S. Powers — Notes on Hawaiian Petrology. 271 



Microscopic examination of thin-sections of the grey, 

 banded trachyte from Waiolai gulch, on the northeast side 

 of West Maui, shows bands of chlorite, biotite, apatite, 

 and limonite and occasional large phenocrysts of olivine 

 in a rock composed principally of oligoclase feldspar laths 

 with minor amounts of orthoclase feldspar, some altered 

 olivines, secondary limonite, hematite, and sericite, micro- 

 lites of unknown composition, and glass. A polished 

 section shows no magnetite. 



Trachyte from Puu Paupau shows less alteration. The 

 feldspar, which composes almost the entire rock, is an 

 albite-oligoclase and the groundmass contains orthoclase 

 feldspar, olivine, apatite, magnetite or ilmenite (identi- 

 fied in polished section), limonite, biotite, and glass. 



A thin-section of the trachyte from Waihee valley 

 shows an alkali feldspar similar to the others. The sub- 

 ordinate constituents are glass, magnetite or ilmenite 

 (quite abundant as seen in polished section), gegirine 

 augite in greenish yellow laths, and biotite and apatite. 



From Molokai a schistose, grey trachyte was collected 

 by the writer. The source of the specimen is believed to 

 have been stream gravel near the mouth of Wailau gulch 

 and the rock is judged to occur in place somewhere on the 

 rim of the precipitous and very large gulch. 



The rock is composed almost entirely of phenocrysts 

 of oligoclase 21 feldspar with subsidiary amounts of brown 

 hornblende (now almost wholly changed to granular ag- 

 gregates of ore, brown hornblende, and to a nearly color- 

 less mineral, probably an amphibole) and augite. The 

 groundmass is finely crystalline and consists of oligoclase, 

 rarely orthoclase, abundant small brown hornblende 

 crystals, augite and magnetite. There is a nearly color- 

 less prismoid material that is probably secondary amphi- 

 bole derived from the breaking-up of the primary brown 

 hornblende. This trachyte differs from those on Maui 

 and Hawaii because of the absence of a plate-like ar- 

 rangement of the feldspar. 



It has been supposed by Daly 22 that the origin of the 

 trachyte on the Hawaiian Islands is connected with the 

 absorption of limestone by the basaltic magma and subse- 

 quent differentiation. While only a small amount of 



21 The writer is indebted to Professor Charles H. Warren, of the Massa- 

 chusetts Institute of Technology, for the description of this rock. 



22 Magmatic differentiation in the Hawaiian Islands, Jour. GeoL, vol. 19, 

 p. 314, 1911. 



