MAGMATIC DIFFERENTIATION IN HAWAII 313 



augite trachyte which is suggestively like the phonolitic trachyte 

 of Puu Anahulu in Hawaii. 1 



Reconnaissances in Kerguelen Island show the intimate asso- 

 ciation of olivine basalt, basalt bearing olivine nodules, trachyte, 

 and phonolite. Basalts and alkaline trachytes are the known 

 species composing Ascension Island. St. Helena shows dominant 

 olivine basalt and olivine-free basalt, with haiiynite basalt and 

 phonolite. 



Without citing other parallels among the oceanic islands, it is 

 now clear that the repeated association of volcanic species, ranging 

 from olivine basalt to phonolitic trachyte or true phonolite, is not 

 accidental. In all essential respects the argument for the gravi- 

 tative differentiation of the salic types from normal basalt seems 

 as strong for the chief Samoan island as it is for the chief Hawaiian 

 island. It is commonly assumed that the subalkaline basalt 

 and the alkaline phonolite originate in separate primary magma 

 chambers. That hypothesis becomes almost, if not quite, incred- 

 ible to any unprejudiced observer of the imposing likeness in the 

 evolution of these distant, deep-sea island groups. 



SUMMARY AND CONCLUSIONS 



The writer's 1909 traverses in Hawaii have led to the view that 

 the average of the many extant, typical analyses of its basalts 

 approximates very closely the composition of the real average of 

 all the basalt exposed in the island. This average is almost 

 identical with that calculated for the world's average basalt. 

 While Mauna Loa and Hualalai are basaltic from base to summit, 

 Mauna Kea is, in a sense, stratified. Up to about the 6,000-foot 

 contour, the broad basal slopes of Mauna Kea are underlain by 

 the normal olivine basalt. From that contour to the summit 

 platform, about 6,000 feet higher, the dominant lava is a basalt, 

 very poor in olivine and, in other respects also, approaching the 

 composition of a basic augite andesite. At the top of the moun- 

 tain are flows and cinder-cones largely consisting of a still less 

 femic type, in which alkaline feldspar (orthoclase or soda- 



1 W. W. Watts, Geological Magazine, XXIII (1896), 358. 



