EXPLANATIONS OF PLATES 



Fig. 1. Ledge of San Pablo sandstone exposed between title levels (viewed 

 from the shore bluff), about one-half mile southwest of Rodeo station, showing 

 the uneven erosion of the beds, the location of the boring isopods in those beds 

 which have been eroded most rapidly, and the scarcity of the borers in the 

 intervening beds. 



Fig. 2. The same ledge as that shown above (viewed from the southern 

 side), showing the erosive action of the isopod borers. A small island of 

 Pinole tuff lies in the background. 



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