Geol.— Vol. I.] SMITH— SANTA C ATA UNA ISLAND. 19 



the cloudy decomposition product of the augite, naturally 

 leads to the conclusion that none of the fibrous hornblende 

 comes from the augite. 



Magnetite is found in all the slides, though not in any 

 considerable amount. It occurs as inclusions in the other 

 minerals, and is generally in the form of grains frequently 

 showing partial crystal boundaries. In size they range up 

 to .3 mm. Besides the grains, there are several very irregu- 

 lar areas of considerable size — up to 2 mm. or more in 

 length — in or near the areas of the ferromagnesian minerals. 



2. PORPHYRITE. 



Following the usage of Iddings^ the term "porphyrite" 

 is here used to include those rocks which are characterized 

 by a medium-grained porph3n-itic structure, and which con- 

 tain among their essential constituents a lime-soda-feldspar. 

 They constitute the connecting link, as it were, between the 

 deep-seated diorites on the one hand, and the surficial 

 andesitic rocks on the other, and pass by insensible grada- 

 tions into either. The physical conditions attending and 

 controlling its crystallization are the prime factor in deter- 

 mining the position of the rock in the scheme of classifica- 

 tion. 



Occurrence. — The porphyrite occurs in a single large area 

 in the southeastern part of the island, and was not found 

 elsewhere by the writer, except as smaller masses in the 

 form of dikes. The main area has an average width of 

 about three miles, with an extreme length of about nine. 

 It is cut by dikes of porphyrite and diorite, from two to 

 thirty feet or more in width, which are shown on the cliffs 

 at a number of places along the shore. 



Character. — The rocks are very much weathered, and 

 even those specimens which appeared to be fairly fresh 

 were seen, when examined microscopically, to be consider- 

 ably altered. In weathering the rocks first break into coarse, 



1 Twelfth An. Kept. U. S. Geol. Surv., Part I, pp. 582-584. 



