304 REGINALD A. DALY 



visible blocks are all angular, quite fresh, and 20 to 50 cm. in 

 greatest diameters. No olivine is visible in the fairly dark-gray, 

 granular rock, either in the field or under the miscroscope. The 

 essential constituents are labradorite, Ab 2 An 3 , and a strongly 

 tinted, greenish-brown, non-pleochroic augite, with an unusual 

 amount of iron ore, probably ilmenite. Apatite in needle form 

 is very abundant; therein this rock contrasts with nearly every 

 Hawaiian rock so far studied in thin section. The stout augite 

 prisms, which lack the diallage parting, reach 4 mm. in length; 

 the thick tables of labradorite are often 5 mm. in length and the 

 plates of ilmenite measure 1 mm., or less, to 5 mm. in length. 

 The structure of the rock is not basaltic or diabasic, but typically 

 hypidiomorphic-granular. 



On the summit of Hualalai the writer sampled three projected 

 blocks which occur in a thin pyroclastic deposit veneering this 

 lava-formed (olivine-basalt) volcano. Two of them are holocrys- 

 talline equivalents of the normal olivine basalt of the island. 

 The third is a coarsely granular rock almost identical in compo- 

 sition and grain with the type forming the laccolith at Uwekahuna; 

 it is an ultra-femic gabbro, with high idiomorphism in the abundant 

 olivine. 



Average composition of Hawaiian basalt: — Of the extant analyses 

 of the basalts from the main island, nineteen, which were made 

 from fresh and typical material, have been selected for the purpose 

 of computing the average composition of the dominant rock type 

 of the island. Most of these analyses are quoted in C. H. Hitch- 

 cock's Hawaii and Its Volcanoes (Appendix D). Ten are taken 

 from O. Silvestri's paper in the Bolletino del R. Comitato Geologico 

 Italiano (XIX [1888], 185); four from E. Cohen's paper in the 

 Neues Jahrbuch fur Mineralogie, etc. (1880; II, 23); and four 

 from A. B. Lyons' paper in the American Journal of Science (II 

 [1896], 424). Mr. Steiger's analysis of the 1852 flow and his 

 analysis of the chemically similar porphyry forming the small 

 laccolith at Uwekahuna, Kilauea, are also included, making twenty 

 analyses in all. 



The calculated average is shown in the first column of Table 

 VI, where the second column gives the writer's result in averaging 



