176 CALIFORNIA ACADEMY OF SCIENCES. 



Pacific Ocean 



The age of these sandstones has not been determined by 

 Prof. Gabb. He suggests that they may belong to the 

 Miocene period from the fact that certain fossil Ostraea, 

 indicative of that period, were once sent to the U. S. Na- 

 tional Museum from the southern part of the peninsula. 

 The sandstones often contain large pebbles of volcanic 

 rocks, suggesting their being deposited near the shore line 

 of some continent, possibly once situated where now the 

 gulf extends. In some parts of the division extensive met- 

 amorphism prevails, but the exact character of the resulting 

 metamorphics is not clearly indicated; the metamorphic 

 action seems to be confined to a belt near the summit 

 and along the eastern shore, and rather appears to be a 

 regional metamorphism than one connected with granite or 

 eruptive rocks. In many places recent dikes and lava flows 

 cut and cover the sandstones. An especially remarkable 

 recent volcanic belt crosses the peninsula from San Ignacio 

 to the volcano of Las Tres Virgines. Post-pliocene deposits 

 of small depth cover the mesa sandstone to an elevation of 

 400 to 500 feet. 



Some distance north of Santa Gertrudis the mesa sand- 

 stones cease to form the major part of the peninsula, but 

 continue to skirt the shore for along distance. The western 

 mountain chain, which now becomes dominant, appears 

 to be mainly composed of granitic rocks, covered in places 

 by recent or older eruptive rocks. This granitic chain con- 

 tinues northward till it crosses the boundary line and joins 

 the San Jacinto Mountains in San Diego County. Prof. 

 Gabb's notes from the northern section are not so complete 

 as those from the southern and central part, and of the 

 region to be described here he has but few words to say. It 

 would be highly desirable to obtain a geological section of 



