66 



CALIFORNIA ACADEMY OF SCIENCES. [PROC. 30 SER. 



shore on the opposite side of the island, at a point to the 

 southwest of the most southern occurrence of the andesite 

 on the map. These sections suggest the possible origin of 

 the island as a tilted orographic block, the rapid descent on 

 the southern side contrasting strongly with the moderate 

 slope on the other. The contrast is similar to that of sec- 

 tions taken on opposite sides of San Clemente Island which 

 is almost certainly such a block. 1 The platform mentioned 



FIGURE 5 Submarine profile, north side of Santa Catalina. 



FIGURE 6 Submarine profile, south side of Santa Catalina. 



as occurring above the 350 feet contour is well brought out in 

 the section from the southern side, though the slight increase 

 in slope above the 200 feet contour is not marked. The 

 features make it clear that before the present sinking of 

 the island began it stood some 350 feet higher than now. 

 That this platform is later than the andesite is seen from the 

 fact that it has been cut in the tufaceous deposits at the 

 isthmus. The island stood for some time at or near that 

 level, while rapid cutting was going on both along the cliffs 

 on the most exposed sides, and in the softer tufaceous 

 deposits near the isthmus, where the erosion of the harder 

 rocks is comparatively slow. When the island had been 



1 "The Post-Pliocene Diastrophism of the Coast of Southern California," by Andrew 

 C. Lawson. Bull. Dept. Geol., Univ. Cal., Vol. i, No. 4, p. 129. 



