PAPERS PRESENTED 95 



confined to any one geological horizon, but they are especially abundant in the 

 Tertiary, where they form the Monterey beds, which are often composed almost 

 entirely of the siliceous skeletons of diatoms. 



Botanical works furnish only scanty information in regard to the life history 

 of the marine diatoms. It is generally agreed that they thrive best in cold 

 seas ; that most of them live near the surface of the water and in the littoral 

 zone. As these organisms perish their siliceous skeletal remains sink slowly 

 to the bottom, especially in those places where the currents are not strong 

 enough to sweep them along. In this coimection attention is directed to the 

 geography of the coast of California during the geologic period, when our 

 greatest deposits of diatomaceous shales were being laid down. The geology 

 of California suggests that the region of the present coast ranges was an 

 archipelago, separating the sea that filled the great valley on the east from the 

 open ocean on the west. The marine currents that flowed southward from 

 Alaska brought down great quantities of marine alga\ the diatoms floating as 

 usual near the surface. Once within the zone of islands these floating ma- 

 terials were probably driven into the cul-de-sac at the lower or southern end 

 of the present San Joaquin Valley, where the granite mountains of the sierras 

 bend westward and northwestward. Here the prevailing winds of the region 

 are from the north during the greater part of the year, and materials carried 

 at or near the surface of the water could not escape if, as is assumed, the em- 

 bayment was fairly well closed at the extreme southern end. It is exactly 

 here, at and around the southwestern corner of the San Joaquin Valley, that 

 the deposits of diatom skeletons are thickest. The total thickness of these 

 shales is here more than 5,000 feet, In general these diatomaceous shales thin 

 out toward the north and are inconspicuous in the geology north of San Fran- 

 cisco. 



It is not to be expected that the theory here put forward to explain accu- 

 mulation under the circumstances mentioned is competent to account for all 

 diatomaceous deposits and much less for all deposits of petroleum. Even in 

 this instance it is evident that all of the diatoms were not caught in the angle 

 of the coast; some of them were never carried to the landward of the coast 

 archipelago, while others escaped to sink farther down the coast. 



It is believed, however, that the prevailing winds, in this instance at least. 

 have been an important factor in the .accumulation of the oil-bearing deposits. 



On the west coast of South America there are petroleum deposits from 

 Payta, Peru, northward to the cape wesl of (Jnyaqnil. The near-shore currents 

 rof the Pacific here flow from the cold antarctic regions northward, and it 

 seems probable that the petroleum deposits of Peru and Kqnador may have 

 originated in some such manner as that suggested above. 



An attempt has been made to apply the theory <<> the explanation of petro- 

 leum deposits of marine origin, in other parts of the world, hot a lack o( 

 knowledge of all the circumstances has thus far prevented progress. 



Presented without notes. 



