GEOLOGY OF BAJA CALIFORNIA, i 177 



the peninsula across the range of San Pedro de Martis, or 

 about 150 miles south of the boundary line. 



The geology of the surroundings of San Diego and the 

 section from there to the Colorado River is known princi- 

 pally from the notes of Prof. Blake while attached to the 

 Pacific Railroad Surveys. A copy of this profile will be 

 found in Plate II, fig. 2. The coast at San Diego is covered 

 by deep post-pliocene strata (sand, clay, etc.), and these de- 

 posits extend for about twenty -five miles inland, forming a 

 very gently sloping mesa, at the eastern edge of which gran- 

 itic rocks make their appearance; in some places there seems 

 to be a narrow belt of porphyritic eruptives at the western 

 edge of the granite.* The now more rapid ascent leads for 

 thirty miles exclusively over granite to the summit, from 

 which there is an abrupt descent to the Colorado Desert, 

 although not quite so marked as further south . The distance 

 from the base of the mountains to the Colorado River is 

 about eighty miles. From near the summit the whole east- 

 ern slope, according to Prof. Blake, is made up of meta- 

 morphic slates of the age of which nothing definite is 

 known. Although the whole long western slope is indicated 

 as granite in the reports mentioned, still it is certain that 

 the granite area contains one, if not several, enclosed 

 masses of metamorphic slates. So, for instance, at Julian, 

 not very far north of the boundary line.t Recent erup- 

 tives are not mentioned, and it would appear, indeed, 

 that they are very scarce in the western part of Southern 

 California; off the coast there is, however, a volcanic belt 

 with basaltic lavas along the islands. :[: 



Going south from San Diego, the level post -plio- 

 cene beds change into a hilly coast, the higher mount- 

 ains extending close to the sea. About forty miles 

 southward recent volcanic flows, coming down to the sea in 



* See Mex. Bound. Survey, Geo!. Report. 



t Report of the State Mineralogist of Cal., 1886. 



i Whitney, Geology of California, Vol. I. 



