L46 



University of California. 



LVol. 3. 



andesine or labradorite. The few untwinned sections generally 

 give interference figures indicating their approximate parallelism 

 to the brachypinacoid, and none could be proved orthoclase. 

 That feldspar is therefore supposed to be absent. 



The fragments that constitute the bulk of the tuff, while 

 comprising a small proportion of compact rock resembling the 

 groundmass of andesite, are in greater part of devitrified pumice. 

 The larger fragments of this material, which always exhibit 

 sharply angular boundaries, are highly vesicular in structure, and 

 the gas cavities, separated by thin partitions, are sometimes of 

 oval or nearly circular outline, sometimes strongly drawn out in 

 one direction. The originally glassy substance of the septa 

 appears, by ordinarily transmitted light, of a pale brownish tint, 

 clouded with whitish dust resembling fine kaolin. Between 

 crossed nicols, the devitrification of the glass becomes evident. 

 Little or none of the original amorphous substance remains; it 

 has been replaced by a microcrystalline aggregate of weakly 

 polarizing minerals. Micro spherulitic structure is occasionally 

 to lie seen. The gas cavities are more or less filled by the 

 minute, colorless tablets and prismatic crystals thickly implanted 

 in the bounding walls. The small dimensions of these crystals 

 hardly permit of quite satisfactory determination, but two 

 species may be distinguished. The tablets, attached by one 

 edge, are always of hexagonal outline, often approaching to 

 rhombic by the almost complete suppression of one pair of faces. 

 Their limpidity, weak refraction and double refraction correspond 

 to tridymite, and they are confidently referred to that species. 

 The prismatic mineral is in the form of slender rods, squarely 

 terminated at the distal end, and showing a longitudinal cleav- 

 age. The refractive index is much below that of balsam, and 

 the double refraction feeble; this mineral is in all probability a 

 zeolite. Needles of similar character, under high-power lenses, 

 may be seen among the devitrification products of the glassy 

 septa. The tridymite is presumably of primary origin. 



Between the larger fragments is some finely comminuted 

 material. The green material that gives the rock its character- 

 istic tint occurs as thin shreds in the devitrified glass and among 

 the crystals in the pumice vesicles, but the greater portion is 



