S8 



)r F.N SKI. 



Fig. 15. \- 



.t !li 



nicrior of Quel^rada do la Loberia vieja towards the shore. 

 Photo C. SkottsberL--. 



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

 is sin(^tilarly rich in the basaltic dikes which in hundredfold vertically traverse 

 the island. 



The rocks |)re\ailin<^ at heights up to c. i ooo m may for the greater part 

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

 (Ab 45, An 55). Olivine is scarce as phenocrysts but occurs in varying amount 

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

 glassy matrix (V\i^. 16). 



In many sj)ecimcns of the vesicular lavas the vesicles are filled or lined with 

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

 fust a coating ot glass, followed by chalcedon and chabasite and a central 

 replenishment of well develoj)ed natrolite spherolites. 



At an elevatif)n of about i ooo m a lava bed of a \ery ditTerent aspect is 

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

 (jiiestion is light gre\- in colour. Large j)henocrysts of olivine (up to 5 mm in 

 diameter) and labrad'jrite are tmiformly distributed in a very fine-grained ground- 

 mass, consistmg 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 asj:)ect. The surface 

 teels rough and grain\- which, together with the light grey colour, at first gives 

 the impression that the rock would ha\e a trachytic or trachy-andesitic compo- 



