310 REGINALD A. DALY 



areas at the present surface of the island. As expected on the 

 hypothesis, the alkaline types more nearly approximating the 

 average basalt in composition are much more voluminous . than 

 the extreme phonolite-trachyte member of the series. In Mauna 

 Kea, at any rate, the trachydoleritic representative of the alkaline 

 species seems to be confined to the summit plateau of the volcano, 

 that is, to the region where it should occur if it were due to the 

 vertical differentiation of the basalt. 



On the other hand, the dominant rocks on the broad summits 

 of Mauna Loa and Hualalai, and of Haleakala, in Maui, are 

 normal basalts, often rich in phenocrystic olivine. 1 There is no 

 doubt that the conditions were unfavorable to important differ- 

 entiation during most of the time engaged in the building of these 

 giant volcanoes. Similarly, the lava of the active vent at Kilauea 

 is basaltic and apparently has always been of that normal com- 

 position. 



One reason for this contrast with Mauna Kea in its latest stage 

 is probably connected with difference of temperature, for the 

 differentiation of any of the commoner earth magmas seems to 

 take place only within a comparatively narrow temperature range 

 occurring just above the "point" of solidification. That the 

 average temperature of the latest Mauna Kea vents was actually 

 lower than that characteristic of the active Mokuaweoweo on 

 Mauna Loa is suggested: (a) by the smaller size of the pipes on 

 Mauna Kea; (b) by the far greater abundance of pyroclastic 

 material on Mauna Kea; and (c) the correlative high viscosity 

 of the short, stubby flows on Mauna Kea. The latter were more 

 viscous than the average flow on the summit of Mauna Loa, not 

 merely or chiefly because of difference in chemical composition. 



But a probably much stronger control is to be found in the 



1 E. S. Dana describes a group of "clinkstone-like basalts" (specific gravity, 

 2.82-3.00), free from olivine or very poor in it, which were collected at the summit 

 of Mauna Loa. These may represent incipient differentiation even at Mokuaweoweo. 

 Another, highly olivinic group of basalts (specific gravity 3 . 00-3 . 20) are, however, 

 associated in great volume. (See J. D. Dana, Characteristics of Volcanoes [New 

 York, 1891], p. 319.) The present writer found a similar variation in the basalts at 

 the summit of Haleakala, which are cut by dikes of compact, olivine-free rock sug- 

 gestively like the trachydolerite of Mauna Kea. 



