is needed to determine the nature and amount of lignite at considerable depths below the surface, but this is a deficiency which time will doubtless remedy. Number of the Seams. A section in part ideal, 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 was seen and measured during the past summer, outcrops in Township 135, Range 101, Section 31, and will be described in connection with the lignite of Billings county. On government land in Township 133, Range 104, 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 are not 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 ten 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 possible to determine the nature of the seams 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 observations. For a given locality the statement holds good. The lignite about Medora, for instance, is present in thicker seams a few feet above water level than at greater elevations; but an elevation which abounds in lignite at one point may be barren in another. The accompanying sketch, Plate VI illustrates conditions as they seem to exist quite generally throughout the lignite area. The number of seams here shown is probably greater than that actually existing along the eastern edge of the area, but does not exaggerate their abundance in the west. A striking instance of the rapid thinning of a seam is given in the forty foot seam already cited, which within onefourth of a mile shrinks to seventeen feet, the top and bottom of the seam drawing together like the surfaces of a lens. Records of two deep wells, one at Medora, and the other atDickinson, bored by the Northern Pacific Railroad, and quoted by * Darton, give an excellent idea of the frequency of lignite seams for a considerable distance below the surface at these points. The well at Medora, situated twenty-five feet above the waters of the Little Missouri and 1,860 feet above sea level, penetrated the Laramie clay for 941 feet without passing through it. Seventeen lignite seams were encountered varying in thickness from a few inches to twenty-three and one-half feet, the total aggregating over sixty feet. Lignite was found but a few feet above the lowest point reached and is distributed nearly uniformly throughout the section. The Dickinson well in going from the surface at that point, 2,400 feet above sea level, to a depth equal to that of the Medora well, 819 feet above sea level, passed through a similar series. Sixteen seams were found, giving a total thickness of lignite of about fifty-five feet. The two points are thirty-nine miles apart, and in view of the data gathered this summer, Darton's attempt at correlation of the lignite seams appears doubtful. Figure 2 is a reproduction of his sketch. Variations in Fuel Value of Lignite in the Same Seam.-Perhaps a majority of the lignite seams examined and tested by analyses showed no great difference in the fuel value of the lignité from top to bottom. In some instances, however, differences in the composition of the lignite in a given seam are noteworthy. Not infrequently the upper foot is inferior in quality and is left as a roof in mining. The loss in this case is not as * Preliminary Report on Artesian Waters of a portion of the Dakotas. |