134 The American Geologist. September, 1904. 
square mile. Over only a small area, which is near the cen- 
tre, are the orbicular rocks found. About two per cent of the 
boulders in the small area, show the orbicular structure. 
Geological relations. The granite through which the gab- 
bro has broken is, with slight variations in petrographical 
character, continuous over a large part of San Diego Co. 
Fairbanks* has done some work on the region and he men- 
tions many phases of the crystalline rocks, but no detailed 
petrographic or chemical analyses have been made of the 
rocks in the locality. 
The granite is evidently older than the gabbro. There 
is no fusion contact visible. The contact can be easily traced 
on the surface and is often found to lie in small water courses, 
showmg less resistance to erosion. .The gabbro is fresh though 
considerably shattered, and exists as huge angular boulders 
lying in irregular heaps and as isolated masses. In no place 
can it be found in a large mass in* situ. The mass was evident- 
Iv consolidated before the uplift, and shattered during that 
interval. Owing to this shattering of the gabbro mass, the 
boulders of orbicular gabbro have been isolated. The writer 
spent several days examining the area and in no place could 
the orbicular rocks be found in place. It is evident from an 
examination of the boulders, that the orbicular portion oc- 
curred in the original mass as a dyke, the character of the gab- 
bro on either side being identical. In some of the larger boul- 
ders it is found as a dj^ke, from one to eight feet in thickness. 
In such cases the line between the orbicular portion and the 
barren portions of the rock is clearly defined. Xo orbicular 
rocks were found at any place nearer than 400 feet from the 
contact with the granite, so it is probably not a -contact phe- 
nomenon. 
General Petrographic Characterist'-cs. 
The rionnal gahhro. The main body of the boss is made 
up of a coarsely crystalline, mesocratic, hornblende gabbro. 
The texture varies from microcrystalline to coarsely granitic, 
with large crystals 10 mm. to 15 mm. in diameter. The most 
notable petrographic feature is the profusion of these horn- 
blende crystals. Dykes or segregation veins, two to ten inches 
* 11th Ann. Report of the Cal. State Mining Bureau, 1893, ^p. 76-120. 
