S. Powers — Notes on Hawaiian Petrology. 259 



thickness from 5 to 30 feet and are composed of aphanitic 

 basalt, occasionally showing small olivine phenocrysts. 

 The flows dip westward at an angle of only 1 to 2 degrees. 

 Fonr cones are situated on the plateau-like summit of the 

 old Island. The young cones near the Nonopapa land- 

 ing are composed of olivine basalt. Several pieces of 

 white pumice have been found on the island — evidently 

 material drifted from some far-away volcano. 



Kauai. — Kauai is composed principally of olivine 

 basalt. Of the 110 specimens collected on all sides of 

 the island, 82% are found to contain olivine visible with 

 a hand lens. About 30 young cones and craters are 

 found on the east and south sides of the island and these, 

 with the exception of Kilauea Crater, appear to consist 

 of normal basalt and basaltic ash. The cones are notably 

 arranged in lines more or less radial to the center of 

 the original volcano. 



Oahu. — Oahu is composed of the older Waianag (Kaala) 

 Eange, in which feldspar basalt is the conspicuous rock- 

 type, and the younger Koolau Range, concerning the 

 petrography of which little is known on account of its 

 topographic features. Of the older volcano only 5 young 

 cones remain, the Laeloa craters of Hitchcock, 4 com- 

 posed of nephelite basalt and arranged along a line 

 tangential to the range. On the flank of the younger 

 range about 39 distinct cones are found, of which eleven 

 consist of tuff or basalt containing nephelite with or 

 without melilite, the remainder of normal basalt or of 

 basaltic ash. 



Molohai. — Molokai consists of the low Mauna Loa on 

 the west and a fragment of Wailau volcano 5 on the east. 

 The major portion of the latter volcano has subsided, 

 leaving a fault-scarp on the north side of the island. 



Specimens of rock from Mauna Loa, Molokai, are dim- 

 cult to collect on account of the absence of deep valleys. 

 Basalt filled with olivine is found on the south and south- 

 west sides while feldspar basalt with conspicuous tablets 

 of plagioclase in a bluish rock occurs with olivine basalt 

 and aphanitic olivine-free basalt near the center of the 

 old volcano on the east and northeast sides of the moun- 

 tain. The basalt weathers rapidly in this hot, arid region 

 and when the wind sweeps away the red residual soil hard 



4 Geology of Oahu, Bull. Geol. Soc. Amer., vol. 11, p. 36, 1900. 



5 S. Powers : Tectonic lines on the Hawaiian Islands, Bull. Geol. Soc 

 Amer., vol. 28, p. 513, 1917. 



Am. Jour. Sci.— Fourth Series. Vol. L, No. 298.— October, 1920. 

 19 



