68 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 30 SER. 



porphyrite laccolite, possibly preceded by a further tilting 

 of the block. This irruption led to the formation of a struc- 

 tural valley in the Little Harbor region, between the por- 

 phyrite area and the ridge just mentioned. This valley was 

 subsequently enlarged and deepened by an extensive ero- 

 sion which followed. At this time Catalina probably stood 

 some two or three thousand feet higher than now. The 

 mass then had the general form of two long ridges, the one 

 already referred to, and another having the general trend of 

 the porphyrite area as seen on the map, and being possibly 

 connected with the former ridge not far from its eastern 

 end. The drainage of the large valley just mentioned was 

 to the west. Its remnants still exist on the island, forming 

 the amphitheater of the Little Harbor region. 



This period of erosion was followed by the eruption at 

 intervals of andesite, which completely filled a portion of 

 this valley and covered the adjacent ridges. The source of 

 these outpours appears to have been local. They were 

 accompanied by a slow settling of the land area to which 

 this mass then belonged, and Santa Catalina became an island, 

 probably for the first time in its history. The evidence 

 shows that it has remained an island ever since. That it 

 was sinking at this time is shown by the deposits of tuff 

 intercalated with the lavas. 



This submergence continued after the andesite flows had 

 ceased, for the higher portions of the andesite were some- 

 what eroded before the island had reached its lowest level, 

 as is shown by the fact that the shelly deposit near Orizaba 

 (see page 51) lies in a saddle several hundred feet below 

 the peaks bordering it on either side. The amount of this 

 depression was between 1,400 and 1,600 feet below the level 

 at which the island now stands. That it was at least as 

 great as this is shown by this same shelly deposit, which 

 occurs at an elevation of about 1,360 feet; and that it was 

 not greater is shown by the base-levelled summits of the 

 island at an elevation of from 1,400 to 1,600 feet. This 

 took place during Miocene times, as the deposits of shale 

 near the isthmus bear witness. This submergence may 



