Lawson. 



Post-Pliocaw Diastrophism. 



125 



agency of ordinary shore forces, and, therefore, representing ancient 

 shore lines, but advances and argues for the astounding hypothesis 

 that they are due to ice action.*" 



The present writer interprets the terraces of San Pedro Hill 

 without any hesitation or qualification as marine wave-cut terraces, 

 and exceedingly fine examples of the class. The considerations 

 which establish this interpretation may be briefly outlined. f If the 

 present attitude of the land were to be changed by an uplift of say 

 100 feet, the shore line would be removed out to the line of the 

 17 fathom submarine contour, and wave action would there imme- 

 diately begin the work of evolving a sea-cliff analogous to that of 

 the present strand. As the position of this cliff receded landward, 

 it would become higher and higher. Suppose this process to have 

 continued till the upper edge of the cliff had receded to within one- 

 quarter of a mile of the base of the sea-cliff of the present attitude 

 of the land; we would then have a gently sloping shelf or terrace 

 between two cliffs, one rising from a strictly horizontal line at its 

 rear, and one dropping away abruptly at a line, not necessarily hori- 

 zontal, at its front. But while under our supposed conditions of 

 uplift the lower cliff has been in active recession, and kept near the 

 vertical by the constant cutting of the waves at its base, the upper 

 cliff has been exposed to atmospheric erosion, and has, therefore, 

 lost something of its vertical and abrupt character. It has been 

 softened down by rounding of its upper edge and by the accumu- 

 lation of alluvium at its base. The surface of the terrace, when 

 lifted above the water, was strewn with (1) water-worn bowlders 

 and pebbles chiefly accumulated in the bays, and (2) marine organ- 

 isms; both plant and animal. The bowlders and pebbles, having 

 resisted the attrition of the shore action, would endure for a long 

 time. The soft parts of the plants and animals would rapidly dis- 

 appear; and later the calcareous shells and other hard parts, being 

 exposed at the surface, would, for the most part, be dissolved and 



*For Mr. Davidson's views in detail, see his paper, "The Abrasions of the 

 Continental Shores of Northwest America, and the Supposed Ancient Sea 

 Levels," Proceedings of the California Academy of Sciences, May 5, 1873. 



f Mr. Davidson says, "The upheaval of the continental shores by sub- 

 terranean action cannot produce such terraces and plateaus. . . . The 

 action of water will not account for them." Loc. cit. 



