192 CALIFORNIA ACADEMY OF SCIENCES. 



observations and some data taken from a Mexican grant 

 map of the "Rancbo Santa Catarina." It is more of a 

 sketch than an accurate map, but it is at least approximately 

 correct. 



The granite of the plateau is petrographically very uni- 

 form. It is throughout a coarse-grained hornblende gran- 

 itite, or a hornblende-biotite granite, with but small local 

 variations. 



Before closing tlie description of the profile a few obser- 

 vations must be recorded which throw an additional light 

 on some geological problems connected with the plateau. 



Near the summit, about four miles east of the dividing 

 range of hills, lies at an elevation of 4,750 feet, the little 

 mining camp, Campo Nacional; it is directly on the line of 

 the profile. It is situated in the pine forest of the level pla- 

 teau, although in its immediate vicinity the canons and ravines 

 of the eastern slope begin to cut into the granite. Stand- 

 ing on a small elevation nothing but granite can be seen as 

 far as the eye reaches, in all directions. Within a radius 

 of a few miles the gulches leading down to the desert have 

 been worked and yielded a considerable quantity of gold. 

 The latter is coarse and well worn , and the gulches are 

 filled with well rounded smooth pebbles of white quartz, or 

 a dark quartzite. 



This, in itself, is remarkable, as there certainly are no met- 

 amorphic rocks anywhere in the vicinity. It was soon found 

 that all these gulches led up to a small flat-topped hill about 

 200 feet above the plateau, called the Black Hill. This hill 

 is about one-half mile long, east and west, and one-eighth to 

 one-quarter mile wide; it is made up of a well-packed mass 

 of auriferous metamorphic gravel in very smooth boulders, 

 often six inches in diameter. Its depth is uncertain; I 

 was told that once a shaft had beea sunk 18 feet without find- 

 ins; bedrock, and Ishould think the mass would be about 50 

 feet thick. The mining has been entirely confined to the 

 ravines leading down from the hill. 



