THE 



JOURNAL OF GEOLOGY 



MAY-JUNE, ign 



MAGMATIC DIFFERENTIATION IN HAWAII 



REGINALD A. DALY 



Massachusetts Institute of Technology, Boston, U.S.A. 



Introduction 

 Special Petrography 



Porphyritic Gabbro of the Uwekahuna Laccolith 



Ultra-femic Olivine Basalt, Flow of 1852 



Andesitic Basalt, Upper Slope of Mauna Kea 



Trachydolerite of Summit Flows, Mauna Kea 



Lherzolitic Nodules in the Summit Lavas of Mauna Kea 



Notes on Other Lava Flows, Studied Microscopically 



Projected Blocks at Kilauea and Hualalai 



Average Composition of Hawaiian Basalt 

 Theoretical Considerations 



Origin of the Ultra-femic Types 



Origin of the Less Femic Types 



Parallel Differentiation in Other Oceanic Islands 

 Summary and Conclusions 



INTRODUCTION 



Though the Hawaiian Islands are largely basaltic, it is already 

 apparent that they contain igneous types of considerable petro- 

 graphic diversity. The species so far discovered range from ultra- 

 femic basalts and intrusive porphyry, with less than 46 per cent 

 of silica and less than 2 per cent of alkalies, to the phonolitic 

 trachyte of western Hawaii, with 62 per cent of silica and more 

 than 13 per cent of alkalies. No sediments of the ordinary silicious 

 kinds appear to enter into the composition of the islands or of their 

 basement. Acid crystalline rocks of the gneissic or granitic order 



Vol. XIX, No. 4 289 



