58 



PERCY QUENSEL 



Fig. 15. View of the interior of Quebrada de la Loberia vieja towards the shore. 



Photo C. Skottsbergr. 



of Masatierra, olivine is more scarce in the dominant basalts of ^Masafuera, but 

 is singularl}- rich in the basaltic dikes which in hundredfold vertically traverse 

 the island. 



The rocks prevailing at heights up to c. i 000 m may for the greater part 

 be classified as dark vesicular feldspar basalts, with large phenocrysts of labradorite 

 (Ab 45, An 55). Olivine is scarce as phenocr}'sts but occurs in varying amount 

 in the groundmass together with augite, labradorite and magnetite in a dark, 

 glassy matrix (Fig. 16). 



In many specimens of the vesicular lavas the vesicles are filled or lined with 

 zeolite minerals. In some samples the vesicles present, from the outward rim, 

 first a coating of glass, followed by chalcedon and chabasite and a central 

 replenishment of well developed natrolite spherolites. 



At an elevation of about i 000 m a lava bed of a very different aspect is 

 met with. In contrast to the dark basalts of lower horizons, the rock now in 

 (jLiestion is light grey in colour. Large })henocrysts of oli\ine (up to 5 mm in 

 diameter) and labradorite are uniformly distributed in a \er\' fine-grained ground- 

 mass, consisting of augite, slender laths of labradorite, abundant small grains of 

 magnetite and a small amount of a nearly colourless glass (Fig. 17). 



In outward appearance this lava has a very singular aspect. The surface 

 feels rough and grainy which, together with the light grey colour, at first gives 

 the impression that the rock would have a trachytic or trachy-andesitic compo- 



