256 S.'Poivers — Notes on Hawaiian Petrology. 



Art. XIX. — Note.s on Hawaiian Petrology; by Sidney 



Powees. 



Contents. 

 Introduction. 

 General Observations. 



Necker. 



Mhoa. 



Niihau. 



Kauai. 



Oahu. 



Molokai. 



Lanai. 



Kahoolawe. 



Maui. 



Hawaii. 

 Distribution of rarer types. 



Trachyte. 



Nephelite basalt, 



Olivine nodules. 



Gabbro intrusives. 

 Age relations. 



Introduction. 



The Hawaiian Islands are composed principally of 

 olivine basalt with subordinate amount of olivine-free 

 basalt and minor occurrences of trachyte, nephelite basalt, 

 gabbro, trachyandesite, and a few other types as de- 

 cribed by Dr. Whitman Cross. 1 The distribution of the 

 rarer types and their relation to phases of vulcanism has 

 not been emphasized and forms the subject of a portion 

 of the present paper. There is also given a description 

 of new occurrences of trachyte on Molokai and on the 

 north, northeast, and south sides of West Maui, of very 

 large olivine nodules in a picritic basalt on Kauai, and of 

 extensive deposits of volcanic breccia on West Maui. 

 Attention is also called to the arrangement of subsidiary 

 cones on the younger volcanoes. 



All the islands of the group were visited by the writer 

 in 1915, with the aid of a Sheldon Travelling Fellow- 

 ship from Harvard University, and a few notes concern- 

 ing the rocks found on them are given below. The 

 islands Niihau and Kahoolawe were never before exam- 

 ined geologically. It is shown that Haleakala, the re- 

 cently extinct volcano forming East Maui, was last active 

 about 1750. Hualalai, on Hawaii, was probably active 

 as late as 1840-41. 



1 U. S. Geological Survey, Professional Paper 88, 1915. 



