the Western, South Atlantic and Gulf States. It will then become possible, by a comparison of the really cognate phenomena, to trace more definitely the history, both general and local, of that turbulent period, without the confusion attending the use of a word to which each observer attaches, more or -less, a different meaning. If it be fully ascertained that in its lower course the Ohio is (sensibly) the extreme southern limit of the "glacial" Drift, while the stratified Drift is substantially continuous from the lakes to the Gulf shore, we have before us a definite problem as to the causes, that can probably be solved by a close examination of the critical region, viz: the southern border of the Paleozoic in the Mississippi Valley. The general relations of the Drift to the Allegheny range are thus far, unfortunately, involved in great obscurity; yet a knowledge of these, especially on the western slope, seems almost a necessary condition precedent of any probable hypothesis regarding the history of the Drift period West and South. It is only thus that the possible existence of an ancient barrier across the Mississippi Valley, at the head of the embayment, may be either established or disproven. The next formation laid down on the map is the Port Hudson group, of which, however, the outcropping littoral portion only is here represented. Properly speaking, it should be shown as occupying also most of the space colored as alluvial, since it underlies everywhere, not only the marine alluvium (and a portion of the Gulf itself), but also that of the Mississippi and its chief tributaries, at least as high up as Memphis, and on Red river, nearly if not quite up to Shreveport. It seems to exist equally in the valleys of other larger rivers tributary to the Gulf; notably in that of the Pascagoula, up to one hundred miles (in a direct line) from the coast.* Having discussed this formation somewhat in detail in papers recently published, I will merely state that it embraces a group of partly littoral and estuarian, partly swamp, lagoon and fluviatile deposits, whose thickness and location is manifestly dependent upon the topographical features of the continent, then (during the "Champlain " period) in progress of slow depression; as shown by the nature of the deposits, and the numerous superimposed generations of large cypress stumps, imbedded in laminated clays exhibiting the yearly fall of leaves. These beds overlie those of the Orange Sand or Stratified Drift, while themselves overlaid by, not only the river alluvium, but also by the Loess or Bluff silt or its equivalents; as well as where this is absent, by the Yellow Loam of the surface. *See Miss. Rep., 1860. p. 153. The reference of the outcrop at Powe's to the Grand Gulf group is, I think, undoubtedly erroneous, and the same may be true of part or whole of the Dwyer's Ferry section, p. 154. It would seem that here also, during the latter portion of the Drift period, most of the larger river channels were already impressed upon the surface, though not always coincident with the present immediate valley; as Newberry has observed in relation to some of the northern rivers. A depression of the land would gradually transform these channels into inlets filled with more or less stagnant fresh water down to a greater or less. distance from the then existing coast line; and thus opportunity would be afforded for the formation of the swamp and lagoon deposits into which both the Mississippi and Red river have subsequently cut their channels. The banks of the Red river as well as, outside the present alluvial area, those of the many lakes and bayous which border that stream, exhibit strata absolutely identical in character with those observed near the coast, yet. of course, totally different from either the alluvial deposits of the present time, or the adjoining tertiaries. The same holds true, more or less, of the Mississippi and its mighty "bayous." According to the observations of Dr. E. A. Smith in the Yazoo bottom, and my own in that of the Tensas, not only do the clays with calcareous concretions (as characteristic of the Port Hudson age as they are foreign to the alluvium of to-day) frequently crop out in the beds of the streams; but much of the best lands of the "buckshot" kind, now situated above overflow, have clearly been formed by simple disintegration of these strata, altogether independently of the river alluvium. These results fully confirm, therefore, the statement made by Gen. Humphreys,* that the Mississippi does not, as a rule, flow in a bed formed of its own deposits, but has excavated it in an older geological formation. Wells exceeding fifteen or twenty feet ordinarily strike these clays throughout the bottom, as they do in the delta; and the analogy has been completed by the repetition of the phenomena observed in driven wells at New Orleans,† at a point about fifty miles above Vicksburg (Gen. Wade Hampton's plantation), where a tube well has furnished a copious flow of combustible gas undiminished for many months. The swamp clays form, however, only the lower portion of the Port Hudson beds. Higher up, as shown at the Port Hudson bluff, there lie yellow or whitish silts and "hardpans. These form, also, a level terrace some miles in width, bordering the Tensas bottom; while high above it, on the hill. tops of Sicily Island, on the Washita, lie the remnants of the Loess formation, the main body of which has succumbed to the erosive influence of the Terrace epoch of elevation. It has, however, left a belt a few miles wide on the eastern side of the valley, as shown on the map. * Rep. on the Mississippi river, p. 98, et al. See profile in this Jour., Jan., 1869. This Jour., vol. i, 1871, p. 345. It would thus seem that during the latter portion of the period of depression, the rate of sinking became, at times, too apid to allow of the accumulation of swamp deposit. The ndurate silts are mostly void of fossils of any kind, but are ccasionally traversed by fluviatile beds with pebbles, driftwood, etc. Then there is a recurrence of the swamp deposits; hen again silts; and finally, the calcareous, silty loam of the Bluff formation, with its numerous terrestrial fossils and "Löss puppets," ends the deposits clearly referable to the epoch of lepression. The Loess differs little from its equivalents farther north, save in being utterly devoid of stratification as well as of any Auviatile organisms. It is not easy to imagine the modus operandi by which a deposit of this kind, sometimes 70 feet thick, and of dead uniformity from top to bottom, could be produced. Its equivalents farther north exhibit very distinctly the structure resulting when deposition takes place in (gently) flowing water; at the south it was probably substantially stagnant, save as regards the tidal flow. Perhaps the latter may serve to explain both the absence of fluviatile as well as marine life, and the uniform intermixture, without any semblance of arrangement, of material varying from the finest silt to pebbles half an inch in diameter. A strong tidal wave running up a deep inlet of this kind, would naturally sweep away, in its periodical rushes, many members of the terrestrial fauna, whose remains are in a marked degree the more abundant the nearer we approach to the edge of the formation. Overlying the Loess we find, wherever opportunity is afforded, a stratum of yellow loam or brick clay, which near the larger valleys is often as much as fifteen to twenty feet in thickness. It is altogether devoid of stratified structure, as well as of fossils, and forms the surface layer, and in most cases the subsoil of the Gulf States. If, as I am inclined to believe, its presence as a connected, though very undulating sheet, on all but the most elevated uplands of these States, necessitates the assumption of submergence, however brief, to the highest level at which it occurs; the changes of level heretofore alluded to would be shown to have exceeded by 600 to 700 feet the estimate given above. The succeeding (Terrace) epoch of elevation has not, so far as I am aware, left any marks in the way of beach-lines or terraces, unless the second bottoms or "hommocks" be accounted such. They, however, belong to a very modern epoch, for they occur on streams no larger than what is usually called a "creek," and are most marked on the smaller rivers; while, apparently absent from those of the largest size, such as the Mississippi, Red and Arkansas rivers. The elevation at which, on the very Gulf shore, we find deposits of the Port Hudson 0 age (180 feet at the Five Islands on Vermillion Bay) shows th nevertheless, that a stupendous amount of erosion was accom re plished during the time that the Mississippi occupied in scoope ing out its channel, to a depth which, even below the norther ei boundary of Louisiana, cannot be estimated at less than 50 el feet. P As regards the modern epoch, I will merely remark that while in the axis of the ancient embayment the Mississippi river, through the singular instrumentality of mudlumps up- ti heaval, is rapidly pushing out the land into the Gulf waters the latter are nevertheless gaining ground on almost the entire coast of Mississippi and Alabama; and the same is true of ac portion of Vermillion Bay. Yet on the whole, the coast of Louisiana, as well as that of Texas and Florida, is more than holding its own; and the shallowness of the water, even where encroachment does take place, will necessarily restrict the latter within narrow limits hereafter. t ART. L.-On the Astromonical Proof of a Resisting Medium in Space; by ASAPH ÉALL. THE return of Encke's Comet during the present year and its very favorable position for observation will attract the attention of astronomers to this, one of the most interesting bodies of our solar system. Besides the interest belonging to all periodic comets since the establishment of the probable connection of their orbits with those of meteoric streams, this comet has a peculiar interest, since from the singular anomaly in its motion Professor Encke drew his inference of a resisting medium in space. Encke's labors on the orbit of this comet were begun in 1819; and immediately after his discovery of its periodicity, he found by comparing the observations of that year with those of 1786, 1795 and 1805, and taking careful account of the planetary perturbations, the remarkable circumstance that the periodic times were diminishing. The following values of these times were found: 1786-1795 periodic time=1208-112 days 1795-1805 1805-1819 66 =1207.879 In order to account for this diminution Encke adopted the hypothesis of a resisting medium in space. He appears to have been led to this hypothesis in the first place on account of its inherent probability, and in this view he was sustained by lbers. But the strongest proof of its truth lies in the fact hat the analytical investigation shows, that a tangential force esulting from a resisting medium would produce secular hanges in the mean motion of the body and in the excentriity of its orbit, leaving the other elements unchanged or hanged by periodic inequalities only. This conclusion is independent of the law of density of the medium. Now this was what Encke needed in order to account for the anomalous motion of the comet, the change falling almost entirely on its mean moion, that of the excentricity being quite insignificant. Other hypotheses were suggested to explain the diminution of the periodic time, and especially that of internal changes in the comet itself, but nearly all of these, besides being less simple than the assumption of a resisting medium, would necessitate the introduction of forces acting in various directions, and producing anomalous changes in all the other elements of the orbit, contrary to what was required by the observations. Encke therefore, notwithstanding the doubts of Bessel and other astronomers, continued steadfast in his theory of a resisting medium in space, and for more than forty years, and until within a short time before his death in 1865, pursued his calculations with wonderful zeal and industry. Between the years 1829 and 1859, he published in the volumes of the Berlin Academy eight memoirs on the orbit of this comet, and also other investigations on the same subject in the Astronomische Nachrichten and in the Berlin Jahrbuch. He assures us, what we can easily believe, that he spared no labor and despised no precaution that could give completeness and surety to his computations; and besides being an excellent mathematician, Encke possessed, in a degree rarely equaled, the skill of adapting formula to convenient and safe forms for numerical calculations. He has given in the Berlin Jahrbuch for 1861 a résumé of his labors, and the proofs presented there, taken simply by themselves, seem to put beyond the shadow of a doubt two conclusions: first, that the periodic time of this comet is diminishing; and secondly, that this diminution is satisfactorily accounted for by the assumption of a resisting medium in space. I will now state the reasons that throw doubt on the preceding conclusions, and which, I think, require that Encke's results should be tested by an independent calculation. The position and dimensions of the orbit of Encke's comet are such that the comet can approach very near to Mercury, so near indeed, that notwithstanding the small mass of this planet, the perturbations which it may produce in the motion of the comet can exceed the greatest ever produced by Jupiter. On account of the rapid motion of Mercury, the calculation of |