78 WARREN N. THAYER 



California until recently." 1 The fact that it has but recently 

 emerged does not make it a constructional plain, for only a super- 

 ficial bed of sediments covers the surface, denuded before the last 

 submergence. Northward into Sonora, Arizona, and California this 

 subdivision of the province grades quite imperceptibly into the 

 lowlands of the Colorado Desert, where it loses its featureless 

 coastal plain identity and is characterized by old valley floors with 

 intermittent streams, old valleys filled with alluvium, and isolated 

 mountains half-buried under alluvium, plains eroded by sheet 

 floods descending from adjacent mountains, and true desert areas, 

 of which the Mojave Desert is typical. 2 



The foothill region represents a less advanced stage of erosion 

 of the edge of the old plateau. It is also wedge-shaped, its width 

 varying from 20 miles in the south to 100 miles in the north. Its 

 average altitude is less than 2,000 feet, though here and there peaks 

 rise to 5,000 feet or more. 3 These are outliers, representing the 

 former extent of the plateau, and are genetically analogous to the 

 lost mountains of the Anahuac Desert Plateau province, previously 

 discussed. 



Northward these foothills grade into mountains of a somewhat 

 different kind — the Basin Range or fault-block type. They still 

 may be regarded as remnants derived from the old plateau but 

 which have suffered recent faulting, as well as the erosion of two 

 cycles. 4 The mountains of the peninsula are counterparts of the 

 foothills of the mainland. According to Botsford the correlation 

 of the two is perfect, as indicated by the "extent and characteristics 

 of the Miocene rhyolytes and dacites on both sides of the gulf." 5 

 Northward it is probable that these peninsular mountains grade 

 into the type of which the San Bernardino Mountains are char- 

 acteristic, and not into the Coast Ranges, as they are generally 

 mapped . 6 



The average altitude of these peninsular mountains is higher 

 than the mainland foothills, on account of less active erosion due 



1 C. Botsford, Eng. and Min. Jour., LXXXIX, 223. 



1 Bowman, loc. tit., p. 237. * Bowman, op. tit., p. 236. 



3 Botsford, loc. tit. 5 Botsford, loc. tit. 



6 Diller, loc. tit., and Ordonez and Aguilera, op. tit., p. 52. 



