198 



University of California. 



[Vol. 2. 



The descent from the upper or lower platform to the ocean floor 

 seems abrupt, on the map, and is suggestive of a great fault or 

 flexure, dying out to the northward. Estimated in degrees, the 

 slope is seen to be very gradual, the maximum being about seven 

 degrees, found at several points off the California coast. In general 

 the slope is considerably less than that, the average probably lying 

 between three and four degrees. 



The broad lower shelf opposite the embayment of the southern 

 California coast appears to have been an area of great orogenic 

 disturbance, giving rise to many irregularities in the surface form 

 of the shelf. Locally, many parts of this great area are broad and 

 platform-like, but as a whole it is quite uneven, with many depres- 

 sions and elevations. The most pronounced of the latter are, of 

 course, the islands and banks. 



The marked depressions in this area, as given on the map, are 

 as follows: To the north of San Nicolas Island, at a distance of 

 about twenty miles, there is a depression with a depth of more 

 than 5,400 feet; between San Nicolas and San Clemente Islands, a 

 broad depression with a depth of about 5,700 feet; southeasterly 

 from San Clemente a depression with a depth of more than 6,000 

 feet; northwesterly from San Nicolas, and near the outer edge of 

 the platform, is a long but not pronounced depression with a depth 

 of about 4,000 feet. 



On the other hand, there is apparently a broad submarine ridge 

 between San Nicolas and Santa Rosa Islands, with a maximum 

 depth of water of some 1,200 feet. About thirty-five miles south- 

 west of San Nicolas there is a shoaling of the water, to a depth of 

 about 2,400 feet. About thirty-five miles west of the coast of San 

 Diego County is a submarine elevation of considerable size, with a 

 depth of water of from 1,200 to 1,800 feet. Southwesterly from 

 San Clemente Island, and at a distance of about twenty-five miles, 

 another shoaling of the water is indicated by a depth of only 1,944 

 feet. Cortes and Tanner Banks, and Osborn Bank to the south of 

 Santa Barbara Island, have been already mentioned. Southwest of 

 Cortes Bank, and evidently near the outer edge of the lower plat- 

 form, we find a depth of only 1,770 feet. 



Considering the upper platform in more detail, for the southern 



