PHYSICAL STRUCTURE OF THE REGION. 491 



of the Pacific slope. Its coast outline, characterized as it is at many 

 points by long sweeps of reentering curves, with outlying islands and 

 projecting points partly enclosing oval, valley-like basins, is at once sug- 

 gestive of a partially submerged series of mountain chains. 



The peninsula is divided by Gabb into three geographical provinces : 

 A southern^ extending from cape Saint Lucas to be3^ond La Paz, charac- 

 terized by irregular granite mountain chains up to 5,000 feet in height, 

 and with deep valleys containing considerable fertile arable land ; an 

 intermediate desert region, characterized by tablelands and flat topped 

 ridges, with a considerable extent of interior valleys, and with isolated 

 mountain tops and ranges projecting above the general mesa level, which 

 rarely reach an elevation of more than 3,000 to 4,000 feet ; this region 

 has no running water and springs are very scarce ; a high northern por- 

 tion from 5,000 to 10,000 feet above sealevel, forming a southern contin- 

 uation of the mountain region of southern California, which has a number 

 of running streams and large valleys susceptible of cultivation, while the 

 higher portions contain considerable extents of pine forests. 



The limits of these three provinces are not sharply defined, but may 

 be taken at about 200 miles in longitudinal extent for the northern, 450 

 miles for the intermediate desert region, and 100 miles for the southern. 



North of the national boundary, in the latitude of San Diego, Lawson 

 recognizes three physiographic provinces : (1) the central mountainous 

 range of the peninsular Sierra, (2) the Colorado desert on the east, and 

 (3) the coastal slope on the west, locally known as the " Mesa." 



The Mesa is a terraced plain, sloping gently upward from the coast 

 eastward and extending 12 to 18 miles into the interior, which surrounds 

 some of the outlying points of the central Sierra that project above it. 

 Toward the desert, on tlie other hand, the mountains present an abrupt 

 escarpment to the east, and the desert plain is in places depressed below 

 sealevel. This escarpment, as Lindgren remarks, lies nearly in a south- 

 southeast line witli the eastern face of the Sierra Nevada, with which its 

 general topographical form, as well as its geological structure, more 

 closely allies it than with the Coast range. 



While, therefore, from a first glance at existing maps it might appear 

 that the depressions of the Mohave and Colorado deserts and of the Gulf 

 of California were the normal southern extensions of the great depression 

 of the San Joaquin and Sacramento valleys, and that the Peninsular 

 range was therefore the normal southern continuation of the Coast range, 

 there is some reason to be found in its topographical form, and still more, 

 as will be seen later, in its geological structure, for the assumption that 

 the peninsula more properly represents the southern extension of the 

 Sierra Nevada uplift. On this assumption the connection between the 



