184 CALIFORNIA ACADEMY OF SCIENCES. 



cretaceous sandstone, which contains numerous pebbles of 

 the same porphj^rites. On the other hand, they are most 

 likely younger than the highly compressed metamorphic 

 slates of uncertain age in the interior. 



For the present the origin and source of these enormous 

 flows of mesozic eruptives must remain uncertain. Along 

 the section from Ensenada eastward there are no eruptives 

 to be seen, and it is probable that they were poured out 

 over the granite from vents near the coast. 



The presence of these effusive rocks in so large quanti- 

 ties forms an important feature in the geology of the penin- 

 sula, the more so as they appear to be absent, or at least 

 little developed, in Alta California. 



Eecent Eeuptiyes. — At Sausal Beach a great number of 

 pebbles of a black vesicular basaltic rock are found, evident- 

 ly derived from the large recent volcanic flows, beginning at 

 Cape San Miguel and extending for about twenty miles up the 

 coast. The exact age of the flows cannot as yet be deter- 

 mined, but judging from structure and form it must be quite 

 recent There are no other neo-volcanic rocks in the limits 

 of the sketch map, and probably none for a long distance 

 south of the bay. One hundred miles further down the 

 coast there is a small volcanic mountain at the bay of San 

 Quentin; from there southward, the recent eruptives are 

 apparently more common. 



Section peom Todos Santos Bay E. N. E. to the Des- 

 ert AND THE Colorado Eiver. — (See Plates II and III.) 

 The profile represented in fig. 1, Plate III, begins at En- 

 senada about fifty miles south of the boundary line and 

 extends to the Colorado Kiver. My own observations did 

 not extend any farther than to the desert, and the eastern 

 part of the section is drawn from the comprehensive view 

 over the desert gained from the last escarpment of the 

 mountains, aided by some notes of Professor Blake in the 

 Pacific Railroad Reports, relating to the same territory. 



