Geol.— Vol. I.] SAriTH— SANTA CATALINA ISLAND. 41 



Analysis. — The following analysis was made from a 

 particularly fresh specimen of the andesite, obtained from 

 the ridge forming the eastern boundary of the valley back 

 of Swain's Landing: : 



Si O2 61.05 



Ti O2 09 



AI2O3 18.30 



Fe^ O3 3.49 



Fe O 1. 1 1 



Mn O trace 



CaO 7.75 



Mg 2.59 



Naa 4.06 



K2O 1.36 



H2O 71 



P2 O5 trace 



100.51 

 Sp. gr. . 2. 668 



5. Relative Age. 



The porphyrite was nowhere found in a fresh condition, 

 while the andesite at many points is very fresh. The pro- 

 nounced difference in the amount of alteration which the 

 two rocks have undergone would suggest that the andesite 

 is the younger. When the feldspars of the two are com- 

 pared this difference amounts to more than a mere sugges- 

 tion. These feldspars are closely related chemically, and, 

 other things being equal, those of the older rock should 

 show a greater amount of weathering. In the fresher an- 

 desite the feldspars are remarkably free from decompo- 

 sition products, while those of the porphyrite always show 

 a greater or less degree of alteration. In none of the spec- 

 imens do the feldspars compare with those of the andesite 

 in freshness. 



More positive evidence as to the relative age of the two 

 rocks was obtained on the western slopes of the andesites, 

 in the Little Harbor region. Here, at an elevation of about 

 1,200 feet, numerous inclusions of the porphyrite are found 

 in the andesite. They are of very irregular shape, averag- 

 ing two or three inches in diameter. Differential weathering 



