13th.--There being no wood here, we used last night the _bois de vache_, which is very plentiful. At our camp this morning, the barometer was at 25.235; the attached thermometer 60 deg. A few clouds were moving through a deep-blue sky, with a light wind from the west. After a ride of twelve miles, in a northerly direction, over a plain covered with innumerable quantities of _cacti_, we reached a small creek in which there was water, and where several herds of buffalo were scattered about among the ravines, which always afford good pasturage. We seem now to be passing along the base of a plateau of the Black hills, in which the formation consists of marls, some of them white and laminated; the country to the left rising suddenly, and falling off gradually and uniformly to the right. In five or six miles of a northeasterly course, we struck a high ridge, broken into conical peaks, on whose summits large boulders were gathered in heaps. The magnetic direction of the ridge is northwest and southeast, the glittering white of its precipitous sides making it visible for many miles to the south. It is composed of a soft earthy limestone and marls, resembling that hereafter described in the neighborhood of the Chimney rock, on the North fork of the Platte, easily worked by the winds and rains, and sometimes moulded into very fantastic shapes. At the foot of the northern slope was the bed of a creek, some forty feet wide, coming, by frequent falls, from the bench above. It was shut in by high, perpendicular banks, in which were strata of white laminated marl. Its bed was perfectly dry, and the leading feature of the whole region is one of remarkable aridity, and perfect freedom from moisture. In about six miles we crossed the bed of another dry creek; and, continuing our ride over high level prairie, a little before sundown we came suddenly upon a beautiful creek, which revived us with a feeling of delighted surprise by the pleasant contrast of the deep verdure of its banks with the parched desert we had passed. We had suffered much to-day, both men and horses, for want of water; having met with it but once in our uninterrupted march of forty miles; and an exclusive meat diet creates much thirst. "_Les bestias tienen mucha hambre_," said the young Spaniard, inquiringly: "_y la gente tambien_," said I, "_amiago_, we'll camp here." A stream of good and clear water ran winding about through the little valley, and a herd of buffalo were quietly feeding a little distance below. It was quite a hunter's paradise; and while some ran down towards the band to kill one for supper, others collected _bois de vache_ for a fire, there being no wood; and I amused myself with hunting for plants among the grass.