Lawson ] 



Geology of Carmelo Bay. 



17 



translucent in dark brown colors. The ground-mass is not uniform 

 in texture. Some parts, constituting definite areas in the section, 

 are very dense, and are crowded with opaque granules, while the 

 surrounding parts are coarser and more transparent. The denser 

 areas appear to be of earlier consolidation, since the feldspars of 

 the more transparent portions of the ground-mass show a tangential 

 disposition with reference to them. Glass is present in the ground- 

 mass, and in it are very numerous feebly polarizing globulites of 

 higher refractive power. In most of the slides from this locality no 

 iddingsite can be detected in the ground-mass. In one slide, how- 

 ever, a few minute shreds of this mineral were found to constitute 

 a part of the ground-mass. In addition to the phenocrysts observed 

 macroscopically, augite is also revealed in thin sections. The 

 augite is frequently nearly colorless, but has a faint reddish or violet 

 reddish tinge. It is perfectly fresh, and is sharply idiomorphic, 

 showing in sections transverse to the prism the characteristic octa- 

 gonal outlines, with nearly rectangular cleavages. Twinning is 

 common on the orthopinacoid. The plagioclase phenocrysts are 

 also perfectly fresh and glassy. They show in some cases a dis- 

 tinct zonal structure. The twinning lamellae are usually broad, and 

 the value of the extinction angle, when this is nearly the same for 

 odd and even numbered lamellae on opposite sides of the cross hair, 

 indicates that the feldspar is labradorite. In some instances the 

 labradorite and augite are associated in ophitic aggregates. The 

 iddingsite is abundant in all sections. 



In addition to the general description of this mineral given in 

 the previous pages, some local features may be noted. The color 

 is not constant. The prevalent color is a deep chestnut brown, but 

 in a few cases the same mineral is of a clear yellowish green, as if 

 quite free from limonite pigment. This variety has the same ab- 

 sorption and other optical characters as the brown varieties. In 

 the green varieties the color is usually uniform, but in the brown 

 crystals the coloring pigment is often distributed in streaks and 

 in irregular patches of varying depth of color. In other sections 

 the brown color is uniform, and in some of these the pigment is 

 so dense as to render the mineral scarcely translucent. Both the 

 brown and the green varieties were observed in one slide to be 



