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University of Calif or 7iia. 



[Vol. 2. 



exposures in the ocean cliffs show this eruptive mass to be an 

 exceedingly complicated one. While it is divided in a general way 

 into two parts, the one chiefly peridotite, the other gabbro, yet the 

 irregular junction between the two and the latter dikes of diabase, 

 norite, gabbro and intermediate types all together constitute a com- 

 plex whose history would be impossible of elucidation were it not 

 for the clean surfaces afforded by the action of the waves. 



Probably the oldest eruptive complex is that formed of the sphe- 

 roidal basalt, diabase and gabbro. The spheroidal basalt forms the 

 surface of the ridge for a distance of nearly three miles, extending from 

 the head of Corralitos Creek continuously northwest to the end of 

 Point Sal. The peculiar spheroidal character is developed mainly at 

 the extreme end of the Point. On the steep southern slope of the 

 Point the basalt is followed downwards first by dikes and then by a 

 massive body of diabase, which in turn finally gives place to gabbro 

 along the ocean cliffs. The relation existing between these different 

 bodies of rock is quite complex. While all evidently constitute a 

 geological unit, different periods of eruption and intrusion are clearly 

 shown. The upwelling body of diabase and gabbro has broken 

 through the banded lavas and have in turn been cut by later dikes 

 of the same character. At one spot appears what is evidently a 

 volcanic chimney filled with an agglomerated breccia of dark 

 aphanitic lava mingled with flows of the same material. The age of 

 these rocks is not definitely known, but for reasons which will be 

 given later they are believed to antedate the Knoxville. 



The smallest eruptive body present is one somewhat laccolitic in 

 character, which has been forced up into the Knoxville shales. 



PLEISTOCENE. 



Extent and Character. — The largest area of Pleistocene is that 

 lying west of Corralitos Creek and north of the summit of the ridge. 

 Its surface is sandy and barren, its topographic features being in 

 most marked contrast to those of the other formations. The strata 

 consist chiefly of indistinctly stratified sand, which shows but a 

 slight degree of consolidation. These beds are seen to the best 

 advantage in a canon emptying into the ocean a mile and a half 

 north of Point Sal, where a thickness of about three hundred feet 



