GEOLOGY OF BAJA CALIFORNIA. ^I 



reddish, decomposed olivines. Of most interest is, how- 

 ever, a basaltic, dark, brownish-gray, very vesicular rock 

 of remarkable freshness and beauty, collected in the 

 vicinity of Calamajuet. In the pores and cavities have 

 crystallized a great number of hexagonal foils of brown 

 mica of an almost metallic, bronze-like lustre. 



In thin section the rock proves to be a nearly normal 

 feldspar-olivine basalt of very fine-grained structure. 

 The feldspar is a'plagioclase, generally in the form of 

 more or less slender laths with fine twin striation. The 

 augite occurs as irregular grains or imperfectly idiomor- 

 phic in short prisms, and has the bamboo color so com- 

 mon among andesites and basalts. The brown mica, 

 coating the cavities, is very sparingly represented in the 

 mass of the rock; occasionally a foil of light-brown color, 

 and not very strongly dichrotic, may be observed imbed- 

 ded in the feldspar mass. The olivine, often partl}^ de- 

 composed into a dark-brown ferruginous mass, has the 

 usual characteristics. Magnetite is frequent in smaller 

 or larger crystals and grains, as is also apatite in long, 

 colorless prisms. 



In the feldspar, and apparently also in the augite, may 

 be seen a considerable number of microlites of a rust- 

 brown to coffee-brown color; they are of minute size, 

 and when their thickness amounts to about half of that of 

 the section, they are usually so dark in color as to be 

 almost opaque. A distinct striation, evidently indicating 

 a cleavage, may be seen on most of them; the refraction 

 is of medium strength, the double refraction quite strong; 

 the extinction is very nearly parallel to the principal ex- 

 tension and the cleavage, but can seldom be accurately 

 measured on account of the interference of the feldspar 

 mass. Pleochroismus, very slight; in fact, hardly ap- 

 preciable. There is considerable difficult}" in the inter- 



