VOL. IV. | Trees of Southern Calzfornia. 333 
Valleys, and the palm and mesquite groves of the deserts are 
examples. The belt of Blue Oak (Quercus Enge/manni) which 
stretches across the hill country of San Diego County, and the 
park like growth of Quercus agrifolia which covers the slopes in 
the neighborhood of Pasadena, are perhaps to be attributed to 
the moisture supplied by the ocean fogs which roll in and con- 
dense upon the seaward exposures which they occupy. The 
exception to the rule is found in that peculiar forest of yucca 
and juniper which fringes the northern base of the San Bernar- 
dino Range from its eastern extremity to the upper end of 
Antelope Valley, and whose existence or limitation seems to 
have no perceptible connection with hydrographic conditions. 
Its constituent trees are the only ones that have solved the 
problem of arboreal sow without a continuous supply of 
moisture. 
At higher altitudes the cooler air and greater humidity afford 
more favorable conditions for tree growth; the chaparral itself 
becomes denser and larger, and at an altitude of between 4000 
and 5000 feet a coniferous forest begins which reaches nearly 
to the summit of the highest mountains.* This belt, which 
occupies the higher parts of the San Bernardino Range and its 
continuation, the San Jacinto and Cuyamaca Mountains, is by 
no means a continuous one. It rather consists of a series of 
forested tracts limited in area in accordance with their altitude 
and slope-exposure; some mere patches measured by acres, while 
the largest extends from near the Cajon Pass to Grayback 
Mountain. West of this main forest there are small bodies of 
coniferous trees in the Cucomonga and San Antonio Mountains, 
in the so-called Sierra Madre, and in the Liebre Mountains, and 
to the south larger and more valuable forests occupy the San 
Jacinto and Cuyamaca Mountains. No accurate measurements 
of these forest areas have ever been made, and, indeed, could not 
be made without great expense and difficulty, so rugged and 
* There are but two bald-topped mountains in the whole region; San 
Antonio, 9630 feet high, and Grayback, 11,725 feet high. The latter is 
pine-clad to within 200 feet of the summit, and covered with the standing 
trunks of dead pines to the very top, so that there cannot be said to be any 
point above tree line. ' 
