Nothing is more probable, than that impressions of human feet made upon that thin stratum of mud, which was deposited on the shelvings of the rocks, and left naked by the retiring of the waters, may, by the induration of the mud, have been preserved, and at length have acquired the appearance of an impression made immediately upon the limestone. This supposition will be somewhat confirmed, if we examine the mud and slime deposited by the water of the Mississippi, which will be found to consist of such an intimate mixture of clay and lime, as under favourable circumstances would very readily become indurated. We are not confident that the impressions above mentioned have originated in the manner here supposed, but we cannot by any means adopt the opinion of some, who have considered them as contemporaneous to those casts of submarine animals, which occupy so great a part of the body of the limestone. We have no hesitation in saying, that whatever those impressions maybe, if they were produced, as they appear to have been by the agency of human feet, they belong to a period far more recent, than that of the deposition of the limestone on whose surface they are found. The country about St. Louis, like that in the rear {54} of Fort Chartres, and indeed like the horizontal limestone country generally, abounds in sink holes sometimes of great depth. These are very numerous, from five to seven miles back of the town. They are in the form of vast funnels, having at the surface a diameter of from twenty to fifty yards. Mr. Say descended into one of these, for the purpose of ascertaining the medium temperature below the surface of the earth. This sink opens at the bottom of a deep ravine. It has two apertures near each other, through which water is admitted, and each large enough to afford passage to the body of a man. Within are two chambers from six to twelve feet in breadth, and thirty-five feet long. At the bottom of the second chamber is a pool of water rather difficult of access. In this apartment the mercury stood at 60° fah.: in a shady part of the ravine about twenty-five feet below the general surface at 75°. The grassy plains to the west of St. Louis are ornamented with many beautifully flowering herbaceous plants. Among those collected there, Dr. Baldwin observed the aristolochia Sipho, cypripedium spectabile,[080] lilium catesbeiana, bartsia coccinea, triosteum perfoliatum, cistus canadensis, clematis viorna, and the tradescantia virginica. The borders of this plain begin to be overrun with a humble growth of black jack and the witch hazel,[081] it abounds in rivulets, and some excellent springs of water, near one of which was found a new and beautiful species of viburnum. On the western borders of this prairie are some fine farms. It is here that Mr. John Bradbury,[082] so long and so advantageously known as a botanist, and by his travels into the interior of America, is preparing to erect his habitation. This