186 Tertiary. 



Not that the whole Post-plioceue epoch was passed without produc- 

 ing its effects: denudation on an extensive scale, lacustrine deposits, 

 immense deposits of cla}', sands, and gravels attest the long period 

 alike of action and of repose which characterize the later Post-pliocene 

 period, when the effects were more local, and ever}^ valley and plain had 

 its beds of gravel and cla_y formed from its mountain margins. 



Considerations founded on the zoological characters of the mollusks 

 of the Miocene period of Europe, have led to the belief that the tem- 

 perature of that epoch approached verj' much to that of Spain and 

 Italy at the present time, or a mean temperature about 66 deg. Fahren- 

 heit. As that temperature is almost the exact figure for a great por- 

 tion of the area observed, it follows that there is little, if any, difference 

 between the climate of the Miocene of Eui'ope, and the present period 

 in those places; and since the drift of California is local, and not gen- 

 eral, and there are no traces on the surface of rocks exposed, of scratch- 

 ing or grooving, no moraines, no polished rocks (roches moutonnees), 

 no traces of glacier action, perhaps it may be asserted with safety that 

 the climate and temperature of this region, from the Miocene period 

 to tlie present time, has preserved a constancy and equality, which 

 latitudes more, polar than 40 deg. never posse_ssed. 



Artesian boring through the Post-pliocene beds, in the Los Angelos 

 valley, showed : 



1. Alluvium, 6 feet. 



2. Blue clay, 30 feet. ^ 



3. Drift gravel, 22 feet. 



4. Arenaceous clay, 16 feet. 



5. Tenaceous blue clay, 300 feet. 



Such a thickness of deposit might be attributable to the local circum- 

 stances, nameh^, a deep trough in the sandstone strata under an eleva- 

 tion, almost vertical, close by ; yet that these incoherent beds are 

 usually of great depth is evident from the smooth surface of the whole 

 plain, which preserves its gradual slope f'-oni the Cordilleras to the 

 ocean, independent of the dip or upheaval of the strata beneath. Again, 

 when looking from the south entrance of the Cajon Pass toward San 

 Bernardino, at an altitude of 2,000 feet, there may be perceived a broad 

 terrace at the base of the mountain, consisting of loose conglomerates, 

 gravel and clay beds, l^^ing at an elevation nearly 200 feet above the 

 present level of the plain in its neighborhood, and which are the only 

 remains of a series of beds which have been removed from the lower 

 and more exposed parts of the plain. Its average thickness, perhaps, 

 might be about 200 feet; the other beds would preserve throughout a 



