Lawson.J 



Post-Pliocene Diastropliism. 



133 



cutting erorees back across the terraces of medium altitude. On 

 the lower terraces these minor water courses flow over the surface- 

 in very shallow trenches. 



Thus the results of stream action, like the character of the ter- 

 races, testify abundantly to the youth of the topography of San 

 Clemente and to the recency of its emergency from the waters of 

 the Pacific. 



The Rocks of San Clemente. — From what has been said of the 

 physiography of San Clemente Island it will be apparent that in 

 its main features it resembles San Pedro Hill. The difference in 

 the two cases lies chiefly in the stronger accentuation of the relief 

 on San Clemente. Notwithstanding this similarity of topogra- 

 phy, the rocky substructure of San Clemente is totally different 

 from that of San Pedro Hill. Instead of the rather soft, white, sili- 

 ceous shale of which the latter is composed, San Clemente is built 

 up of volcanic lavas with some intercalated volcanic breccias, and a 

 very limited proportion of fossiliferous, white limestone, which may 

 be the equivalent of the Miocene of the coast. These limestones are 

 seen at Wilson's Cove, where they are fossiliferous and repose in a 

 disturbed condition upon the volcanic beds. The latter are thus 

 apparently of not later age than Miocene. They appear to be mostly 

 basalts, more or less vesicular, with lighter colored rocks of rather 

 open texture, which are probably andesites. The bedding of the 

 volcanic flows is usually more or less distinct, and is generally 

 inclined, so that the terraces bevel their edges. Observations suffi- 

 cient to warrant a statement as to the stratigraphical structure of 

 the island were not attempted. The general impression was 

 obtained, however, that on the southwest side of the island the dip 

 was to the southwest. On the cliffs and stream canons of this side 

 of the island there are numerous caves and cavernous recesses. 

 These appear to be an original characteristic of the lava flows, and 

 are only exposed, not formed, by erosive agencies. An important 

 lesson is learned from a comparison of the geological character of 

 San Clemente Island with that of San Pedro Hill, viz., that within 

 certain limits the forces which have been active in the formation of 

 the great terraces, as well as the forces of subaerial erosion which 

 tend to their destruction, have effected the same result, practically 



