a 

 varying content of olivine. 



