l.AWSON ~l 



Pai.acheJ 



The Berkeley Hills. 



419 



Under the microscope the thin sections exhibit a hypocrystal- 

 line structure with occasional porphyritic crystals. These pheno- 

 crysts, of which more than two or three seldom occur in a slide, 

 consist of labradorite feldspar and augite in idiomorphic crystals. 

 The ground-mass is a uniform mixture of feldspar microlites, 

 grains of augite, and interstitial glassy matter in nearly equal pro- 

 portions, with scattered grains of magnetite. The microlites show 

 more or less perfect flow structure and are tangentially grouped 

 about the vesicles, which are frequently drawn out into long wedge- 

 shaped forms, clearly by differential movements of the viscous 

 magma. The vesicles are generally filled with secondary minerals, 

 thus forming the amygdules, which give the rock its distinctive 

 aspect. These amygdules are of all sizes, from that of a pin head 

 up to six inches in diameter, the average size being less than an 

 inch across. Their shape is infinitely varied. The more regular 

 forms are spherical, ovoid, or disc-shaped, cylindrical, wedge- 

 shaped, or almond-shaped; irregular and fantastic shapes are quite 

 as numerous, especially in the more highly vesicular lavas, where 

 the vesicles have moulded one another in expanding. Most of 

 them, however, are chalcedonic only in their outer part, and are 

 then lined with clear vitreous quartz crystals, leaving a cavity at the 

 center, or are filled in solidly with quartz of the same character; or 

 the quartz at the center may be a sugary aggregate. When hollow 

 and lined with brilliant quartz crystals, the latter are often remark- 

 able in being terminated by the positive rhombohedron alone. 

 The centers are sometimes filled with large crystals of calcite, and 

 that mineral has in some cases formed the first layer in the cavity, 

 followed by chalcedony. When such forms are exposed to the 

 weather, the calcite is quickly dissolved, and leaves the outer sur- 

 face of the chalcedony covered with sharp-cut moulds of the cal- 

 cite crystals. Calcite forms the sole filling of some cavities. 

 Natrolite occurs frequently alone, and with calcite, chalcedony, and 

 opal, in these amygdules. It is always finely fibrous in radial aggre- 

 gates. Under the microscope it is cloudy, owing to its exceedingly 

 fine fibration, gives rather high interference colors, and extinguishes 

 parallel to the fibers, the radial form giving neat interference crosses 

 in parallel polarized light between crossed nicols. It gelatinizes 



