86 
CIRCUMSTANCES ATTENDING THE DEPOSIT. 
deposit of the Quaternary period, has not been considered in calculation of duration. Yet 
such a deposit must have existed over the plain, and must have been removed afterwards ; so 
that two additional periods would still require to he added to make the calculation complete, 
namely, the period occupied by the last deposit, and the period occupied by its removal. 
From a review of the physical structure of these drifts, it would appear that the upper blue 
clay bed was deposited after the first or lowest bed had been lowered by a gradual subsidence 
of the whole, and this subsidence was probably repeated. The denudation of the upper con¬ 
glomerates may have been effected by the later upheavals, at which time the base of the sierra 
may have been at the water’s edge. During the greater portion of this period the waters east 
of the Cordilleras communicated with the ocean on the west through various channels, of 
which the chief were the present passes—the Cajon, San Gorgono, and New pass. 
Sections of these plains are given in fig. 3, plate 5, and fig. 1, plate 7. 
