PETROLEUM ROCKS OF THE GREAT BASIN.
"rre have given a representation of both the ancient and modern formation of the eastern part of the Great Basin in figure 4, Chapter III. In this figure, the dark lines underlying the white rocks which support the coal are designed as the Devonian oil bearing rocks or strata: they are thicker in the engraving than the proper proportion, but exhibit correctly their position, from the steep eastern basins under the anthracite coal-fields to the wide and shallow basins of Western Pennsylvania and Ohio. This picture, however, is an imaginative one, and only given to illustrate the natural pro cesses by which the Palaeozoic formations of the Great Basin grew into present shape and form.
Figure 117, in Chapter XVII., illustrating the Groat Basin in its actual or present condition, and relative depreciation of strata, westward, conveys a general idea of the subordinate or intermediate basins, and the succession and comparative thickness of the succeeding formations overlying the granite and composing the metamorphic and This representation, however, exhibits the formations west of the Alle ghanies to the Rocky Mountains ; while figure 4 is designed to illustrate the succession from the granite of the East and the Chesapeake Bay to the Ohio. The Devonian oil formation is shown beneath the sandstones and limestones supporting the coal; but the relative thickness of this formation is proportionally less in thickness in its westward spread than represented in figure 117.
Figure 2, in Chapter II., representing the Palaeozoic column in the vicinity of the anthracite coal-fields, gives the thickness of the Devonian rocks from the Ponent, or old red sandstone, to the Meridian or Oriskany sandstone inclusive, at 15,000 feet. These rocks include the Catskill, Chemung, Portage, Genesee, Hamilton, Marcellus, Upper Helderberg, Schoharie, and Oriskany, of New York.
In Venango county, and Western Pennsylvania generally, the Ponent entirely die• appears, and all the formations thin rapidly in that direction, and the probable thick ness of the Devonian oil-rocks in that locality may not be more than from 1000 to 1500 feet in thickness. In Illinois, and the Great Central coal-field generally, the thickness
of the formations making up the Devonian is not over 300 feet, as shown by figure 128. In Missouri, within the same coal-field, it is about the same, as shown in figure 131. But the distance from the coal or surface to these oil-bearing rocks is much greater on the Great Kanawha, in West Virginia, in Illinois, and Missouri, than in Venango. There the Devonian rocks come to the surface in the deep valleys, and the millstone grit of the coal formation caps the highest hills from 400 to 600 feet above the level of the streams. But on the Great Kanawha the millstone grit is under the bed of the river, and the geological horizon is, consequently, from 500 to 700 feet higher on the Kanawha at Charleston than on Oil Creek at Sheafer; and, while the sandstones thin in a south western direction, the limestones increase. In Venango, the limestone strata are thin plates of only a few feet in thickness, while below the Kanawha, in West Virginia, they range from 500 to 1000 feet in thickness, and occur between the upper or heavy oils and the middle oils, as illustrated in figure 188.
The foregoing figure illustrates the gradual thinning or decrease of the sandrocks, and the thickening or increase of the limestones, towards the centre of the Great Basin. Perhaps in no part of the Appalachian formations are the conditions necessary for the existence of oil so favorable as in Northwestern Pennsylvania, as we may here briefly describe before tracing the formations farther west ; but we may state, as preliminary, the fact of the intervention of the Carboniferous or Mountain limestone, as illustrated in figures 128 and 131, between the upper and lower oils in all the Western States. This limestone is only 3 feet thick on the northeastern escarpment of the Alleghanies, about 200 under the Ohio at Wheeling, and over 1000 at the mouth of the Great Ka nawha, and through the West.