8 4 



University of California. 



[Vol. i. 



border of quartz, while others have a final interior filling of calcite. 

 The groundmass shows nothing worthy of note beyond an indis- 

 tinct flow structure. A chemical analysis of the rock is given in 

 column I of the table. It is seen to be a decidedly basic rock, 

 and would probably fall into the class of olivine-free basalts, the 

 magma having solidified before the separation of the pyroxene. 

 The considerable proportion of titanic acid shown in the analysis 

 is interesting, as no titanium mineral is recognizable in the slides. 



Down on the beach to the west, this porphyritic rock can be 

 traced through several gradations into the non-porphyritic, apha- 

 nitic variety forming the lenses at its southern end. It is accord- 

 ingly to be regarded as a local variation of the usual compact phase. 



The spheroidal rock at F (Fig. 3) is a light gray compact rock, 

 showing no trace of crystalline structure to the eye, and veined with 

 pinkish white calcite. It does not appear amygdaloidal, but some 

 of the smooth water-worn surfaces show a minute laminated struc- 

 ture resembling that observed in many rhyolites. The microscope 

 shows that it was quite a glassy rock, with small lath-shaped feld- 

 spars, but is now much obscured by calcite and chlorite. Between 

 the spheroidal basalt and the pyroclastic layer above, is a small len- 

 ticular mass of mottled, light gray, rather spongy-looking rock, 

 which apparently belongs to and is continuous with the former. 

 In thin sections it is seen to be a mass of vesicles, separated by walls 

 of microlitic glass and filled with chlorite and calcite. At the time 

 of its eruption this rock must have been of a very light pumiceous 

 haracter. 



At B, the last point, going north where the spheroidal basalt 

 can be sharply distinguished from the diabase, the rock is gray and 

 compact and very similar to that at F. Under the microscope 

 (Fig. 6) it is seen to be made up of lath-shaped feldspars with jagged, 

 irregularly terminated ends, and a glassy base. Many of the smaller 

 plagioclases are aggregated into brush-like or feather-like forms, 

 suggesting somewhat the grouping of orthoclase crystals figured by 

 Iddings,* and theaugiteand feldspar groupings described by Danaf 

 in the Hawaiian lavas. 



*Spherulitic Crystallization, Bull. Philos. Soc. Wash., Vol. XI, p. 45, PI. 8. 

 t Characteristics of Volcanoes, pp. 320-324. 



