1903 | FLORA OF SOUTHERN CALIFORNIA 209 
between Los Angeles and Santa Ana, the San Bernardino, San 
Jacinto, and many smaller valleys are Quaternary and Recent. 
They include sandy and gravelly mesas, as well as rich loams, 
ferruginous clays, and black adobe. Almost without exception 
they contain soda salts, from the small percentage which increases 
fertility, to the excess which forbids the growth of all but a few 
Specialized plants. Recent formations also prevail throughout 
the Colorado Desert. 
The formations of the Mojave Desert are less well known. 
The mountains are, for the most part, granitic, the intervening 
mesas and valleys being of recent deposition. Evidences of 
former volcanic activity are exhibited in places. This is most 
marked in the country between Point of Rocks and Bagdad, on 
the Santa Fé Railway, an air-line distance of some 80 miles 
(130 km) Here the rocks are metamorphosed, and often display 
commingled bands and patches of reds, blues, greens, purples 
and yellows.2 The mesas are strewn with scoria, the hills have 
black lava caps, fissure lines are seen, and lava streams may be 
traced for miles. Not less than a dozen volcanic cones are 
known in this region. One of the largest is 450° (137™) high 
and 3,000 (g14™) in diameter at base; the extinct crater at the 
summit is 750% (228™) in diameter and 150* (45™) deep. 
The Providence Mountains limit this volcanic area on the east, 
but northward other less known centers exist. 
CLIMATIC CHARACTERISTICS. 
Southern California possesses a variety of climates, but 
throughout the larger part of it aridity and high temperature are 
the dominant features. There are few, if any, absolutely frost- 
less localities, but there are many where only light frosts ever 
occur, and which may be quite untouched for several successive 
years. At altitudes below 1,500* (457™) the midwinter tem- 
perature rarely falls below 25° F. (—4°C.), and the same alti- 
tude marks the ordinary limit of even light snowfalls. 
But ascending to higher altitudes a different climate and a 
Cooler temperature is soon reached. Thus at San Bernardino 
* This peculiar coloration gave the Calico Mountains their prosaic name, in refer- 
€nce to their variegated tints. 
