University of California. 



[Vol. i. 



The post-volcanic portion of the delta which reposes upon the 

 lava sheet appears to differ somewhat from that portion which is 

 pre-volcanic, in that it is more of an incoherent gravelly conglom- 

 erate, showing, so far as can be discovered, no bedded structure. It 

 has at present a maximum thickness above the lava sheet of about 

 1 20 feet. The crest of the delta as left by erosion is a short, nearly 

 level-topped ridge, which not improbably closely corresponds to the 

 original surface. The situation of this upper, or post-volcanic, 

 portion is such that it, also, could only have accumulated during a 

 sinking of the coast. The slopes of the delta are terraced. On the 

 northwest side there is a terrace at 570 feet above tide, and on the 

 side overlooking San Jose Canon there is a terrace at 500 feet, while 

 on the northeast side of the hill, just beyond the edge of the delta 

 formation, Pholas borings were observed at an elevation of 470 feet. 

 The terraces could only have been formed at stages of an uplift of 

 the coast subsequent to the depression to which the delta owed its 

 origin. 



Putting together the various facts which have thus far been 

 advanced, we seem to have the following sequence of events fairly 

 clearly outlined. It is possible, however, that our interpretation 

 underestimates the complexity of the problem considered. As a 

 starting point, in such a picture as we are able to sketch, it may be 

 affirmed that prior to the subsidence which caused the accumulation 

 of a heavy delta in the old valley of the San Jose there had been a 

 prolonged post-Miocene period of uplift and erosion, which gave to 

 the country the main features of the sculpture it now exhibits. 

 This is established by the fact that the delta in its outer portion 

 spreads down over a steep antecedent slope worn partly out of the 

 Santa Lucia granite and partly out of the Monterey shale, and by 

 the fact that a mile and more up the valley of the San Jose, where 

 there are high walls of rock on either side, the delta reposes upon 

 a wide stream-trench floor. At the point examined this trench 

 floor is now 265 feet above the sea, but if the land were now de- 

 pressed till it stood at sea level, the valley of the San Jose would still 

 be an imposing feature of the topography. Such a feature it must 

 have been before our delta began to accumulate. 



This early uplift, which gave rise to the main topographic features 



