S. Powers — Notes on Hawaiian Petrology. 277 



ture and most of the xenoliths should therefore be sharply 

 separated from the matrix. 



Gabbro intrusives. — Intrusive gabbro has been de- 

 scribed by Lindgren 31 from the Wailau Canyon on Molo- 

 kai, by Cross 32 from the Waimea Canyon on Kauai, and 

 by Daly 33 from Uwekahuna, at Kilauea, Hawaii. In the 

 first locality a tropical jungle conceals the parent body, at 

 the second' locality the rock (probably a dike) has not 

 been seen in place, but at Uwekahuna the intrusive is 

 seen in cross-section with upper and lower contacts 

 within reach in a few places. 34 An inclusion of gabbro in 

 basalt Avas found by the writer in the Hanalei Valley, 

 Kauai. Very ample petrographic descriptions of the 

 Wailau and of the Kauai gabbro s under the name kauaiite 

 (Iddings, Cross), and of the typical Uwekahuna gabbro 

 have been published. 



The Uwekahuna intrusive occurs in the walls of the 

 Kilauean sink north of the crater Halamaumau and the 

 base of the olivine gabbro is 40 feet above the floor of 

 the crater. The intrusive is about 650 feet long and 68 

 feet in maximum height, but it may be composed of two 

 masses. Chilled upper and lower contacts with a uni- 

 form-grained rock between, prove the intrusive character 

 of the mass and suggest rapid cooling. Olivine is lacking 

 at the upper contact in the chilled phase which is one 

 foot thick, but in the second foot the olivine begins to ap- 

 pear abundantly as tiny phenocrysts and within three feet 

 the rock assumes its normal grain. Gas tubes are very 

 abundant at the upper contact. Olivine phenocrysts 1/16 

 inch in diameter are present in the chilled base of the 

 intrusive, while they are % inch in diameter in the normal 

 rock. 



Thin sections of the upper contact show it to be re- 

 markably sharp considering the porous nature of the 

 overlying pahoehoe. The latter is largely glass, with 

 magnetite, augite, and tiny feldspar laths. Alteration of 

 the magnetite and glass has produced limonite and hema- 

 tite, staining the feldspar laths and giving the rock a dull 

 reddish color. Frozen to the pahoehoe is a vesicular gab- 

 bro consisting principally of glass with specks of augite 



31 U. S. Geol. Surv., Water Supply Paper 77, p. 14, 1903. 



32 Op. cit., 1915, p. 14. 



33 Jour. Geol., vol. 19, pp. 291-4, 1911; Proc. Amer. Acad. Arts & Sci., 

 vol. 47, pp. 115-6, 1911. 



34 S. Powers. Intrusive bodies at Kilauea, Zeitschr. Vulk., Bd. 3, pp. 

 28-33, figs. 1-5, 1915. 



