262 GEOLOGY. 



Farther west the Miocene is represented by the Pascagoula forma- 

 tion (generally a greenish-blue clay) of Alabama ] and adjacent States, 

 and by the Oakville beds on the coastal slope of Texas. 2 In the hitter 

 State there is little Miocene of marine origin exposed, but from borings 

 it is known that marine Miocene beds underlie some parts of the coastal 

 region. Such beds are said to be 1500 feet thick at Galveston. Non- 

 marine beds have extensive development in the northern part of Texas, 

 and will be referred to in connection with the other terrestrial forma- 

 tions of the period. Much of the oil of the Texas-Louisiana coastal 

 plain (Beaumont, Sour Lake, Saratoga, Jennings, etc.) comes from 

 dolomized limestones which overlie Eocene (or Oligocene) clays (Frio). 

 The limestones and associated clastic beds are probably Miocene. 3 



The Pacific coast. — The marine Miocene of the Pacific coast is 

 restricted to a relatively narrow belt. In California, the sea locally 

 invaded the central valley, but the position of the coast line appears to 

 have varied during the course of the period, as a result of crustal move- 

 ments, sedimentation, and the ejection of igneous matter. 



Where the marine Miocene of California (the Monterey series) rests 

 on the Eocene (Tejon), the relation is generally one of unconformity, 

 and where the former overlaps the latter, it often rests on metamorphic 

 rocks. The Monterey series consists of shales, sandstone, and volcanic 

 debris, but varies notably from point to point. Its composition and 

 history in the San Luis region 4 may serve as an instructive illustration 

 of the marine Miocene of the Pacific coast (Fig. 444). Early in the 

 Miocene period, the sea transgressed most of the central and southern 

 parts of the Coast range, but before sedimentation had proceeded far, 

 volcanic activity began and a large amount of pyroclastic rock was 

 extruded from many vents. A notable feature of the sediments of this 

 stage is the abundance of diatomaceous matter with the volcanic ash. 

 In one place, fully a third of a 20 feet thick bed of fine ash, etc., is said 

 to be made up of diatoms. Later, volcanic activity subsided and lime- 

 stone deposition followed. Still later, organisms secreting silica re- 

 placed those secreting lime carbonate, and 4000 feet of shale, largely 



1 Smith, Geol. Surv. of Ala., 1894. See also Reports Geol. Surv. of Texas; also 

 Dall and Harris, loc. cit. 



2 Dumble, Jour. Geol., Vol. II. 



3 Hayes, Bull. 213, U. S. Geol. Surv., p. 346. 



4 Fairbanks, San Luis folio, U. S. Geol. Surv. 



