GEOL. VOL. I.] SMITH SANTA CATALINA ISLAND. 13 



front of the platforms on either side of the canon. Streams 

 have cut into the surface and along the sides, forming sev- 

 eral comparatively broad watercourses. Some of these 

 streams have not yet reached the level of the main canon 

 where they debouch upon it, and have formed small, rather 

 low and broad alluvial fans beyond their mouths. 



The material composing the platforms consists of both 

 rounded and angular fragments. From the form of the 

 structures and the form and arrangement of their material, 

 it is evident that we have here, not a stream terrace, as its 

 appearance might at first indicate, but undoubted alluvial 

 fans. 1 These have been dissected and cut away in recent 

 geological times, owing to the drowning of the stream valley 

 at Avalon, with a consequent shortening of the stream 

 courses, and a deepening of the channels. 



III. GEOLOGY. 



The basement series of Santa Catalina consists of crys- 

 talline metamorphic rocks, principally quartzite. This series, 

 with the hornblendic rocks, the talc-schist and the serpentine, 

 covers in a general way the whole western half of the island. 

 Besides the main occurrence, there are patches of these 

 rocks along the main ridge to the west of Avalon Canon. 

 The basement rocks are cut by occasional dikes, which are 

 principally at the west end, and have a general northeasterly 

 trend. 



The eastern end of the island is occupied by diorite and 

 porphyrite. Bordering this area on the north, and of later 

 age, is an area consisting of numerous flows of andesite, of 

 which there are several other smaller occurrences besides. 

 A particularly interesting area of these rocks is found to the 

 east of Isthmus Cove, where, interbedded with the volcanics, 

 is seen a band of tuff and diatomaceous earth. 



The lower slopes of the andesites in the Little Harbor 



Alluvial fans are by no means uncommon on the island, but no other case presents 

 any similar terracing. 



