The sandstones vary in composition, some of them being micaceous, the common mica being biotite. A calcareous sandstone of this description occurs at Velva, where it is but a local hardening of an extensive sand layer. Within a mile the stone. thickens from two to ten feet. Outcroppings of a similar sandstone occur on the Missouri river at Washburn and it has been used to some extent as a foundation for buildings. At Sims and Dickinson, at elevations considerably above that of the stone at Washburn, sandstone is found in strata of considerable thickness. While these sandstones are similar in composition, the black mica being a conspicuous element in each, there is little ground for the assumption that they represent a single stratum, the only conclusion to be drawn from their similarity of composition being that they probably had a common source.

The most notable sandstone stratum in the Laramie is that which caps it, and appears at the tops of some of the high buttes in the western part of the state, as in Sentinel and Square Buttes. As developed in Sentinel Butte it is fifty feet thick and presents a vertical face for hundreds of feet on the north side of the butte. On the east and west ends great masses have fallen, forming talus slopes of magnificent proportions. This sandstone is light colored, shows but the slightest trace of lime, and is free from mica. The natural exposures show that the rock presents great resistance to the weather, the great blocks

being but slightly. eroded by wind or water and are very firm, when their long period of exposure is considered. Situated three miles from the railroad, with an abundance of good coal at hand, the quality and vast quantities of this sandstone would make it the most promising quarry proposition in the state, if shipping rates and a market were favorable. It is illustrated in place near the top of Sentinel Butte in plate V. Todd finds a similar sandstone at the top of the Laramie on the high buttes of the northwestern part of South Dakota.*

Sandstone nodules and concretions occur in great abundance in the sands and sandy clays of the Laramie. Their shapes are ⚫ often grotesque, suggesting forms of plants and animals and they are frequently mistaken for fossils. They usually show plainly the concentric structure which reveals their their origin. Many of them are remarkably round while others are elongated, all of the nodules in a given locality tending to take a common form. They are generally arranged in layers, a fair illustration of their mode of occurrence being given in the center of the frontispiece. Great disc-like bodies are not uncommon, the short axis of the disc being vertical, while the other axis may reach a length of twenty feet. Thin though very persistent bands of clay-ironstone partly oxidized to hematite occur frequently in the Laramie clays. The greatest thickness observed was two inches, the mineral at no point appearing in quantities of economic significance.

Limestone is equally rare, and when it does occur is highly argillaceous. The frontispiece shows a thin layer directly over the coal. Beautiful leaf prints are abundantly preserved in the ironstone layers.

The general characteristics of the lignite will be briefly described, detailed descriptions of exposures being reserved for the subsequent text.

NATURE OF THE LIGNITE DEPOSITS.

A careful consideration of the nature of the lignite deposits is important for practical and theoretic reasons. The numerous natural exposures of lignite in the driftless area and the re markably fine opportunity to study the Laramie strata offered. by the bad lands make it possible to draw, even from a pre. liminary study of the region, rather far reaching conclusions.

* South Dakota Geological Survey, the First and Second Biennial Reports, with ac

companying papers, pages 52-53.

A large collection of records from deep wells or prospect holes. is needed to determine the nature and amount of lignite at considerable depth below the surface, but this is a deficiency which time will doubtless remedy.

Number of the Seams.-A section in part ideaf, through the Laramie strata in the western part of the state, given in plate VI illustrates the nature of the lignite seams and probably does not exaggerate their number in the region between Sentinel Butte on the west and Fryburg on the. east. The five well developed seams shown in the sketch as outcropping in the bluffs of the Little Missouri actually exist and may be traced for miles north and south of Medora. The seams shown in Sentinel Butte are not imaginary, and it is highly probable that further search between the base of the butte and the top of the bluffs will reveal other seams. Along the Missouri river south of Williston five and six seams are easily traceable for long distances, while from the Berthold agency Mr. Wood reports nine well defined seams in a single exposure. These records are in part due to the excellent opportunity for observation given by the bluffs of streams in the driftless area where vertical sections of nearly two hundred feet are not uncommon. They are due also to the abundance of the seams. Near the eastern edge of the lignite area their number is greatly reduced, but even here two and three exist at certain points, though probably only one is workable. This portion of the area will be more minutely considered in Mr. Wood's report on Ward county.

On account of the nature of the seams, described in another paragraph, their number does not remain constant through large stretches of territory, and it is possible, though hardly probable, that in a few localities in the very heart of the lignite area a prospect hole passing completely through the Laramie would not encounter a lignite seam. This probable variation in number and thickness of seams often within comparatively short distances will be a factor of great practical importance in opening up on a large scale a new lignite field. In the first place it will largely increase the number of prospect holes which must be sunk to determine the amount of available coal in a given area. To offset this, however, are a number of conditions which make prospecting unusually easy.

Thickness of the Seams.-The lignite seams vary in thickness from an inch to forty feet. This maximum thickness, which

in

was seen and measured during the past summer, outcrops in Township 135, Range 101, Section 31, and will be described connection with the lignite of Billings county. On government land in Township 133, Range 101, Section 20, and in Sentinel Butte, three miles south of the town of the same name, are extensive seams twenty-five feet thick. Seams fifteen feet thick uncommon and are exposed frequently along the Little Missouri river and more rarely to the north and east. In the subsequent text a large number of seams between six and tea feet in thickness will be described. In the western part of the lignite area seams two feet or less were so abundant that in the preliminary work covered by the present report it was not practical to note them all minutely.

Extent of Seams.-As important from an economic point of view as their thickness is the extent, or persistence, of the seams laterally. The impression is somewhat prevalent that a single seam may be recognized at points separated from each other by a hundred or more miles. Special attention was directed to this point during the past summer, and the elevations of many of the coal seams in adjacent regions were noted in order to correlate the seams in one area with those of another, if this should be possible. Generally such a correlation was impossible on account of variations in thickness and elevation, unless a greater dip to the strata or more irregularity in the level of the surface on which the lignite was formed was supposed than actually seems to exist. If the nature of the seams admitted of such a correlation it would be very useful, for by prospecting along streams where natural exposures occur, it would often be impossible to determine the nature of the seams between between two exposures widely separated. In some instances the lateral persistence of seams is considerable and can be demonstrated. In general, however, it was not found practical to correlate the seams in different exposures if the exposures were separated more than four or five miles. While a single seam was often traced for six miles or more along the banks of the Little Missouri, others seen at the same time thinned out and gave place to new ones a little above or below them. The interrupted seam often developed farther on after a break of perhaps half a mile, where the lignite was replaced by a bituminous, or even by a nearly pure clay. The deduction, which in the early part of the season's work seemed true, that certain horizons in the Laramie are notably richer in lignite than others, was not borne out by later