Lawson.1 



Orbicular Gabbro. 



385 



of about a mile and a quarter and from north to south of about 

 three-quarters of a mile. The summit of the hill is about 

 1800 feet above sea level or 1200 feet above the bottom lands at 

 Dehesa. The area is bounded on the north and west by granite, 

 and on the east and south by the alluvium of the valley. As 

 is indicated by Fairbanks' statement, there are probably other 

 areas of this rock to the southeast of Dehesa which may be 

 portions of the same geological mass separated from it super- 

 ficially by stream erosion. These other areas were, however, not 

 visited by the writer. 



Petrographic Features of the Normal Fades. — In general the 

 main mass of the basic rock presents the characters of a coarse 

 gabbro, consisting, so far as can be determined by an inspection 

 of hand specimens, of a granular aggregate of light gray, striated 

 feldspar, a dark ferro-magnesian mineral and some black iron 

 ore. The feldspar in most specimens preponderates but in some 

 it is equaled in quantity by the dark constituents. The cleavage 

 faces of the feldspar are occasionally an inch in extent and are 

 very commonly half an inch, although the bulk of the mineral is 

 of smaller dimensions. The cleavage faces of the ferro-magnesian 

 constituent are also occasionally as much as an inch in length, 

 but are prevailingly smaller than those of the feldspar. The 

 rock weathers to a tawny yellow color which gives the hill a 

 reddish appearance at a distance. The rock is, however, not 

 deeply decomposed and fresh specimens may be taken anywhere 

 at the surface. Several thin sections of each of the common 

 varieties of the rock were studied. The variations of the main 

 mass of the rock are of a minor character, and it may be charac- 

 terized in general as a hornblende-gabbro with notable propor- 

 tions of olivine and hypersthene. The structure is on the whole 

 allotriomorphic granular except for the tendency to idiomorphism 

 on the part of the olivine. 



The plagioclase is perfectly fresh in thin section and nearly 

 all of it shows twinning on the albite law, while occasionally the 

 pericline twinning is also observable. The maximum extinction 

 angles in thin sections normal to the brachy-pinacoid are about 

 45°, thus indicating that it is a basic labradorite or possibly a 

 bytownite. In some of the sections this basic labradorite exhibits 



