6 4 



University of California. 



[Vol. i. 



was subjected to chemical analysis, is a compact lithoidal rock, 

 with ill-defined banded structure; the ground-mass is white, but is 

 rendered grayish by crowds of minute dark-colored dots, with 

 numerous phenocrysts of milky white feldspar. 



Under the microscope the ground-mass is seen to be largely 

 composed of glass with occasional irregular patches which have 

 become partially devitrified. It is in parts clouded with yellowish 

 iron oxide and contains a few scattered grains of magnetite. Feath- 

 ery radial aggregates of chalcedony are present in small amount. 

 The minute dots which arescattered irregularly through the matrix 

 are spherulites, exhibiting in greater or less perfection the character- 

 istic black cross between crossed nicols. The spherulites are 

 rarely circular, but have commonly an irregular or wavy outline. 

 Numerous radial cracks divide them, but the highest powers failed 

 to separate the individual fibers, and therefore the mineral character 

 could not be determined. In color they are yellow or brownish; in 

 nearly every case the body of the spherulite is surrounded by a 

 border showing stronger interference, optically continuous with the 

 center, but lighter than it in color, and shading off imperceptibly 

 into the still lighter glass of the ground-mass. 



Manjrof the spherulites have phenocrysts of feldspar at their cen- 

 ters, and nearly all of the larger feldspar phenocrysts are surrounded 

 by groups of spherulites, radiating from points on the edge of the 

 crystal, and so intergrown as to produce a festooned effect. 



Fig. 3, Plate 5, illustrates such a crystal with its encircling 

 spherulites. 



The feldspar is mostly plagioclase, twinned and sometimes zoned. 

 Orthoclase is also present. 



There is a small amount of free quartz in grains and patches, 

 the corroded remains of original idiomorphic forms. 



No ferro-magnesian silicates are present. 



Fig. 4, Plate 5, illustrates a characteristic aspect of this rock 

 between crossed nicols. 



The Laminated Sphemlitic Variety. — The most abundant and 

 most noticeable variety of the spherulitic fades is a form which 

 is very distinctly laminated. This rock varies in color from dull 

 white to gray, yellowish, and rarely purplish shades. In the more 



