Geol.— Vol. L] SMITH—SANTA C ATA UNA ISLAND. 67 



reduced to the form approximately shown by the 200 feet 

 contour the present sinking began. This is shown not 

 only by the increase of the submarine slope, but also in the 

 isthmus chart, by the preservation of the main features of 

 the present drainage system, showing that the recent stream- 

 valley flooding took place at that level. The information 

 on this point obtained from this latter source, however, is 

 reliable only within certain limits, as a rapid, partial or com- 

 plete filling up of these sunken channels in their lower 

 levels might easily cause an error in the interpretation of 

 the facts. But, taken in connection with the other evi- 

 dence, they may be considered as trustworthy to a certain 

 extent at least. 



2. Outline of History. 



The alteration and deformation of the basement rocks of 

 Santa Catalina probably took place before the individualiza- 

 tion of the mass now forming the island. In the opinion of 

 the writer, the history of Santa Catalina began with the 

 tilting of an orographic block formed of the already altered 

 basement rocks. This view is based on the character of 

 the submarine contours and the slight recent tilting shown 

 in the slope of the summits (see figs. 2 and 3, page 7). 

 The tilting was no doubt gradual, and has continued inter- 

 mittently to comparatively recent times. This is shown in 

 connection with the figures just referred to, as it was pointed 

 out that the angle of the slope of the crest is about 1° from 

 the horizontal, and in the direction of tilting toward the 

 north. No further evidence of the original crust-block is 

 seen on the land, owing to the extensive erosion to which 

 the mass has since been subjected. 



The time of the original tilting is not known, but the 

 crust-block must have been at that time a part of the main- 

 land. By long continued erosion the crest of the mass was 

 carried northward so that it occupied a position now approx- 

 imately represented by the main ridge from Whitley's Cove 

 to the west end. Following this came the irruption of the 



