Monday, September 16.  Whilst some of the party were engaged in the same way as yesterday, others were employed in examining the surrounding country.  About a quarter of a mile behind our camp, and at an elevation of twenty feet above it, a plain extends nearly three miles parallel to the river, and about a mile back to the hills, towards which it gradually ascends.  Here we saw a grove of plum-trees loaded with fruit, now ripe, and differing in nothing from those of the Atlantic states, except that the tree is smaller and more thickly set.  The ground of the plain is occupied by the burrows of multitudes of barking squirrels, who entice hither the wolves of a small kind, hawks, and polecats, all of which animals we saw, and presumed that they fed on the squirrel.  This plain is intersected nearly in its whole extent by deep ravines and steep irregular rising grounds from one to two hundred feet.  On ascending the range of hills which border the plain, we saw a second high level plain stretching to the south as far as the eye could reach.  To the westward, a high range of hills about twenty miles distant runs nearly north and south, but not to any great extent, as their rise and termination is embraced by one view, and they seemed covered with a verdure similar to that of the plains.  The same view extended over the irregular hills which border the northern side of the Missouri; all around the country had been recently burnt, and a young green grass about four inches high covered the ground, which was enlivened by herds of antelopes and buffaloe; the last of which were in such multitudes, that we cannot exaggerate in saying that at a single glance we saw three thousand of them before us.  Of all the animals we had seen the antelope seems to possess the most wonderful fleetness:  shy and timorous they generally repose only on the ridges, which command a view of all the approaches of an enemy:  the acuteness of their sight distinguishes the most distant danger, the delicate sensibility of their smell defeats the precautions of concealment, and when alarmed their rapid career seems more like the flight of birds than the movements of an earthly being.  After many unsuccessful attempts, captain Lewis at last, by winding around the ridges, approached a party of seven, which were on an eminence, towards which the wind was unfortunately blowing.  The only male of the party frequently encircled the summit of the hill, as if to announce any danger to the females, who formed a group at the top.  Although they did not see captain Lewis, the smell alarmed them, and they fled when he was at the distance of two hundred yards:  he immediately ran to the spot where they had been, a ravine concealed them from him, but the next moment they appeared on a second ridge at the distance of three miles.  He doubted whether it could be the same, but their number and the extreme rapidity with which they continued their course, convinced him that they must have gone with a speed equal to that of the most distinguished racehorse.  Among our acquisitions to-day was a mule-deer, a magpie, the common deer, and buffaloe:  captain Lewis also saw a hare, and killed a rattlesnake near the burrows of the barking squirrels.