abstracts: geology 131 



torical record begins with the Franciscan rocks, marine Jurassic forma- 

 tions, which, probably in late Jurassic time, were extensively intruded 

 by basic igneous rocks. Since then they have repeatedly been exposed 

 to erosion and have furnished materials for some of the Tertiary strata. 

 The lower Cretaceous record is scant, the region having been a land area 

 during much of that period. During the upper Cretaceous on the other 

 hand this region was covered far and wide by the sea and the great mass 

 of conglomerate, sandstone and shale then deposited is in places still 

 represented by a thickness of nearly 10,000 feet. The Tertiary section 

 is especially complete, with a thickness of 13,000 feet of sedimentary strata 

 separable by unconformities into six or seven important formations 

 containing abundant fossils. It shows that this region was land from the 

 beginning of the Tertiary until the commencement of estuarine deposi- 

 tion in the Tejon period. Not long after this, in the late Eocene or Oli- 

 gocene, came the deposition of a great body of diatomaceous shales, of 

 which a thousand feet are still preserved beneath the important erosional 

 unconformity which separates them from the marine Vaqueros sandstone 

 of the lower Miocene. After the lower Miocene there was again a land 

 interval, followed by submergence beneath the sea in the middle Miocene 

 Santa Margarita period, which in turn was brought to a close by uplift 

 and was succeeded at least locally by an interval of erosion. During 

 the upper Miocene and perhaps also early Pliocene epochs fluctuating 

 littoral marine conditions prevailed, with the resultant deposition of 

 variable sediments that make up the Jacalitosand Etchegoin formations. 

 Part of the Pliocene and probably also the early Quaternary was taken 

 up by the deposition of several thousand feet of freshwater, fluviatile 

 and marine sands, gravels and clays composing the Tulare formation, 

 after which there came one of the principal epochs of orogenic movement 

 to which the Coast Ranges have been subjected, with the consequent 

 uplifting and folding of all the strata that precede in time the later 

 Quaternary valley fillings. 



Among the more interesting facts brought out in the study of this 

 region are the following: (1) the upper Cretaceous age of an organic 

 shale formation hitherto thought to be Eocene; (2) the presence of a 

 thick diatomaceous shale formation in the Eocene or Oligocene, and of 

 a similar formation in the middle Miocene — the one earlier and the other 

 later than the well known Monterey shale, with which both had been 

 correlated before: (3) the probable middle Miocene and pre-San Pablo 

 age of the Santa Margarita formation; (4) the Miocene age of the greater 

 part of the Etchegoin formation, hitherto regarded as Pliocene; (5) the 

 unconformities of the Eocene on the Cretaceous, of the Miocene on the 



