University of California. 



[Vol. i. 



ing a tendency to take on lath-shaped forms, and the groundmass 

 containing a noticeable amount of glass. Both contain scattered 

 patches of pyrite, and the Point Knox rock shows some iron ore 

 accompanied by gray cloudy leucoxene. The blue amphibole 

 already described in the sill rock is also present in all slides from 

 this area in which the augite is well developed. 



Near the extreme end of Point Stuart, specimens of the eruptive 

 may be taken only a few yards apart which show complete grada- 

 tions between the usual granular, augitic fourchite, containing no 

 glass, and a highly glassy fades, in which no complete crystal forms 

 are visible. In the various intermediate stages the augite loses its 

 granular form and takes on an elongated habit, the crystals showing 

 a tendency toward radial and brush-like groupings. Plagioclase 

 also appears, both as porphyritic crystals and in the usual lath- 

 shaped forms, but is always much decomposed. Nearly all slides, 

 both from the Point Stuart and the Point Knox area, show the 

 presence of the small prismatic crystals of zoisite, either in the 

 groundmass or in the altered porphyritic plagioclases. 



The most glassy fades yet found is represented by a small 

 hand specimen, collected by Professor Lawson at the base of the 

 cliff near the extremity of Point Stuart. In general color and 

 appearance this glassy form is not very different from the rest of 

 the fourchite, but a close examination of its weathered surfaces with 

 a lens, indicates that it is finely spherulitic. A thin section, under 

 the microscope, shows that it is made up of small, roughly poly- 

 gonal areas abutting against each other and separated by what 

 appear to be shrinkage cracks. Each polygon is surrounded by 

 a border of varying width of a yellowish brown polarizing material, 

 which is apparently radially fibrous, the fibrous extinguishing at 

 considerable angles between crossed nicols. This translucent brown 

 material, which is probably a mixture of augite crystallites and glass, 

 sometimes fills the whole polygonal area, but more often the center 

 is occupied by a gray cloudy substance, almost dark between 

 crossed nicols. which was at first taken for unindividualized glass, 

 but which appears to be a secondary product resulting from the 

 decomposition of bunches of radiating plagioclase microlites, 

 although it may also be partly original glass. The specific gravity 



