298 CLARK AND ARNOLD MARINE OLIGOCEXE 



Tertiary deposits on the West Coast were, for the most part, laid down in 

 geosynclinal depressions which paralleled the axes of the present moun- 

 tain ranges. In these slowly sinking troughs the sediments sometimes 

 accumulated to enormous thickness before deposition was interrupted, 

 and the Tertiary deposits in the Coast Eanges have a maximum thickness 

 of over 40,000 feet, of which fully 10,000 feet belong to that portion of 

 the section here referred to the Oligocene. The position of the axes of 

 these troughs varied during the different epochs of deposition, as did also 

 the areas covered by the different seas. Therefore on the "West Coast we 

 find the conditions of deposition during the Tertiary paralleling very 

 closely those which existed in the Appalachian province during the Paleo- 

 zoic. One of the noteworthy differences between the Appalachian prov- 

 ince and the Coast Eange province is that in the latter the geosynclinal 

 condition of deposition is still in operation, as seen in the Great Valley 

 of California, where several thousand feet of Pleistocene and recent de- 

 posits have accumulated and deposition still continues, while the deposi- 

 tion in the last Appalachian trough was discontinued many ages ago. 



Up to the present time the problems of local sequence and correlation 

 of Tertiary deposits on the West Coast have been the paramount ones, 

 and as yet very little evidence has l)een produced to establish an exact 

 correlation of the larger part of this section with the corresponding hori- 

 zons of either the East Coast of America or with Europe. This is espe- 

 cially true of that portion of the section referred to the Oligocene. The 

 problems of local sequence^ and local correlation of the different Tertiary 

 sections on the West Coast have not been simple ones, and there still re- 

 mains much to be done before the stratigraphic and faunal divisions of 

 these series will be adequately known. 



1 The following table shows the general stratigraphic divisions of the West Coast ma- 

 rine Tertiary, as recognized by the writers : 



Pliocene : 



Merced Group. Includes Jacalitos. Etchegoin. Fernando. Purissima. Empire. Mon- 

 tessano. 



Upper Miocene : 



San Pablo Group. Includes Brioues and Santa Margarita formations. 



Middle and Lower Miocene : 



Monterey Group. Includes Temblor and Vaqueros. 



Oligocene : 



San Lorenzo Group. Includes Sooke formation on Vancouver Island and Lincoln, 

 Porter, and Blakeley horizons of Weaver. 



Eocene : 



Tejon Group. Meganos Group. Martinez Group. 



