424 



University of California. 



[Vol. x • 



ring in such small isolated basins.* But there is nothing to show 

 that these depressions are actually subsiding. We are accordingly 

 li.nited in our search to those portions of the ocean-bottom that are 

 adjacent to land, in order that their movements may be recorded by 

 the relations of some subaerial land-mass to the general sea-level. But 

 as in general the subsidence of a restricted portion of the coast means 

 a conveyance of sediment to that point, the favorable conditions 

 desired can rarely be found. It is this which makes the study of 

 Catalina Island so interesting. As shown by Professor Lawson.f 

 this island has subsided, while the strand of the mainland, twenty- 

 five miles to the north, has risen 1,240 feet, and the island of San 

 Clemente, twenty-five miles to the south, has risen 1,500 feet. This 

 subsidence can not be regarded as the result of sedimentary loading, 

 as no large rivers deliver their sediments into this basin, and the 

 water about Catalina Island is remarkably blue and transparent, 

 with none of the discoloration which is noticeable for miles outside 

 of the Golden G.ite. The movement, too, is out of all proportion 

 to any conceivable weight of sediment that may have been deposited 

 upon the sea-bottom, within the subsiding area. We have here, 

 then, a striking example of the subsidence of a restricted area of 

 the earth's crust which is not due to superficial loading. 



At Seven Mile Beach, on the ocean side of the San Francisco 

 peninsula, Professor LawsonJ has described Pliocene strata com- 

 posed of arenaceous beds of shallow-water origin, having an aggre- 

 gate thickness of over 5,000 feet. "The basal bed is a stratum of 

 partially carbonized forest material," which "rests directly on a 

 seemingly even surface of volcanic rocks which are of Mesozoic 

 age." The theory of isostasy fails to explain such a sequence of 

 events as is here recorded. The forest-bed can not be regarded as 

 being of sufficient weight to depress its basement below sea-level, 

 and there was probably a local subsidence which was independent 

 of sedimentary loading, and which carried the forest-bed down below 

 sea-level, and allowed it to be buried beneath marine sediments a 

 mile in thickness. It must be borne in mind, however, that the 



*. Mamie, Vol. Lil.I, p. 392 (iScp). 

 t/-oc. cii., p. 135, et. seq. 

 %Loc. cit., p. 142, el. seq. 



