1913] Louderlack: The Monterey Series 187 



enous shales which corespond to his lower Monterey shales in 

 the San Luis area, or Arnold's upper Vaqueros shales in the 

 Santa Maria district, then siliceous (diatomaceous) shales corre- 

 sponding to the siliceous shales generally recognized throughout 

 the Monterey-San Luis Obispo-Santa Barbara region as Mon- 

 terey. 



These siliceous, biogenic shales are separated in the region 

 north of Santa Clara Valley by a thick sandstone (Lower Modelo 

 sandstone), from other siliceous shales above, and these by an- 

 other sandstone (Upper Modelo sandstone) from a third body 

 of siliceous shale still higher. Of this last shale, it is said: 14 

 "This shale is indistinguishable from that separating the two 

 Modelo sandstones already described. . . . Were the upper sand- 

 stone to disappear, the shales above and below would become a 

 single mass, uniform in their general features from top to bot- 

 tom?" were both sandstones to disappear it would be difficult 

 to distinguish these rocks from the upper portion of the Vaqueros 

 formation." In other words, were the two sandstones not 

 present, the upper part of the series would become a uniform 

 succession of siliceous diatomaceous shales and correspond in 

 character and stratigraphic position exactly with the siliceous 

 Monterey shales of the coast region to the west and north. 



The Modelo Sandstones. — A study of the rield relations 10 of 

 these sandstones leads to suggestive results. In upper Hopper 

 Canon the lower Modelo sandstone is well developed and is 

 probably over 2000 feet thick. This thickness diminishes rap- 

 idly towards the south, so that in 5 or 6 miles it is only a few 

 hundred feet and within 8 or 9 miles it has apparently disap- 

 peared or is so insignificant as to attract no attention. No for- 

 mation corresponding to it has been recognized south of the 

 Santa Clara Valley, nor to the west in the Sulphur Mountain 

 district. 



The upper sandstone has not been recognized over so large 

 an area, nor is its maximum thickness so great nor its tapering 

 so rapid, but its relationships seem to be similar. It lies in 



~* Bull. 309, p. 19. 



is The italics introduced by present writer. 



i« These descriptions may be followed on map, plate I, Bull. 309, U. S. 

 Geol. Surv. 



