130 
CAJON PASS.—GEOLOGICAL FEATURES OF THE COUNTRY. 
The ascent from the river’s hank has been quite gradual. Tall yucca trees soon made their 
appearance. Afterwards we entered thickets of cedars interspersed with artemisia and green 
hunch-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 he 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 thermometer at 38° Fahrenheit, hut sufficiently calm and 
clear to afford an opportunity for making astronomical observations. 
March 16 —Gamp 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 bills, 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 tbe Mojave river. P>ut 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 Ave 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 tAvo 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, Ave 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 Avas slightly less than 100 feet per mile. 
The geological formation of this range of mountains is quite interesting ; Avliite 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 pinons, oaks, and sycamores, 
* The following letter from Baron Humboldt, describing the chemical composition of some of the volcanic rocks collected by 
our geologist, will be read with great interest: 
a Potsdam, au Chateau de la Ville, d 18 Aout, 1855. 
Monsieur: Si j’ofifre si tard I’hommage de ma vive et affectueuse reconnaissance a 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 geologue, 
que j’ai voulu faire jouir de l’examen des roches volcaniques que nous devons a vos bontes Monsieur. C’est un monde nou¬ 
veau pour le rapport de la geologie que votre important ouvrage nous fera connaitre. Mr. Mollhausen * 
vous dirait avec quel scrupuleux interec nous avons clierches sur les cartes les localites des sites ou M. 
Jules Marcou, savant et zelc gbologue, a recueilli Its roches, choisies avec cette intelligence qui en facilite l’etude du composi¬ 
tion mincralogique. Cet aperyu geologique devient doublement interessant lorsqu' on le voit complete par 1'examen des vues 
pittoresques tracees avec un vrai talent par votre topographe et habile dessinateur, Mr. Mollhausen. On possbde par la 4 la fois 
les contours des montagnes, les traits physionomiques des formations de roches et la possibility d’en determiner la composition 
miuferalogique. Le “Resumd of a geological reconnaissance between the junction of the Arkansas and the Mississippi and the 
Pueblo de los Angeles, in California” (Chapter VI of Doc. 129) embrasse, d’une maniere tres piquante; l er , Fetendue des form¬ 
ations secondaires et tertiares que l’on ne connaissait pas a cette latitude dans ses frappautes analogies avec l’Europe; et 2 eme ; 
les cones volcaniques qui les out perces et recouverts d’immeuses “overflowings of lava.” La collection de roches que nous 
devons 4 yqs bontes par la gcncreuse permission de Monsieur le Secretaire de la Guerre a ete deposbe 4 votre nom au cabinet 
