9 6 



University of California. 



IVol. r. 



ification or basic segregation of the more normal diabase, is evident 

 from the fact that it can be traced both east and west from the 

 middle line (see section B' B"), into rock indistinguishable from the 

 latter; moreover, a single hand specimen was obtained near F, 

 showing both aspects of the rock present in the same piece. 



At the turn of the path just south of the spheroids shown in 

 Plate 7, Fig. 2, the diabase has a somewhat lighter green color and 

 appears more granular in texture. In thin section it shows a 

 nearly holocrystalline ophitic structure, made up of plagioclase, 

 augite, opaque iron ores, and green serpentinous patches which are 

 evidently remnants of iddingsite. There are no amygdules, but 

 secondary alteration is well advanced, and chlorite and serpentine 

 are scattered through the slide. The iron oxides are only moder- 

 ately abundant and are accompanied by gray clouds of leucoxene. 

 Apatite was not detected in the slide examined, but may be present 

 in the rock, as it r shows a tendency to occur in groups or colonies 

 of crystals, so that different slides from the same rock often show it 

 in varying abundance. The augite is light pinkish red, with little 

 or no pleochroism. It is the freshest mineral in the slide, showing 

 only slight traces of decomposition in the presence of a yellow stain 

 bordering some of the cracks. 



At the point where the path crosses the contact with the sand- 

 stone, the diabase is exposed as a crumbling mass in the cutting by 

 the path side. The undecomposed nodules show that it is a dark 

 green distinctly crystalline rock of granular, almost gabbroitic, 

 appearance. Under the microscope it shows a beautifully sharp 

 ophitic structure (Fig. 9), with not a trace of glass, and is made up 

 of plagioclase, augite, iddingsite, brown and green hornblende, and 

 brown mica. There are present also a few crystals of apatite, and 

 occasional grains of magnetite. The plagioclases are generally 

 cloudy, but the augite is clear and fresh, being colorless to very 

 light green or pink. The iddingsite occurs in rounded decomposed 

 crystals of considerable size and is very abundant (Fig. 10), giving 

 to the rock its green color. One crystal was observed surrounded 

 by magnetite as an original inclusion. It is in this phase of the 

 diabase that the intergfowths of hornblende and augite occur, as 

 described in a former paragraph. A chemical analysis of this rock 

 is given in column III of the table of analyses. 



