CHAPTER II. 
GEOLOGY OF ROUTE FROM SAN DIEGO TO FORT YUMA. 
GENERAL FEATURES.—THE PENINSULAR MOUNTAINS. ante RELATIONS TO THE SIERRA NEVADA.—PREVALENCE OF GRANITIC AND 
GNEISSOID ROCKS.— CHARACTER OF THE GRANITES. RTID BASES, PROOFS OF RECENT ELEVATION KIN 
CIPAL UPHEAVAL AFTER EocENE AND BEFORE MIOCENE PERIOD. er ey GEOLOGY. i cliearaael OF SAN DIEGO. ah amslar 3 STRATA AND 
FOSSILS —MODERN TRAP ERUPTION EAST OF 1H® MISSION.—SAN DIEGO TO THE DESERT.—TRAP NEAR PENASQUITAS.---GRANITES WITH 
FELSPATHIC VEINS OF SAN PASQUAL AND LAGUNA.—SANTA ISABEL.—WéARNER’S.— THE sila Piuns _—TerTiarRirs or Ca- 
RISSO CREEK.— OsTREAS.—ANOMIAS AND GNATHCDONS CONTAINED IN THEM.—THE COLORADO DESERT —ARROYOS OR “WASHES.”— 
TRANSPORT KS.—ERODED AND POLISHED PEBBLES.—STRUCTURE AND ORIGIN OF THE DESERT.—ANCIENT FRESH-WATER LAKES.— 
_ 3 rad RIVER.—ALAMO MOCHO.—COOKE’sS WELL.—DESERT GRAVEL.—TRANSPURTED FROM UPPER COLORADO. 
GENERAL FEATURES.. 
The geology of this region, like that of the coast of Southern California, has been so freely 
treated of in the reports of Dr. Antesill and Mr. Blake, that little remains for me to say in 
reference to it. 
The trail which leads from San Diego to the Colorado, almost immediately after leaving the 
- Pacific coast, ascends the foot-hills of the Peninsular mountains, and thence for a distance of one 
hundred miles is constantly involved in the labyrinth of ranges and valleys which compose this 
broad mountain belt. At Carisso creek the road emerges from the mountains, and for another 
hundred miles leads across the level and barren wastes of the Colorado desert, when, without 
other preparation than that afforded by a belt of timber visible at the distance of many miles, 
the traveller finds himself standing upon the low, alluvial banks of a broad and rapid river, 
the waters of which are loaded with a reddish sediment, suggestive of the name it bears. 
Both portions ef the route, the mountain and desert, though contrasting so strongly in their 
physical features, are surprisingly harmonious in the desolate monotony of their geological 
structure and their animal and vegetable life. 
THE PENINSULAR MOUNTAINS. 
This mountain belt forms, in Southern California, the main prolongation of the Sierra Nevada 
system, and, to the eye of the common observer, would seem to be its only representative, 
Crossing a line drawn eastward from San Diego towards the interior of the continent. As will 
be seen, however, in the description of the country bordering the Colorado, there are many 
other and important mountain ranges which properly belong to the Sierra Nevada system ; : 
having much in common, both in their structure and trend, with the Peninsular ranges, and, in 
fact, uniting with tem at a point further north. 
The great mass of the Peninsular mountains east of San Diego is composed of granitic and 
gneissoid rocks, to the almost entire exclusion of unchanged sedimentary strata, or other forms 
of metamorphosed material than such as the former present. These granitic rocks, in nearly all 
localities, exhibit a common character, which they share, as will be seen hereafter, with most 
of the granites of the other ranges of the same system on the Colorado ; that is, a predominance 
of the felspathic over the hornblendic ingredients, and particularly in the quantity of albite 
which they contain. As a consequence of their mineral composition they are usually light- 
colored and soft, yielding readily to the decomposing action of the elements, and presenting 
few of the bold and picturesque escarpments usually associated with mountain masses of like 
