The outside granite edge of the great basin is 6000 feet above the sea in Southwestern Virginia, at the head of the New River,—a continuation of the Great Kanawha,—and from 10,000 to 15,000 feet high in the Rocky Mountains. The succeeding gneissic belt has less elevation, and succeeds the granite not only on its western but on its eastern side, and laps over the granite on the margin of the great basin almost vertically, and in the interior horizontally. Derived from volcanic eruptions or the line of volcanic vents existing in the granite belt of the east, it naturally over spread the granite on both sides of this elevated belt, and covered it in all the deep valleys or lower basins. This belt of gneissic or azoic rocks is succeeded by the Paleozoic or sedimentary strata d, represented in the Alle ghanies on the east and the Black Hills on the west; and on or in this exists the coal.
We must here state that our section is conjectural in its western margin so far as the coal is concerned, and also, to some extent, the form of the Paleozoic strata. The general form and character of the western margin of the great basin must partake to a great extent of its eastern features; but we cannot state from personal observation that the effects of volcanic action were the same, or that basins for the formation of coal were created in the same manner as they were to the east. It is possible that the greater elevation of the western margin left the larger portion high and comparatively dry before the advent of the coal era, or the period of time when the heat was tempered to the proper degree to permit the growth of vegetation.
The section we give is miniature in scale, since we have to comprise 1500 miles in seven inches: we can, therefore, only portray the chief points within the great basin.
The Alleghany coal-field, which is 180 miles wide, is the first, and lies along the eastern side. It is separated from the Central coal-field by the Devonian and Silurian formations of Ohio and Indiana, which are over 100 miles wide. The Central coal-field is as wide as the Alleghany. It will be noticed, in the small maps which we give of these fields, that the scale in the Alleghany map is 100 miles to an inch; while it is only 50 miles to an inch in the Central, and / of an inch to 50 miles in the Western field. The Western field in Iowa and Missouri is also from 150 to 200 miles wide. The extent of the coal in Kansas is conjectural; and the same may be said of all to the west of the Missouri River where it leaves or bounds the Great Western coal-field in Missouri. It is, perhaps, scarcely to be doubted that the Central, Western, and Arkansas coal-fields were once united and formed one immense area of coal nearly, if not fully, 500 miles square, or embracing an area of 250,000 square miles.
The coal depreciates rapidly westward, and only two or three small seams exist in Western Missouri. Westward still, however, we find coal and coal formations; but most of the coal west of Missouri and east of the Black Hills is tertiary coals or lignites, formed, in all probability, in the marshes and bogs of that higher region; while the coal formed in the deeper basins to the east.
The Eastern coal-basins dip below the sea-level; while the Western basins, even in Missouri, lie from 400 to 800 feet above it; and while the Silurian rocks descend from 20,000 to 30,000 feet below the coal in the East, they do not, perhaps, reach the sea-level in the West. It. will be noticed that the ascent is gradual towards the West, and that the Western coal-fields are more uniformly level or partake less of the basin-shape than the Eastern fields. While the Alleghany field presents the. basin formation prominently, the Central coal-field is only moderately basined, and the Western coal-field is nearly horizontal. The evidence presented here is conclusive as to the formation of coal in water; and the deeper the basins the more extensive are the measures and the more numerous the coal-seams. The sections presented farther on demonstrate this fact con clusively. Vertical section, figure 118, through the Alleghany coal-field, presents 50 feet of workable coal; while that of the Central coal-field, figure 128, shows only 20 feet, and that of Missouri, figure 131, only 10 to 15 feet.
We must conclude, therefore, that a shallow depth of water, forming bogs or swamps, was not a favorable condition for the production of coal; and this fact is strong evidence that true coal is not the production of an arhorescent flOra. The basin-shape, as presented in irregular formation, also condemns the theory of the elevation of the submerged portions or the deep basins above water-level. It is not probable that they would be elevated and depressed in spots: such a phenomenon would be un natural. The elevation or depression must be gradual and general. But we discard all such unnatural processes in the formation of coal. The general depression of certain portions of the earth's crust we showed to be consistent with the natural processes, and, though not absolutely necessary to the formation of coal, yet not inconsistent with its formation in deep basins; while the facts here presented demonstrate the propositions formerly set Torth,—that coal is formed in comparatively deep basins from the bituminous results of naphtha or carbon oils in connection with an aqueous flora, and perhaps the oils of an arborescent flora, compressed within the coal-strata and rising to the surface of the water, as all oils must do when released from confinement.