SOUTHERN CALIFORNIA ACADEMY OF SCIENCES. 99 



angular fragments by the later intrusion of molten lava from be- 

 low, which enveloped the fragments of the earlier lava, forming 

 a volcanic breccia, in which the later or cementing material, is 

 softer and more readily disintegrated when exposeu to the ele- 

 ments, and allows the included fragments of the earlier flow to 

 weather out and become separated from the mass. {See Plate 2). 



Either the shrinkage of the mass, or the irregularity of the 

 elevation and depression of the region caused the lava to crack 

 across the stream (the line of least resistance) and these cracks 

 or fissures allowed the elements to act more readily upon the lines 

 of the fissures, until openings were formed which resulted in the 

 breaks separating the different islands. {See Plate j). 



Soft places in the lava rock have allowed the waters of the 

 ocean to form the numerous caves, ocean-floored caverns, col- 

 umns, arches and fantastic outlying rocks, for wdiich the islands 

 are noted. 



Professor Lawson (loc. cit.) in describing the rocks of San 

 Clemente Island, says : "On the clifl^s and stream canons of this 

 side of the island, there are numerous caves and cavernous re- 

 cesses. These appear to be an original characteristic of the lava 

 flows, and are only exposed, not formed, by erosive agencies." 



An extensive study of the islands lying west of the San Cle- 

 mente, including the Anacapas, Santa Cruz, Santa Rosa, and San 

 Miguel, have convinced me that, however weL his theory may 

 fit to San Clemente, it will not apply to any of the other islands 

 I have mentioned ; for where so many miles of perpendicular vol- 

 canic blufifs are exposed there would, on this theory, certainly be 

 many of these caves visible above the ocean level, whereas of the 

 large number of caves visited by me, I know of two only which, 

 in ordinary times, have any portion of their floors exposed above 

 the ocean level, and one of those is covered at high tide, and the 

 other is dry by reason of its lying back of a mass of rocks which 

 have fallen from the bluff. 



The beating of the waves has worn these caves and tunnels 

 into the vesicular basalt, carving out chambers whose roofs are 

 supported by grand pillars ; or into low, cavernous, arched tunnels 

 which extend to unknown distances under the island. In some 

 instances the openings are high, gradually decreasing in height 

 until the roof becomes so low that the crests of the waves touch the 

 roof and fill the caves, the confined air causing reverberations 

 similar to the discharge of artillery. 



In time, some of these caves or tunnels are cut throu?;h the 

 islands, in the narrowest places, and later the roof falls and the 

 Island is divided. 



After the roof has fallen, the passages wear away more rapidly, 

 and form passages of widths varying from a few yards to three 

 or four miles in width, as may be seen in the channels which sepa- 

 rate the various islands from each other. 



