COLORADO DESERT — UNUSUAL TEMPERATURE. 125 



calcareous paste, occurs ; it forms in places a compact rock, and, lying upontlie tertiary clays of 

 the desert, may belong to the modern period. 



From Algodones to Fort Yuma the trail is along the river hank, (right,) which yields a dense 

 growth of willow and cotton-wood. Continual inroads were being made by the river sweeping 

 away the right bank, destroying the old trail ; as much as six feet of the bank has been re- 

 moved during one night, (June 8.) The river waters were very turbid with (red) mud, high, 

 though declining, and flowed at a rate of five to six miles an hour, ten feet below the level of 

 the bank, two miles below the fort. 



It is to be regretted that the rapid transit over the desert, and the period of the year (June) 

 in which it occurred, rendered it, from the insupportable heat, impossible to make a more 

 detailed examination than that contained in these pages. Owing to the excessive heat the 

 marches were made in the evening and night time, and the day was passed in tent to seek 

 repose and avoid the temperature, which, commencing at sunrise with 85° or 90° Fah., 

 would reach 100° at noon, and 117° at 3 o'clock p. m. A hasty and necessarily imperfect 

 survey was all which could be made under such circumstances. The increased temperature 

 of the desert may be due to two causes : one, the level and low surface of the region, 

 causing reverberation of the rays of heat from the lofty hills on the west side, and from the 

 occasional covering of fine, bright sand on its surface ; the other, the chief one, is the 

 presence of the sand in the air, which, as so much solid matter in a heated fluid, conveys the 

 warmth more readily to those surfaces which it touches. The southeast wind, which springs 

 up in the afternoon, always carries with it large quantities of sand, blinding the eyes and 

 scorching the skin. The highest ascertained temperature of air alone, examined under 

 circumstances free from the efi"ects of radiation or convection, has been that of 110°. 6 Fah., 

 determined by Ruppel at Ambokul, Abyssinia; while on the great African desert the fine 

 grains of sand floating in the air form centres of radiant heat, elevating the temperature to 

 122°, and even 133° Fah., in the shade, in the oasis of Mourzouk. The horizontal tabular 

 masses of granite and syenite which form flat expanses of naked rock — expanses some thousand 

 feet in diameter, may serve to aggravate the temperature. 



Of the injurious effect of the excessive heat and long marches, without water, upon cattle and 

 sheep, there are abundant evidences in the Colorado desert. Skulls, limbs, and whole skeletons 

 of animals lie strewed about along the trail near the watering places. Most of these were ou 

 the way to California from Santa Fe and the plains of New Mexico and Sonora, and, overcome 

 by the fatigue and drought, succumbed to nature. Their bleached bones and preserved skins 

 have rendered their last remains unpalatable to the coyote and wolf, and are a valuable, though 

 melancholy, testimony of the dryness of the air. 



The Colorado desert, where crossed by this trail, must be looked upon as a deep trough, 

 scooped out at the western side of the continent ; its boundary on the west is the only well 

 defined one. There the Cordilleras rise abruptly like a wall ; to the north the line of demarca- 

 tion may be drawn by the termination southwards of these isolated ranges and lone hills, 

 which dot over the surface northeast of Carrizo ; these extend northward into a hill country not 

 yet explored, but which can difi'er but little from the districts south and north of it ; more north 

 of tliis hill country is the Mojave valley, a region of small ranges and isolated valleys, whose 

 general character, during the greater portion of the year, is sterility ; and such is the whole 

 character of the country to Salt Lake valley, the only difi'erence in climate and vegetation being 

 due to the gradual ascent from 100 feet above sea level to nearly 4,000 feet. 



On the east the desert is considered to terminate at the Colorado, but for 100 miles east of 

 that river the general sterile aspect still remains, except in situations where water abounds or 



