ORIGIN OF AUGITE ANDESITE 415 



This rock appeared to be quite holocrystalline and was essentially- 

 composed of feldspar microclites, augite and iron oxide; olivine was a 

 rare accessory. The augite and feldspar appear in large part to have 

 crystallized at the same time. 



The lava from the flanking fissure was examined and found to be 

 very dense (sp. gr. 2.97) and much more basic than the lavas at the 

 edge of the crater "qui datent pourtant de la meme phase eruptive. " 

 This basic lava carried only 48.98 per cent of silica. It is rich in 

 olivine, magnetite and augite, with which less abundant crystals of 

 anorthite are associated. 



Ainsi, deversement d'une lave vitreuse, relativement riche en silice, par-dessus 

 les bords du cratere comme un trop-plein, puis soutirage par des crevasses late- 

 rales, ouvertes a une assez grande distance du sommet, d'une lava tout a la fois 

 plus basique et plus dense, chargee de peridot, mais se rapprochant de la prece- 

 dente par la nature de son element feldspathique (anorthite), tels sont les deux 

 phenomenes consecutifs presentes par I'eruption de 1874. On peut les con- 

 siderer comme representant la marche habituelle des eruptions de ce volcan. ' 



The basic lava was observed to flow slowly, so that one may conclude 

 that the temperature was well below 1200° C. or the point at which 

 olivine begins to form in a cooling basalt. Other recorded flows of 

 basaltic nature, such as that of 18 12, ran with great speed, indicating 

 higher temperatures and much lower kinetic viscosity. The tempera- 

 ture of the lavas within the vent is, thus, sufhciently variable to allow 

 of their intermittent differentiation by the sinking of phenocrysts. In 

 the process of time the island has been built up by alternating flows 

 of olivine basalt, basic and acid augite andesites, and either true 

 picritic rocks (described by Velain) or their near relatives, ultra- 

 basic basalts. Normal olivine basalt formed the original lava, 

 whence, through fractional crystalhzation, the other types were 

 derived. 



4. General field relations. — The hypothesis implies that, in general, 

 the derivation of augite andesite and olivine rocks from olivine basalt 

 takes place in the relatively small openings represented by volcanic 

 vents ; secondly, that the effective differentiation occurs only when the 

 lava column stands in the vent for a considerable time, and at tempera- 

 tures ranging between 1200° and 1050° C. ; thirdly, that the peri- 



^ C. Velain, op. cit., p. 182. 



