Smith! 



Islands of Southern California. 



continued below sea-level to a depth of at least 300 fathoms. 



Begg's Rock, forty feet in height, and lying nine miles north- 

 west of San Nicolas, has been already spoken of as a distant 

 off-shore stack belonging to that island. It lies within the 300-foot 

 submarine contour. 



San Pedro Hill has been described by Dr. Lawson.* It has a 

 length of about nine and a half miles, and an average width of 

 nearly five miles, its highest part having an elevation of 1,482 feet. 

 The rocks of the hill are largely Miocene shales, and are charac- 

 terized by rather gentle subaerial slopes, showing moderate con- 

 trasts due to dissection. The forms of San Pedro Hill, like those 

 of the Coast Range in general, were doubtless mature previous to 

 the Pliocene depression of the coast. These matured forms have 

 been modified by wave action, which has given rise to a series of 

 well-developed and well-preserved elevated terraces. This modi- 

 fication of the subaerial forms has been greatest at either end of 

 the hill. Near the middle of the hill on the seaward side, and on 

 the landward side, the modification has been least, and the condi- 

 tions most nearly approach the original mature forms. The 

 valleys due to dissection are not deep, nor would they ever, it is 

 probable, become large and open, on an isolated hill of this size, 

 and in rocks of such a character, during the complete physio- 

 graphic cycle. Considering the softness of the rocks of which 

 the bulk of the hill is composed, the general effect of the topog- 

 raphy is seen to be at least that of adolescence. The great 

 terraces on the seaward flanks of the hill are still fresh and young, 

 as terraces, but the topographic age of the surface as a whole, that 

 is, its progress from infancy to old age, is fairly well advanced. 



San Miguel, the westernmost island of the northern group, lies 

 about twenty-six miles from the mainland, and about three and a 

 half miles west of Santa Rosa, from which it is separated by San 

 Miguel Passage, with a maximum depth, in the middle, of less than 

 IOO feet. So shallow are the channels between all the islands of 

 this group, that an elevation of from 160 to 180 feet would make a 

 single island of them all. 



*The Post-Pliocene Diastrophism of the Coast of Southern California, 

 Bull. Dept. Geol., Univ. Calif., Vol. I, No. 4, 1893, pp. 122-128. 



