Kinds of Rocks 



65 



61) occur at about 90 per cent of the stations 

 having any kind of rock in place, with igne- 

 ous rocks at 7 per cent of the stations and 

 metamorphic rocks at 3 per cent. These 

 percentages probably serve as an indication 

 of the nature of rock bottom in the areas of 

 densest sampling more than they do for the 

 region as a whole. If the three groups of 

 rocks were considered on the basis of num- 

 ber of individual specimens, including trans- 

 ported pebbles, the percentage of igneous 

 rocks would be approximately doubled, owing 

 to the greater resistance to abrasion offered 

 by igneous as compared with most sedimen- 

 tary rocks. 



The sedimentary rocks consist of many 



types, one of the most abundant of which is 

 phosphorite. Because of its special interest 

 and problems phosphorite will be considered 

 in a separate section. Shale and mudstone 

 together are even more abundant than 

 phosphorite (Emery and Shepard, 1945). 

 They are followed in order by limestone, 

 sandstone, conglomerate, and chert. These 

 sedimentary rocks are widespread through- 

 out the entire continental borderland just as 

 they are on the adjoining mainland. Geo- 

 logical ages of most of the sedimentary rocks 

 in place have been determined on the basis 

 of Foraminifera (mostly in mudstones and 

 shales) or on hthological similarity to strata 

 of known age on nearby shores. By far, the 





Figure 61. (a) Large quantity 

 of Chico (Cretaceous) shale 

 broken from walls of La Jolla 

 Canyon (FPS 23, lat. 32°51.4', 

 long. 117° 16.3', 210 feet). The 

 large piece is 45 cm long. In 

 contrast, note the small re- 

 covery from two nearby sam- 

 ples in right foreground. The 

 group of kelp holdfasts, picked 

 up on the adjacent beach, are 

 of similar shale. 



(b) Sample from San Clem- 

 ente Rift Valley (FPS 45, lat. 

 32°47.1', long. 118° 15.7', 3600 

 feet). The pieces on the right 

 are Mohian (Miocene) mud- 

 stone; because of their size (45 

 cm), number, and fragileness it 

 is believed to be the local bed- 

 rock. Middle-sized pieces of 

 limestone and phosphorite are 

 believed to have undergone 

 little transportation, whereas 

 small and partly rounded vol- 

 canic rocks are from more dis- 

 tant outcrops. From Emery 

 and Shepard (1945, PI. 2, 3). 



