been able to notice any irregularity in these changes except as to the time. The irregular disturbances are very interesting, and may be identical with the magnetic storms of Gauss. Upon certain days they are hardly perceptible, though they never cease altogether. On some days they are violent, if I may be allowed the expression. The needle does not take a sudden start and return as when influenced by lightning, but moves gradually without oscillation to some fixed point, from which it will return sometimes in two minutes and sometimes in ten or fifteen minutes. An extended series of observations will be necessary before any deductions can be safely made. If the wires should be separated by a slight interval during a thunder storm, doubtless electrical sparks would be visible. During heavy storms, a flash of lightning twenty miles distant from the wires of Morse's telegraph will induce electricity in the wire sufficient to operate the magnets, and work the telegraph, sometimes recording several signals. A flash of lightning in Baltimore, forty miles distant from this place, will operate the magnet at this end of the line. Washington, D. C. Appendix.-Mr. Lane remarks, in his communication on the electric conduction in metals,* that in his experiments he found no confirmation of the statement made in this Journal † a few years since, "that the conducting power of a wire is greatly impaired by bending or twisting it." Mr. Lane has somewhat magnified my meaning by introducing the word greatly, which I did not use, and it is probable that he has really confirmed my statement, as he remarks, that after winding and unwinding a thick wire several times over a cylinder less than an inch in diameter, the conducting power appeared scarcely affected, implying that some effect was produced; and if in Mr. Lane's ingenious and well arranged experiments, which were made with only a few feet of wire, he found even the slightest loss of conducting power, it is reasonable to suppose that in many hundred feet of wire the loss would be a very notable quantity. A statement which I also made in addition to the above, should have been explained. The statement was that " a wire which has once been wound upon a magnet is not fit for the same purpose again." There are * See this Journal, Second Series, i, 230. two reasons why such wire would not answer as well as before. First, its conducting power would be impaired, and secondly, it is extremely difficult to straighten such wire, so as to wind the several turns as closely as before. In regard to the tables which have heretofore been made for the relative conducting powers of different metals, it must be observed, that they cannot be regarded as any thing more than approximations to the truth. What is true of many practical applications of science, is particularly so of galvanism, viz. that laws, rules, or principles deduced from miniature experiments, have entirely failed when applied to operations upon a large scale. As for instance, the law of Pouillet, that wires conducted directly as their cross sections and inversely as their length, was admitted until the experiments in telegraphic operations upon hundreds of miles of wire, disproved this law and confirmed the law of Ohm. I have some interesting facts hereafter to be communicated, showing the importance of operations upon grand scales, in establishing general principles. Most of your readers will remember the signal discovery of Liebig of the existence and economy of ammonia in the atmosphere, which he owed to his departure from the limited eudiometrical experiments of other philosophers and investigating large quantities of air instead of small. The effect before alluded to of bending and twisting wires is much more apparent in some species of wires than others. There has been a spurious copper wire now for some time in the market, and it has been my misfortune to purchase about ten thousand feet of it.* Its conducting capacity as measured by the axial galvanometer, in lengths of 1000 feet, is about one-third less than pure copper wire. It is well known that a wire may soon be broken by bending it back and forth for a few times. Each bend of course approaches to this solution of continuity, and must interrupt that integrity of molecular arrangement which is essential to good conduction. Now a few such short bends may not be appreciable by ordinary tests, but when multiplied several hundred times, the resistance to the current becomes of sufficient moment to be a subject of attention. C. G. P. Washington, D. C., July 3d, 1846. * It is the same wire that has so perplexed the telegraphic operations between New York and Philadelphia, breaking almost daily merely by its own weight. ART. XIX.-Eocene Formation of the Walnut Hills, &c., Mississippi; by T. A. CONRAD. THE elevated bluffs on the Mississippi above New Orleans, on which stand the towns of Natchez, Rodney, Grand Gulf, and Vicksburg have a similar geological origin, and these interesting hills present sections of two formations or depositions widely different in age, as well as in the phenomena presented to our notice and in the causes which produced them. The lower strata are wholly of marine origin, and as I stated in a former publication, they are members of the Eocene, admirably characterized by the peculiar forms which existed in the seas of that period. The shells are extremely abundant, and as in all other localities of the Union which have been explored, the line of demarcation between this formation and the Miocene is as strongly marked as a total diversity of species can make it. My investigations during a late tour in Mississippi, have been chiefly confined to the vicinity of Vicksburg. The hills here rise steeply from the Mississippi river and are some miles in extent. They have been washed into frequent and sometimes very profound ravines which cut through and expose the Eocene strata, and the sides of the hills and ravines are whitened in many places by the shells which have been washed out of the ferruginous marl or fossiliferous mixture of sand and clay. The strata appear to be nearly horizontal and the greatest elevation about sixty feet above the ordinary high-water level. The lowest stratum exposed is a bluish compact limestone, which is quarried for the purpose of paving the streets of Vicksburg. It is full of shells and casts of shells of such species as are common in the marls above. One of the most abundant bivalves is Pecten Poulsoni, (Morton,) a species occurring in the white limestone near Claiborne, Alabama. A very thin wafer-shaped Nummulite, described by Dr. Morton, is common in the limestone as well as in all the strata above, and connects the formation of Vicksburg with the Eocene white limestone of St. Stephen's, Alabama. A new species of Pinna is one of the most striking fossils of the limestone at Vicksburg, and which is rare above it. Over this rock are various strata of sandy marl, sometimes indurated and ferruginous, clay, and clay and sand mixed, all of which are very prolific in fossil shells. Near the summit of the Eocene are beds of coarse gravel mixed with whole shells and fragments, from which many fine agates have been procured by Mr. Anderson of Vicksburg. These strata do not appear to have been much disturbed or inclined by the force which elevated them to their present level. The group of fossil shells though as strongly marked in its Eocene character as any in the Union, is yet remarkably distinct from that of Claiborne or of any other locality which I have seen, as out of about sixty-two species, thirty-eight are new, and I am confident that ten species will on comparison be found identical with Claiborne shells. In most of the strata here, small and large fragments of shells are very abundant, some of which are water-worn and others not in the least abraded. Occasionally we find a black water-worn shell just in the proportion to the others as we see them on the sea beach of New Jersey. The vicinity of an ancient sea beach is strongly indicated by the phenomena of these strata, which is not the case at Claiborne. Bivalves with connected valves are rare, fragments abundant, and the many waterworn specimens all tend to prove the action of the surf. In the clay stratum of the upper portion of the formation, the shells in some rare instances retain a trace of their original colors and their polish is fully equal to that of recent shells, though they become chalky on exposure to the sun. The large Cardita planicosta, which so generally prevails in Eocene deposits, is unknown here, and the Crassatella alta of Claiborne is also absent, but there is an allied though very distinct species. There is one bivalve here, a new Panopea which is common, and yet unlike the other bivalves is almost in every instance entire, and placed vertically in the strata, just as it occurred when living and burrowing in the bed of the sea. It therefore lived and died on the spot where it is now found, whilst nearly every other shell had been abraded by the surf or transported by currents. This is precisely the case with all the various species of Panopaa in the Miocene formation, and it is clear that they burrowed deeply in the mud or sand beyond the influence of the agitated waters which scattered the various shells at that time existing near the surface. The principal development of the Eocene is north of Vicksburg, and every ravine cuts through its various strata, but it is almost impossible to procure an accurate section of them, as they are universally sunk and displaced by land slides and subsidence. As near as I could ascertain, the Eocene strata rise to a level of sixty feet above the river when there is an ordinary freshet. The limestone, nearly on a level with the water, is the lowest stratum known, as debris and deposits from the river cover and conceal whatever may be at a lower level. At the plantation of Dr. George Smith, five miles northeast of Vicksburg, a ravine cuts through the Eocene strata and exposes about the same elevation of horizontal beds, and between them and the loam with land shells, is a stratum of loam with coarse gravel without a trace of organic remains. This gravel is also visible at Vicksburg, but the thickness of it, or its relation to the Eocene is not yet determined. The only abundant shell of the Vicksburg Eocene, common also at Claiborne in Alabama, is Dentalium thalloides. It is probable that the deposits of the Walnut Hills were made in shoaler water, and nearer the shore of the Eocene ocean than those of Claiborne, but it is remarkable that no species of Cerithium occurs, a genus so abundant in species in the Eocene of France, and which is supposed, where it abounds in a fossil state, to indicate an ancient estuary. Vicksburg Fossiliferous Loam. Above the Eocene of Vicksburg, Grand Gulf, Rodney and Natchez, there is a thick deposit of loam of uniform composition and appearance, which is at least fifty feet thick in many places, and probably much more in others; but owing to land slides and the vast accumulation of debris between this loam and the Eocene, the depth of the former is uncertain, and there may be a distinct deposit between the two. The loam is apparently identical in composition with the cane-brake lands of the Mississippi, and abounds in land shells of such species as exist plentifully in the alluvial flats subject to the overflow of freshets. I have collected Helix thyroidus, H. ligera, H. concava, H. setosa, H. arborea, H. perspectiva, &c., together with Succinea ovalis and Helicina orbiculata. Of fresh water shells, there is a small Cyclas and Paludina which I have not seen in the rivulets near Vicksburg, though two other species of Cyclas abound in one of these small streams. The fossiliferous loam has a very undulating summit line, following the outline of the innumerable hills, and it is covered by six to eight feet of a different kind of |