188 Tertiary. 



posits of clays and gravels are of great depth. The borings which 

 have been made in Sacramento and San Joaquin plains have revealed 

 a similar structure of basin, while that in Santa Clara valley, Santa 

 Clara count3% shows that the deposit has not been to so great a depth 

 in that plain. Thus, at the Stockton well boring, after passing through 

 red cla3'S, sands, and gravel, the blue clay was met with at the depth 

 of 400 feet. On the Sacramento valle^^, between the city and Pit river, 

 the lava clays and sands cover the blue clay to the depth ot 358 feet. 

 In the Santa Clara valley, the covering of clay and light sand above 

 blue clay is from 80 to 115 feet. 



Blue clays are found 465 feet below the surface at Los Angelos, and, 

 therefore, below the present sea level; while the surface of the terrace 

 on San Bernardino is somewhat above 2,000 feet in altitude, and as the 

 beds are horizontal, or nearly so, it follows that near Los Angelos the 

 deposit took place when the water was over 2,000 feet deep at that 

 point. All the low Tertiary hills were ledges of rock, several hundred 

 feet below low water. The ocean then rolled up east of the Cordilleras, 

 occupying the Colorado desert and the Mohave valley; and the Cor- 

 dilleras stood up like a peninsula in the great mass of waters, with its 

 crests from 3,000 to 5,000 feet above the surface, and with a breadth 

 liot more than 60 miles from S. W. to N. E. From the wearing down 

 of the felspathic rocks, the granitic porphyries, and the dark colored 

 shales, arose the blue clays, while the trappean and hornblende rocks 

 formed the material of the coarser drift, transported by currents pro- 

 duced by the elevations. The carriage of such coarse matter would 

 inevitably remove lai'ge portions of the Tertiary hills of the plain, and 

 form the breaks which now occur in what was once a continuous chain, 

 the denuded matter itself going to form the bed of arenaceous clay. 



It has been calculated that the deposit going on at present in the 

 Gulf of Mexico, produced both by the alluvium of the Mississippi and 

 the transported mud of the Amazon, does not exceed more than half 

 an inch 3'early. There is nothing in the topographical condition of 

 Southern California to warrant the belief that the slow deposit could 

 have occurred to a greater depth in the same space of time; for there is 

 no evidence of the double influence of a large river and a strong current 

 of sea water coinciding. Admitting, however, that the same rate of 

 deposit occurred then as now in the two localities, the period of 

 deposit of the lower blue clay bed would be 7,201* years, and of the 

 upper blue clay and gravels above 1,600 years, making a total of 8,800 

 years of perfect repose. If to this we add the periods of elevation, 

 both rapid and slow, the total period occupied by the deposit of Post- 



