278 Si Powers — Notes on Hawaiian Petrology. 



and magnetite, minute laths of feldspar, and small augite 

 phenocrysts. One and one half inches below the contact 

 the gabbro is a fine-grained, glassy rock showing feldspar, 

 augite, and magnetite, bnt no olivine in thin section. 

 Some of the magnetite is in rods with spicular out- 

 growths. The glass occurs in streaks and bands of dark 

 color between bands filled with feldspar, augite and inter- 

 stitial, translucent glass. In the glassy areas small 

 augite phenocrysts occur, but they are filled with and sur- 

 rounded by magnetite grains. Seven and one-half 

 inches below the contact the gabbro is a uniformly me- 

 dium-grained rock with a little glass in patches. The 

 component minerals are feldspar, augite, and magnetite. 

 No olivine is present. 



A laccolithic origin has been ascribed to the Uweka- 

 huna intrusive by Professor Daly in view of what he took 

 to be doming in the overlying pahoehoe beds. This ap- 

 pearance may be induced by erosion and it is not seen 

 when the body is viewed from its own level. 



A comparison of a lava-tube at Haena, Kauai, with the 

 above intrusion has led the writer to suggest 35 that the 

 latter may be a filling of a lava-tube or of two tubes side 

 by side. The Haena tube is at least 240 feet long, 200 

 feet wide, and 25 feet in height. The floor is covered by 

 a deposit of sand and debris of unknown thickness, 

 washed in from a talus cone at the entrance. Both the 

 Haena tube and the Uwekahuna intrusive have a roof of 

 finely-vesiculated pahoehoe of great rigidity. 



In the Hanalei valley, Kauai, near the falls, the writer 

 found an inclusion of gabbro in basalt. The gabbro is a 

 coarse-grained rock with large feldspars and less con- 

 spicuous augites. Under the microscope the minerals in 

 order of abundance are seen to be feldspar (partly oligo- 

 clase with a secondary generation which may be ortho- 

 clase), purple augite, limonite, magnetite, apatite, and 

 biotite. The iron ores occur as skeletal crystals resem- 

 bling Chinese characters, as in the Wailau and Kauai gab- 

 bros. No olivine was observed. The rock is free from 

 the vesicles so abundant in the greater part of the Kauai 

 gabbro and is finer grained and contains less augite than 

 the Wailau gabbro. 



Age Eelations. 



A certain sequence of events can be traced in the life- 

 history of volcanoes, and especially in basaltic volcanoes, 



^Loc. cit. 



