238 



RALPH ARNOLD 



geology has heretofore been considered as a unit, is believed to be 

 related to the positive or upward-tending forces accompanying or 

 immediately preceding the important volcanic activity which took 

 place during early Miocene' time adjacent to the Sacramento Valley, 



and northward into Washington, but 

 which are absent or insignificant in 

 the region contiguous to the San 

 Joaquin. In this connection it is 

 also worthy of note that the greater 

 part of the Willamette Valley was 

 also out of water during the lower 

 Miocene.^ 



DISTRIBUTION AND CHARACTER OF 

 SEDIMENTS 



The Vaqueros or lower Miocene 

 proper, and the Monterey or lower 

 middle Miocene epochs have been 

 included in mapping and discussing 

 the lower Miocene, for together they 

 mark by subsidence the beginning 

 of a new geologic cycle following 

 the Oligocene elevation. Locally 

 the Vaqueros and Monterey have 

 totally unlike histories. The Vaque- 

 ros in the Coast Ranges of central 

 California is characteristically con- 

 glomeratic at the base, and sandy, 

 with minor quantities of shale, in 

 its upper portion. In the northern 

 part of southern California it is 

 largely dark arenaceous shale as- 

 sociated with minor quantities of 

 sandstone. The Monterey, on the other hand, is composed 

 largely of diatomaceous material with minor quantities of sand- 

 stone, fine volcanic ejectamenta, and limestone, the last three 



' J. C. Merriam, Bull. Dept. Geol. Univ. Cal., Vol. V, p. 173. 

 * Oral communication from Mr. Chester W. Washburne. 



Lower Miocene 



Fig. 3. — Map showing hypothetical 

 distribution of land and water on the 

 Pacific Coast during lower Miocene 

 time. 



