OBSERVATIONS BY PREVIOUS INVESTIGATORS. 495 



this valley rests in part upon a plateau-like shoulder of the buried range, 

 and that its form was not unlike that of the granite plateau described by 

 Lindgren in the latitude of Ensenada. 



From Gabb's descriptions it would ajDpear that a similar topographical 

 structure obtains for the part of the peninsula stretching south from lati- 

 tude 29° to La Paz. The eastern range has for the most part a mesa- 

 topped crest, broken here and there by projecting ridges which stretch 

 in part across the peninsula and separate the interior valleys. The 

 interior valleys, set off successively a little more to the southward and 

 westward, become more extensive southward, one being described as 

 stretching from La Purissima to Todos Santos (of the south), a distance 

 of 150 to 200 miles, with an average width of 10 miles. The western - 

 range is apparently still more indistinct as a topographical feature and 

 is not recognized by him, but the western mesa region is S2:>oken of as 

 stretching in varying width from Magdalena bay, in latitude 24° 30', to 

 cape Cohiett, in latitude 31°. 



Geological Structure. 

 observations by previous investigators. 



Before giving the facts observed in regard to the geological structure of 

 the region examined by the writers, it may be well to give a brief resume 

 of those ascertained further north and south l)y the observers already 

 mentioned. 



Both Fairbanks and Lawson devote their attention mainly to the later 

 formations composing the coast mesa of southern California, and of the 

 interior Sierra it is only stated tiiat it consists largely of granite, with 

 some small areas of metamorphic slates. The mesa region, according to 

 them, consists at different points of beds of the Chico-Cretaceous, Tejon- 

 Eocene, Miocene, Pliocene and post-Pliocene formations. The Chico and 

 Tejon beds are found in the high peninsula known as Point Loma and 

 at La Jolla, on the coast immediately north of San Diego, between 

 which they are folded into a shallow synclinal and are overlaid uncon- 

 formably by a late Tertiary conglomerate 300 feet thick. Characteristic 

 Chico forms, notably CoraUlochama orcutti, occur in the lowest sandstone 

 at the foot of the cliff of Point Loma, and typical Tejon forms are found 

 at a horizon estimated at 1 ,203 feet higher and apj^arently comformable, 

 though there is no direct superposition. At the highest point of the 

 anticline at La Jolla Cretaceous beds are again exposed with characteristic 

 forms, including Baculites chicoensis. The coast northward from La Jolla 

 shows 400- foot cliffs of the Tejon beds, Avith characteristic Eocene fossils 

 at the base, gradually sinking to the northward. 



