Smith.] 



Islands of Southern California. 



197 



variable. From Point Conception to San Diego it varies from a 

 minimum of about a mile, to the southwest of Point Vincente, to 

 a maximum of ten or eleven miles, found at several points. The 

 usual width is from three to five miles. 



Along the northern coast of California the descent from the 

 platform just described to oceanic depths is without noticeable 

 break (as shown by the few soundings on the published maps 

 of the U. S. Coast and Geodetic Survey). Between Monterey Bay 

 and Point Conception, however, there is in addition to this, which 

 may be called the upper platform, a second and lower platform, 

 more or less pronounced in its development, the outer edge of 

 which is roughly marked by the 3,000-foot submarine contour. 

 The distance of the 3,000-foot submarine contour from Cayucos is 

 about 43 miles;* from Point Sal 40 miles, and from Point Arguello, 

 32 miles. Southward to Point Conception the outer edge of the 

 lower platform, like that of the upper one, follows the general 

 direction of the coast line. South of Point Conception, however, 

 the coast makes an abrupt change from the general southeasterly 

 direction which it has been following for more than 500 miles, to a 

 general direction, between Point Conception and San Diego, of 

 east southeast; while the outer edge of the lower platform still 

 keeps the same general direction as before, to a point about oppo- 

 site the southern boundary of the state, where it, too, makes an 

 abrupt change to an easterly direction. The result is a great 

 curving embayment of the southern California coast line, about 

 250 miles in length, facing a broad submarine shelf (the lower 

 platform), which has a maximum width in this region of about 

 120 miles. From the outer edge of this platform the descent is 

 comparatively rapid to oceanic depths of 10,000 to 12,000 feet or 

 more. 



The depth of the ocean floor, off the Pacific Coast of the United 

 States, varies considerably, showing a gradual decrease as we go 

 northward. Opposite San Diego the depths reached off the coastal 

 platform are from 12,000 to 15,000 feet; while opposite the mouth 

 of the Columbia River the depth is only 9,200 feet at a distance of 

 about 225 miles from the shore. 



*A1I distances are given in statute miles, unless otherwise indicated. 



