(313 ) 
the southward of this promontory turns directly eastward for 
125 kilometers or more, paralleling the cross ranges which lie only 
a few miles distant. The coastal country is therefore protected 
from the northwest coast winds, and its warm southern slopes form 
a striking contrast to the cold bleak wind-swept mesas and hills 
about the promontory. A natural barrier is thus formed where 
a large number of species meet their southern or northern limits. 
South of Point Conception the entire coastal slope below the 
Transition Zone belongs to the Upper Sonoran, except the interior 
valleys and the southwestern part of San Diego County which 
are Lower Sonoran. 
Within this area, which we shall term the Southern California 
District, are over 100 species of trees and shrubs. A study 
of the geographical distribution of these species shows that only 
four extend eastward as far as the Rocky Mountains, less than 
ten per cent. north to the Columbia River, and about the same 
number east to the Great Basin and Arizona. Nearly all the 
remaining species, or fully two thirds of the entire number are 
confined to the California Sub-area, and of these about forty 
are chiefly confined to the Southern California District. 
The most characteristic species of the Southern California 
District is Pseudotsuga macrocarpa. ‘The geographical limits of 
this conifer almost coincide with those of the district. It does not 
extend northward beyond the Santa Ynez Mountains, nor beyond 
Fort Tejon in the mountains connecting with the Sierra Nevada. 
It is confined, however, to the upper altitudes, extending even into 
the Transition Zone. 
The following trees and shrubs are characteristic of the Southern 
California District, being almost or wholly confined within its 
boundaries: 
Pseudotsuga macrocarpa * Ceanothus Orcuttit 
Juglans californica Ceanothus megacarpus 
Eriogonum cinereum Ceanothus crasstfolius 
Eriogonum fasciculatum folio- Vitis Girdiana 
losum Malacothamnus fasciculatus 
Atriplex Brewert Malacothamnus Nuttall 
*Clematis pauctflora Styrax californica flavescens 
Odostemon Nevinii Leptodactylon californicum glan- 
Ribes indecorum dulosum 
Grossularia amara * Eriodictyon crassifolium 
