Smith.] 



Islands of Southern California. 



183 



slopes and the less the contrasts due to subaerial erosion, the greater 

 has been the modification. 



Most of the islands of southern California present forms similar 

 to those found along the mainland coast, with the variations which 

 would' naturally result from the differences in the resistance of the 

 rocks, and the possible modification of the forms by wave action 

 during progressive elevation. For this reason it is believed that 

 most of the islands (among which it is intended to include San 

 Pedro Hill, throughout this paper) possessed nearly mature topog- 

 raphy at the close of the post-Miocene period of erosion, this 

 having since been more or less modified (especially in the case of 

 those islands with less resistant rocks) by wave action, and to a less 

 extent by Pliocene and later deposits. 



Two of the islands with simple topography are exceptional in 

 presenting extremely young topographic forms, their masses not 

 having been elevated till after the close of the post-Miocene erosion 

 period. One of these islands, San Clemente, is developed in hard, 

 the other, San Nicolas, in soft rocks. 



The rugged forms along the coast and among the islands are 

 always nearly mature and developed in rather resistant rocks. The 

 simple forms, however (as indicated by what has been already 

 given) are either (a) very young, in either hard or soft rocks, or {b) 

 originally mature, or nearly mature, forms, in soft rocks, modified, 

 to a greater or less extent, by later deposits and by wave action. 

 This latter class now presents forms which range from those near 

 youth to those well advanced toward maturity, depending on the 

 amount of modification of the original form. 



Classifying the islands on the basis already given, we have 

 belonging to the group with simple topography: San Clemente, 

 with extremely young topography in generally hard rocks; San 

 Nicolas, with young topography in soft rocks; San Pedro Hill, 

 with modified mature topography, now presenting, on the whole, 

 the forms of adolescence, developed in moderately soft rocks; San 

 Miguel, with modified mature topography, now rather youthful in 

 character, developed in soft rocks; also Santa Rosa, in part, shows 

 modified mature topography, though belonging, according to its 

 general character, with the more rugged islands. 



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