ART. V. On the Tertiary Formations of Mississippi and Alabama; by EUG. W. HILGARD, Ph.D., State Geologist of Mississippi.

PORTIONS of the Tertiary formations of Mississippi and Alabama have formed the subject of study of various observers, from time to time, during the past thirty-five years; and the rela tive age and characteristic fossils of three of the most important divisions, have been determined by the labors of Morton, Conrad, Lea, Lyell, Tuomey and others. Most of these observations, however, have been confined to a few localities, or to such as were situated at short distances from each other in the direction of the strike, though sometimes affording complete sections in that of the dip of the strata.

It is my object in the present paper, to review the general results of my own observations, as combined and collated with those of other scientific observers to whose writings I have been able to refer. If in so doing I am led to controvert the opinions of some, it is in the interest of science, my opportunities for ob servation having afforded fuller data for reaching correct conclusions. No one can appreciate more than I do, how much American geology owes to the indefatigable research of Conrad, espe cially. Had he been less active in promoting our systematic knowledge of the Tertiary, I should have had fewer objections to offer to his opinions, and certainly fewer results to science.

Among the sections best adapted to the study of the Alabama Tertiary, are those afforded along the course of the Alabama and Tombigby rivers, by the well-known exposures of Claiborne and St. Stephens, where Sir Charles Lyell first definitively settled the question of the age of the so-called Numamulite, more properly Orbitoides, limestone; and observed the fact, ignored again by some subsequent writers on the subject, that the matrix of Zeuglodon bones always lies below the true Orbitoides limestone. The Vicksburg and Jackson groups. In most respects, the Claiborne and the St. Stephens sections agree so closely, that their character was naturally considered as the type of the Southwestern Tertiary, until Conrad's examination of the Vicksburg bluff showed the Orbitoides to be there associated with a fauna distinct from, yet equalling in variety and peculiarity, that of the Claiborne sand. In view of the coincidence of leading fossils, nevertheless, Conrad at once considered the part of the Vicksburg profile first examined by him (No. 5 of Sec. 31, p. 141 of my Miss. Report) as the near congener of the Orbitoides limestone of St. Stephens. Yet he seems to have retained doubts as to the precise equivalence of the two divisions, which have lately found expression in the separation attempted by him, of the Vicksburg marl and blue limestone from the Orbitoides limestone proper,

and the transfer of the latter to the Jackson group. (This Jour., Jan., 1866.)

The collection of shells upon which Conrad based his determination of the latter group (Trans. Acad. Nat. Sci., 1855, p. 257) was unfortunately a selected one.* It led him to the conclusion that the Jackson beds contained no species in common with those of Vicksburg, and a very few with those of Claiborne. Had he been on the spot he would have found, as I did a few years afterwards, that not only do the Jackson beds contain a goodly proportion of recognized Vicksburg fossils, but that the same fauna (though in an indifferent state of preservation) occurs in the marlstone strata overlying the sandy shell-bed, associated with Zeuglodon bones, Luganum Rogersi and Scutella Lyelli. Moreover, the oyster occurring on top of the Jackson beds, as stated in a previous paper is not O. Georgiana, but a Gryphaea everywhere accompanying the Zeuglodon; but also occurring in the Vicksburg marl and limestone. Nowhere in Mississippi has a single Orbitoid been found associated with either the Zeuglodon, or any of the characteristic fossils of the Jackson group. It is quite possible that in Alabama, Zeuglodon bones may have been picked up in company with Orbitoids, equally as well as with drift pebbles. There, the same ravine often cuts into the strata of both groups, and of course commingles their fossils. In Mississippi, I have found this direct superposition only in a single instance; elsewhere, the regions in which the several groups crop out are so far separated geographically, (in conse quence of the intercalation of lignitic strata,) as to leave the observer no legitimate chance of error in reference to fossils.

Notwithstanding the defectiveness of his materials, Conrad assigned to the Jackson group its proper place, between that represented by the Claiborne sands and the Orbitoides limestone. He still, however (l. c.) thought it most probable that the Zeuglodon was referable to the same age as the latter.

A great deal of the obscurity in which the relative age of the Southwestern Tertiary has been involved, is owing to too great a reliance placed by most observers on lithological characters,

It is impossible to avoid erroneous inferences from the examination of fossils sent for determination by amateurs, and rarely collected with a view to completeness, or general results. From the collection of Jackson fossils submitted to Mr. Conrad, any one would infer that this rich fauna had been totally extinguished by some cataclysm, before the deposition of the Vicksburg strata; whereas in fact, probably more than one-fifth of the former fauna is represented in the latter.

The same has happened with reference to the superior Cretaceous of Mississippi and Alabama, the Ripley group of Conrad, whose fossils as described by him from a selected collection forwarded to him, would seem to constitute an isolated group, almost unconnected specifically, with the lower members of the Cretaceous of the Southwest and elsewhere; whereas in reality it shares the leading fossils of the latter. and is connected with the Rotten Limestone group especially, by transitions both lithological and paleontological, as ascertained by myself a year previous. (Miss. Rep., pp. 79, 84.)

differences as well as resemblances. The "white limestone of Alabama" has so long been quoted as the matrix of the Zeuglodon as well as of the Orbitoides, that no one seemed to question their being contemporaries. Yet in examining all the records of the occurrence of Zeuglodon bones which I have been able to collect, I have no where found a distinct statement that the Orbitoids have been found associated with them in situ. The Orbitoides limestone is mentioned as forming knolls, hill-topsthe Zeuglodon as being found in level fields, or in ravines.

The true position of the Zeuglodon bed did not, however, escape the glance of Lyell (On the Nummulite Limestone of Alabama; this Jour. [2], vol. iv); for he distinctly identifies the upper "Rotten limestone" bed of the Claiborne bluff with that which, at Bettis' Hill, contains Aturea Alabumensis and Zeuglodon, and underlies the Orbitoides rock. The only other observer who seems to have recognized the same fact. is C. S. Hale (this Jour., [2] vol. vi, p. 354). Tuomey, otherwise so accurate in his field observations, ignores it, and speaks only of the "white limestone" in general.

Nowhere has the geologist more need of divesting himself of reliance upon lithological characters, than in the study of the Mississippi Eocene. Not only do the materials of the different groups often bear a most extraordinary resemblance to each other, but their character varies incessantly in one and the same stratum, within short distances. Hale (1. c.) remarks that in Mississippi, the Orbitoides limestone seems to be represented by blue marlstone, and so it is-sometimes. But while on the one hand we see the hard limestone of the Vicksburg bluff passing into blue marl (Byram, Marshall's quarry), we on the other hand find it passing equally into a rock undistinguishable from that of St. Stephens (Brandon, Wayne county); the varied fossils described by Conrad disappearing almost entirely, to be replaced by millions of Orbitoids imbedded in a semi-indurate mass of carbonate of line, interspersed at times with similarly constituted conglomeratic masses of Pecten Poulsoni.

I cannot therefore, with the lights before me, agree to the propriety of distinguishing as separate divisions the Orbitoides limestone and the Vicksburg group of fossils. Even the occurrence of a different species of Orbitoides (O. nupera Con.) at Vicksburg cannot alter the case, for the undoubted O. Mantelli occurs there also, in the solid rock. And there are few of the characteristic fossils of the Vicksburg profile, which I have not on some occasions found side by side with the O. Mantelli and its companions, the Pecten Poulsoni and Ostrea Vicksburgensis.

Of course, the coral had its favorite haunts-the mollusks theirs. There is nothing surprising in the fact that where the one abounds the others are usually scarce, or vice versâ.

The Red Bluff group.—In a late paper, above referred to, Conrad proposed to distinguish the lowest fossiliferous stratum ordinarily visible at Vicksburg, and subsequently studied by him (No. 4 of my Vicksburg section, ut supra), as a separate group, which he considers as characterized by the occurrence of Ostrea Georgiana, and for which he proposes the name of Shell Bluff group. I have elsewhere (this Jour., July, 1866) explained my reasons for dissenting from Conrad as to the position between the Claiborne and Jackson groups, which he assigns to this new division. To the propriety of distinguishing it, however, as a sub-group of the Vicksburg age, I fully agree, though doubting that of giving it the name of a locality from which, as Conrad remarks, but one coincident fossil is known-0. Georgianawhile another also occurring there-0. selleformis-in Mississippi and Alabama is confined to the Claiborne group. In a profile of 80 feet, as occurring at Shell Bluff, loose data like those extant regarding this locality, cannot fairly be made a ground of conclusions contrary to the order elsewhere elaborately observed. For aught that is on record, the whole Jackson group may be represented between the beds in which O. Georgiana and O. sellaformis respectively occur at that place, if (as seems probable from its non-occurrence in the Jackson group of Missis sippi and Alabama) the former shell should be so restricted in its range as Conrad supposes.

I believe the white limestone (No. 1 of my Vicksburg section) which underlies the lignite at Vicksburg, but is visible only at extraordinarily low stages of water, to be of the Jackson age, both from its stratigraphical position and the lithological character of the specimens I have seen. But whether it is or not, there can be no reasonable doubt that the usual Jackson strata, which are largely developed on the Yazoo above Vicksburg, underlie at Vicksburg, as well as on Pearl river and Chickasaw hay, the Vicksburg group.

The Georgiana bed at Vicksburg is preeminently the habitat of a shell common to the Jackson and Vicksburg stages, but most abundant in the former, viz., Meretrix Sobrina Con.; of the two Madrepores described by Conrad, and of Fulgoraria Mississip., all occurring, more or less, in the Vicksburg stage proper. Of the fossil first mentioned, I have after a freshet found hundreds washed out, mingled with numerous masses of Madrepores, sometimes of several pounds weight, with Fulgoraria, Natica? Vicksburgensis, Ostrea Georgiana, etc. The bed has therefore affinities both above and below, and moreover occupies precisely the stratigraphical position of the bed at Red Bluff (Miss. Rept., p. 135). Here the fossils are much more numerous and the affinities in both directions are therefore better expressed. Characteristic and abundant above all, however, is a Plagiostoma, which I

cannot distinguish from figures and descriptions of P. dumosum, but hesitate to refer to that species, since it has not been found in the underlying Jackson strata. Lyell mentions the occurrence of P. dumosum in the lower portion of the Orbitoides limestone at Bettis' Hill; the same is mentioned by Tuomey, moreover, as occupying a corresponding position in the St. Stephens profile, associated with Orbitoids, and even his description of the lithological character of the bed tallies with that of the Red Bluff deposit. Hale, likewise, mentions P. dumosum as one of the prominent fossils of the "white limestone."

The Red Bluff bed seems, therefore, to be more or less coëxtensive with the Vicksburg group, and regularly associated with it as a subordinate feature. Its inconsiderable thickness readily explains its entire absence at many points where, stratigraphically, it ought to appear. Unfortunately, the fossils accom. panying O. Georgiana at the only locality, other than Vicksburg, where it has been found in Mississippi, have not been observed.

The Claiborne group proper.-That the beds of blue marl and white marlstone, which in my Report I have designated as the "Calcareous Claiborne" group, are strictly equivalent to the typical fossiliferous sand at Claiborne, with underlying limestone bed, is probable both from their stratigraphical position and the correspondence of all the fossils thus far observed; though from the indifferent state of preservation in which the latter are found in the Mississippi stratum, these are few in number. Ostrea sellaformis Con. and O. divaricata Lea, are the leading shells; I have also recognized Corbula gibbosa Lea and Voluta petrosa Con. These beds possess fewer good exposures in Mississippi than either of the preceding groups, and may possess many unobserved features. Since publishing my Report, I have received evidence that it extends somewhat farther westward, between the territory of the Jackson and Siliceous Claiborne groups, than it appears on the map. Nor is the division between it and latter groups very well defined, inasmuch as the transition from siliceous to calcareous materials is a gradual one, through strata often very rich in Scutella Iyelli, Pecten Lyelli, Ostrea divaricata and O. Alabamensis Tuo.? I am not aware of the existence of any lignite bed in the dark colored clays which immediately underlie the blue marl.

Siliceous Claiborne, or Buhrstone group.-The precise Alabamian equivalents of my "Siliceous Claiborne" group are not nearly so obvious. The extreme variability of its strata both in Mississippi and Alabama (see Tuomey's first Report, and C. S. Hale, l. c.), which it seems natural should carry with it a corresponding variation at least in the predominance of fossils, and the comparative scarcity and ill preservation of the latter, render its study doubly difficult. I think that, as Tuomey intimates

AM. JOUR. SCI.-SECOND SERIES, VOL. XLIII, No. 127.-JAN., 1867.