S. Powers — Notes on Hawaiian Petrology. 257 



This paper presents the conclusions of the writer on 

 a variety of subjects. It is presented as his contribution 

 to the knowledge of the geology of the Hawaiian Islands. 

 It is by no means complete on any one of the subjects 

 treated. Petrographic descriptions are given only to 

 substantiate conclusions. Dr. Whitman Cross has the 

 collections of rocks and has very kindly offered to study 

 them petrographically. However, the writer prefers to 

 publish at this time the scattered notes which follow 

 rather than to postpone the publication for complete 

 corroboration. Sincere thanks are due to Professors 

 K. A. Daly and J. E. Wolff of Harvard University, for 

 making this investigation possible, and to Professor Daly, 

 to Dr. T. A. Jaggar, and to Dr. Whitman Cross for 

 kindly criticizing the manuscript. 



General Observations. 



Necker. — Xecker Island is one of the western Hawai- 

 ian group, 250 miles northwest of Kauai. The island is 

 described as the ''remains of a soil-capped volcanic 

 crater" 300 feet high, % mile long, and 500 feet wide 

 at the widest part. 2 A specimen of basalt collected on 

 this island for Professor W. A. Bryan, of the College of 

 Hawaii, Honolulu, is a fine-grained rock composed of 

 phenocrysts of feldspar and smaller phenocrysts of 

 olivine in a fine-grained grounclmass. The groundmass 

 shows under the microscope part ophitic, part poikilitic 

 texture and is composed of labradorite feldspar, slightly 

 titaniferous augite, olivine, magnetite, and apatite. 



Xilwa. — Xihoa Island rises abruptly from the sea 120 

 miles northwest of Kauai. Professor Bryan collected a 

 specimen of altered basalt from the island. The rock 

 is an olivine basalt, and consists of large olivine crystals 

 between feebly poikilitic feldspar laths in an aphanitic, 

 partly glassy groundmass in which magnetite and thin 

 feldspar laths are visible. 



Niihau. — Xiihau (fig. 1) is a desert island consisting 

 of a fragment of an original island 3 bounded on three 

 sides by a low plain. The surface of the plain is an ele- 



- W. A. Bryan: Natural History of Hawaii, Honolulu, 1915, p. 93; also 

 described by Carl Elschner, The Leeward Islands of the Hawaiian Group, 

 reprinted from the Sunday Advertiser, Honolulu, 1915, 68p. 



3 S. Powers: Tectonic lines in the Hawaiian Islands, Bull. Geol. Soc. 

 Amer., vol. 28, p. 513-511, 1917. 



