130 CAJON PASS.—GEOLOGICAL FEATURES OF THE COUNTRY. 
The ascent from the river’s bank has been quite gradual. Tall yucca trees soon made their 
appearance. Afterwards we entered thickets of cedars interspersed with artemisia and green 
bunch-grass ; where, about fifteen miles from the point of leaving the river, we encamped. 
There is no water at this point. The Mojave river appears to be several miles distant upon the 
left. Leroux, who has previously travelled the old Spanish trail, which follows that stream to 
its head, says it is divided into three branches; the most eastern having its source near Cajon 
Pass. The evening is cool, with the grgoeengte, at 38° Fahrenheit, but sufficiently calm and 
clear to afford an opportunity for making astronomical observations. 
March 16—Camp 149.—The night had been rendered uncomfortable by a chilling blast from 
snow-covered peaks of a mountain some fifteen miles southwest, called by Leroux ‘‘San Ga- 
briel." We packed up by moonlight, and at sunrise continued the survey. For the first four 
miles and a half we ascended at a nearly uniform rate of eighty-one feet per mile. Thence we 
rose more rapidly, making about 800 feet in 4.5 miles, which placed us upon the summit of 
Cajon Pass. The crest was a sharp and narrow ''divide," formed by a ridge of hills, with 
gravel and sand upon the surface. Westwardly, and thence curving around towards the 
north, there seemed, from the base of the rugged mountains, an extension of the same smooth 
slope that we had traversed from the bend of the Mojave river. But towards the south was 
presented scenery quite different. Between lofty mountains there was a deep ravine, through 
which, in the distance, could be seen a great plain or valley lying thousands of feet below. To 
the foot of the gravelly ridge upon which we stood, the angle of inclination was about 45°. 
But the road followed a slight spur, and by that our descent in one mile was 820 feet. That 
brought us into an arroyo, or creek, which threads the ravine. At this place we were on nearly 
the same level as at a point measured on our trail, five and a half miles distant, upon the 
other side of the summit. A careful survey, tracing upon the ground a horizontal curve through 
either of the points mentioned, would probably discover a place where the width of the crest 
might be somewhat less. But it is evident that, for a railroad through this pass, there would 
be required a long and expensive tunnel. From the southern foot of the sharp crest referred 
to, keeping the dry channel of Cajon creek, we descended, in two miles, 250 feet, and there 
found water. Continuing our march, the ravine became rougher and steeper; the descent 
along the trickling stream being nearly 500 feet in 3.36 miles. Having now travelled about 
twenty-five miles from the Mojave river, we made a halt, to allow the animals to drink and 
graze. We then descended four miles farther down the rocky ravine, and encamped for the 
night. This last declivity was slightly less than 100 feet per mile. 
The geological formation of this range of mountains is quite interesting; white marble and red 
porphyry being found, and indications also of gold and silver mines. The time of its upheaval 
proves to have been later than heretofore supposed, Mr. Marcou placing it somewhere in the 
tertiary period.* We are again in a woodland region. Scattering piñons, oaks, and sycamores, 
* The following letter from Baron Humboldt, describing the chemical composition of some of the voleanic rocks collected by 
our geologist, will be read with great interest : 
à Porspam, au CHATEAU DE LA VILLE, d 18 Août, 1855. 
Monsieur: Si j'offre si tard l'hommage de ma vive et affectueuse reconnaissance à Monsieur le Pr. Lieutenant Whipple pour 
les interessantes communications qu'il a bien voulu me faire, ce retard n'a eu pour cause que l'absence d'un savant géologue, 
que j'ai — faire jouir de l'examen des 1 roches voleani iques que nous deer à vos bontés Monsieur. Ss est un Ben M 
veau — le rapport de la géologie itre. Mr. Móllhausen 
^ vous ër de wt ideas intérêt n nous avons cherchés sur les cartes les localités des sites ih M. 
Jules Marcon, savant et zélé géologue, a recueilli les roches, choisies avec cette intelligence qui en facilite l'étude du composi- 
tion minéralogique. Cet aperçu géologique devient doublement interessant lorsqu' on le voit completé par l'examen des vues 
pittoresques tracées avec un vrai talent par votre topographe et habile dessinateur, Mr. Móllhausen. On posséde par là à la fois 
les contours des montagnes, les traits physionomiques des formations de roches et ^ia possibilité d'en déterminer la composition 
minéralogique. Le “Resumé of a geological reconnaissance between the junction of the Arkansas and the Mississippi and the 
Pueble de los tg in California” (Chapter VI of Doe. 129) embrasse, d'une maniére très piquante; 1°", l'étendue des form- 
ations secondaires et tertiares que l'on ne connaissait pas à cette latitude dans ses piss cie analogies avec l'Europe; et 2*"*; 
les cones voleaniques qui les ont percés et recouverts d'immenses “overflowings of lava." La collection de roches = nous 
devons à vos bontés par la généreuse permission de Monsieur le Secrétaire de EE nom au 
