DURATION OF EPOCH OF DEPOSIT. 85 



not more than 100 feet in the deepest boring ; the depths of blue clay in the other valleys far 

 exceeds that, and has not yet been determined. 



If the periods of clay deposits be assumed (as generally admitted) to be those of repose^ and 

 the deposits of gravel and sand as the indications of periods of elevation or depression of the 

 land, we have in this history of the Quaternary period, derived from the evidence afforded by 

 the artesian boring in the plain, two periods of repose and three periods of elevations, the 

 rapidity and extent of which may be estimated, in some degree, by the textural condition of the 

 gravel. 



Of the duration of these periods of upheaval but little can be safely affirmed ; they throw but 

 little light on the total length of the Quaternary epoch. As the deposit of fine mud out of 

 water has been examined under various conditions, a distant approach to truth may be obtained 

 from a consideration of the blue clays. 



These are found 465 feet below the surface at Los Angeles, 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 Angeles 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 Cordilleras 

 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 not 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 produced by the elevations. The carriage of such coarse matters 

 would inevitably remove large 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 yearly. 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 7,200 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 Quaternary beds would equal the 

 period occupied by some deposits of the secondary age. Yet such a calculation would scarcely 

 give the total period accurately, since neither has the base of the lower blue clay bed yet 

 been reached, nor should the present alluvial surface be looked upon as the last deposit of that 

 epochj or the prelude of the modern period ; since, as has been already stated, the slopes of 

 San Bernardino display a series of conglomerates and gravels 200 feet above the level of the 

 nearest stream, (Cajon creek.) These are coarse accumulations of primary pebbles and 

 granitic clays which have been removed from every portion of the plain where it is exjjosed. 

 In the gorges and caiions it still remains ; and wherever a pass has been travelled, there it is 

 found, as the superficial covering, between 200 and 300 feet deep ; this, the last evidence of 



