30 CALIFORNIA ACADEMY OF SCIENCES. 



once continuous, and perhaps a part of the mesa sand- 

 stones have been derived from the destruction by erosion 

 of this older area. 



An interesting question in this connection is the con- 

 figuration of the land-area, separating the Atlantic 

 from the Pacific in Cretaceous time. Dr. C. A.White has 

 pointed out that fossils of the Atlantic Cretaceous fauna 

 have been found on the western side of the Sierra Madre 

 on the mainland of Mexico, while the shore line of the Pa- 

 cific Cretaceous ocean can be traced as far southward as 

 Todos Santos Bay, at least.* Did this comparatively 

 narrow isthmus continue southward, or did the two 

 oceans meet some degrees south of the boundary line? 

 Dr. White is inclined to regard the first mentioned view 

 as the more probable, mainly on account of the great and 

 marked difference between the fauna of the Atlantic and 

 the Pacific Cretaceous, and it must be conceded that the 

 existence of older ranges of crystalline rocks on the 

 western side of the peninsula also speaks in favor of it. 

 The question can only be satisfactorily settled by a more 

 detailed investigation of the southern peninsula, and 

 especially of the sedimentary series designated under the 

 name of "mesa sandstones." 



II. Basalt. 



One or two specimens from the northern end of the 

 peninsula, near Calamajuet on the gulf side at about Lat. 

 29° 30', deserve special mention. Recent or tertiary vol- 

 canic flows in this vicinity overlie slaty, at least partly 

 metamorphosed rocks. There are specimens of a slaty, 

 dark-gray, fine-grained sandstone, locally used as whet- 

 stones; further of crumpled and folded talcose slates. 



The volcanic rock is a dark-gray basalt with numerous 



^Bulletin 15, p. 30, U. S. G. S. 



