Oreodon beds capping flat-topped buttes south of the town of Scenic and abounding in the tests of ostracod crustaceans, even in completely silicified portions of the rock, they are interpreted as deposits formed in shallow pools on the surface of the level plain on which the Oreodon beds were later deposited, as discussed below. Some have been completely replaced by silica, showing white or brown chert bands. Titanothere bones in place have been seen at several localities but a few feet below these white lenses. Above, the "red layer" is usually overlain by a zone of greenish nodules and sandy lenses in a green clay about 17 feet thick where measured in the eastern part of the basin of Indian Creek. The difference in lithology and color and the extreme poverty in fossils of this green noduliferous zone, in striking contrast with their abundance but a foot or so below, makes the identification of the upper limit of the horizon in question an easy matter, and the collector is always eager to return to it after his excursion into the barren layer above.

In Pennington County the "turtle-oreodon layer" is usually a harsh-feeling pinkish-gray clay with greenish banding and mottling, containing in the uppermost six or seven feet one or more zones of calcareous concretions (Pl. VII., Fig. 1), frequently inclosing turtles or Oreodon and other skulls and partial skeletons. The pinkish tinge is much more apparent when the beds are wet after a rain. They abound in rolled clay pellets best seen in the nodules which are merely local hardenings of the clay without concentric structure. Possibly the inclosed fossils have at times controlled the accretion of calcareous material, but very many contain no fossils nor any visible central nucleus. They vary in diameter from a fraction of an inch to several feet. Their surface may be stained a rusty brown and is pitted with depressions left by the weathering out of the clay pellets just referred to. Sometimes a calcified root tracery is visible on the surface of a concretion. Occasionally, as observed at several places in the basin of Indian Creek, they fuse into a solid sheet of rusty, clayey limestone a few inches to a foot or more thick, from which fossils are absent, but with return to the condition of separate nodules fossils reappear.

In Pennington County the nodules are locally cut out, either alto

gether or in part, by channels of varying depth filled with coarse greenish micaceous sandstone, which I take to be the equivalent of the Metamynodon channel sandstone, as they occupy a position similar to the latter. The nodules do not pass through the sandstones or lie above them, but stop suddenly at the edges of the channels and reappear beyond them, which can only be interpreted as erosion subsequent to the deposition of the nodular zone; but there are other channels clearly contemporary with the "turtle-oreodon layer" and wholly inclosed within it as observed to the east of Chamberlain Pass, about 4.5 miles east of Scenic in the basin of Cain Creek, where the lower levels of the "turtle-oreodon layer" may be diversified by zones of pale green lime-cemented concretions or hard greenish sandy clays cut by vertical joints into spherically weathering blocks and separated horizontally by beds of soft clay. Furthermore, a well-defined green sandstone channel was here noted in the upper part of the "red layer" itself.

In Jackson County, in the vicinity of Cedar Pass near Interior, along the south side of "The Wall," the contact of the "turtleoreodon layer" with the Titanotherium beds is somewhat doubtful owing to the presence in both of greenish nodular zones which look very much alike and to the local absence of fossils characteristic of the Titanotherium beds. My measurements started somewhat arbitrarily at the top of a rather persistent bed of reddish clay above which an Oreodon beds fauna occurs throughout a zone of reddish and greenish banded clays 43 feet, more or less, thick with frequent bands of nodules, greenish in color in the lower part and, at the top, rusty and cellular, inclosing fossils and similar in every respect to those described from the Big Badlands. At the Cedar Pass locality the "turtle-oreodon layer" is followed by 58 feet, more or less, of clay from which nodules are conspicuously absent, faintly colorbanded (red and green) and practically free from coarse material except an occasional thin lens of sandstone. It will be noted that this sequence of beds is somewhat different from that found 40 miles or more farther west, but this is not an infrequent occurrence in Tertiary continental formations. The remarkable thing is the presence of the "turtle-oreodon layer" or "red layer" over such a

vast extent of country, with everywhere the persistence of a rusty nodular zone at its top, and its development over what appears to have been a base-leveled surface (Fig. 2, Pl. VII.).

Before discussing the origin of the "red layer" a novel feature should be mentioned, namely, the presence at several horizons in the upper Titanotherium and lower Oreodon beds in the basin of Indian Creek of calcareous crusts and nodules formed by the functioning of blue-green algæ, similar to the algal balls described by Professor H. Justin Roddy from Little Conestoga Creek, Lancaster County, Pennsylvania.1 One of these algal reefs occurs in the upper part of the "turtle-oreodon layer," probably in Sec. 6, T. 4 S., R. 13 E., Black Hills Mer., in the most easterly part of Indian Creek basin about half a mile west of the narrow mesa known locally as Hart Mountain. It is exposed along the south side of a minor badland draw and occurs both in the pinkish-gray clays of the "turtleoreodon layer" and along the margin of a channel-filling of green. micaceous sandstone interrupting the upper part of the brown nodular zone. Laterally, the reef passes into the sandstone, some of the algal masses rising as miniature islands through the sand. Most of the algal growth here is in the form of a crust, only a foot or so of its width remaining in place, the rest being scattered as talus blocks over the underlying clays. At least three zones of rusty nodules are present in the general vicinity of this reef. The nodules decrease in number as the south edge of the algal crust is approached and almost disappear. The reef first appears as a thin seam in the pinkish-gray clays and rapidly thickens up to a maximum of two inches or so of typically concentrically-banded, Stromatopora-like algal crust and balls. About 50 feet north of the south edge of the algal crust it abuts on the green channel sandstone already mentioned. The whole deposit has a length from north to south of 225-250 paces and is scattered as talus over a width of clay slope from 25 to 50 feet beyond its outcrop. Rusty nodules of the type inclosing rolled mud pellets are much less abundant throughout the area occupied by the algal reef, because cut out by the channel with which it is associated. A few project from their clay matrix below the level of the 1 PROC. AM. PHIL. Soc., Vol. LIV., 1915, pp. 246–258, two figures.

algal crust; none were seen included in it. These alga grew at the edge and partly within a shallow stream cutting the "turtle-oreodon layer". Elsewhere throughout Indian Creek basin algal crusts and balls have been seen at several localities in the upper part of the Titanotherium beds, not far below the Oreodon beds contact, resting on a clay substratum and sometimes growing over bone fragments. Specimens of both types have been sectioned and the material submitted to Dr. Marshall A. Howe, of the New York Botanical Garden. His studies thereon have been most seriously handicapped by recrystallization of the organically formed limestone, almost completely obliterating the original cellular structure. Dr. Howe thinks that one organism dominates, and that there is no very serious admixture, and, furthermore, that "it is most reasonable to suppose that the plant belongs with the Myxophyceæ (Cyanophyceæ or Bluegreen Algæ), and under this class its family is most likely to be the Rivulariaceæ, though one can not feel absolutely confident of it. One would not be justified in referring it to any known genus, living or fossil, and there is nothing very definite, concrete, or detailed on which to establish a new genus."

In formulating a theory of origin for the "turtle-oreodon layer conditions controlling the deposition and preservation of the nunerous fossils, as well as lithologic and stratigraphic data, must be considered. Every collector is at once struck by the enormous numbers of fossil turtles of all sizes. These are not aquatic forms, but, as Dr. Hay2 has pointed out, are closely related to our living land tortoises, so far as can be determined from the material available, mostly empty carapaces. These, quite irrespective of size, do not lie in death pose, but may be upside down or on edge. Heads and limbs are missing and have either been eaten by carnivores or decayed and dropped off when the shells were moved by the transporting agent. The majority of the other fossils found are mammalian skulls, often with lower jaws attached, and lie in all sorts of positions. Loosely inserted teeth are sometimes missing, but there is little or no indication of abrasion by movement in material found outside the sandstone channels. Many of the skulls were reduced

2 O. P. Hay, "The Fossil Turtles of North America," Carnegie Institution of Washington. Publication No. 75, p. 385, 1908.

before interment to the condition of tooth-bearing jaw-fragments, the portion of the body most resistant to decay. Turtle shells and all the cavities in the skulls are completely filled with a harsh-feeling pinkish-gray clay full of small rolled clay pellets. These are not concentrated into lenses, but evenly distributed throughout the clay. Mouse-nibbled bones and abundant undissolved coprolites of carnivores are common features in the same matrix with the skulls and turtle shells. One associated Oreodon skeleton collected by the Princeton Expedition of 1920 had the head and neck bent back in a manner frequently observable in the more or less dried-up carcasses of sheep killed by winter storms. A film of iron oxide, red or yellow, frequently covers the fossil bones, their organic content perhaps acting as a precipitant on iron compounds which subsequently oxidized. Some of the original calcareous material of the bones still remains, but much has been replaced by silica, which may completely fill marrow cavities in limb bones, and pulp canals in teeth, and occurs also in the form of chalcedony veins in the numerous shrinkage cracks which traverse both Titanotherium and Oreodon beds.

Turning now to stratigraphic and lithologic data, in the basin of Indian Creek, when viewed from a distance, the Chadron-Brule contact is seen to be remarkably even and free from sinuosities, apparently a base-leveled erosion plane separating the soft cross-bedded clays and channel sandstones of the Titanotherium beds, with their smooth hummocky hill profiles, from the horizontally stratified, evenly color-banded, harsh-feeling Oreodon clays with their dendritic type of dissection and steeper slopes. The sudden disappearance of titanotheres at the contact is further proof of a stratigraphic break. When examined close at hand, the contact loses much of its distinctness, owing to the softness and similarity of materials on either side of the erosion plane. Over this surface, on which were numerous minor ponds floored with marl (the silicified limestones already described as occurring at the contact), a harsh

The color is about intermediate between "seashell pink" and "pale ochreous-salmon" of Ridgway's Color Standards and Nomenclature, Pls. XIV., XV., 1912, with a pale gray tone and has accordingly been spoken of above as pinkish-gray.