Genesee shale. This bed of gravel is the reservoir, and becomes charged with a supply of thick petroleum called "surface oil." Some wells have yielded thousands of barrels of surface oil. It may be necessary to add that in some portions of Enniskillen the Genesee shale has been removed, and the surface wells are evidently supplied from the lower Marcellus shale, which also stocks the crevices of the Hamilton limestones. In Venango County, Pennsylvania, and Trumbull, and Knox, and contiguous counties in Ohio, the Genesee shale is overlaid by porous sandstones which serve as reservoirs of the oil. In the Glasgow region of Southern Kentucky, the formation overlying the Genesee shale is the Mountain limestone; but this is in places arenaceous, and in others vesicular and cavernous, and thus furnishes the requisite conditions of oil-accumulation. In one instance at least, in that region, the Genesee shale itself affords the reservoir for the storage of its productions. In West Virginia the oil seems to accumulate in the conglomerate at the base of the Coal-measures. The same is the case in Southwestern Pennsylvania, Southeastern Ohio, and Northeastern Kentucky. The reservoir in the Burkesville region of Southern Kentucky is found in the shattered shaly limestones of the Cincinnati group. These are reproduced in physical characters in the shattered shaly limestones of the Hamilton group, which serve as the place of deposit of the oils of Ontario.

I close this sketch of the geological phenomena of petroleum by presenting a synopsis of oil regions and the formations tributary to their supplies.

I. The black shales of the Cincinnati group afford oil which accumulates (1) in the fissured shaly limestones of the same group, and supplies (A) the Burkesville region of Southern Kentucky, and (B) Manitoulin Island in Lake Huron.

II. The Marcellus shale affords most of the petroleum

which accumulates (2) in the fissured shaly limestones of the Hamilton group, and thus supplies (C) the Ontario oil region, locally divided into (a) the Bothwell district, (b) the Oil-Springs district, and (c) the Petrolea district.

The Marcellus shale affords also a large portion of the oil which accumulates (3) in the drift gravel of the Ontario region.

III. The Genesee shale, with perhaps some contributions from the Marcellus shale, affords oil which accumulates (4) in cavities and fissures within itself in (D) some of the Glasgow region of Southern Kentucky.

It affords also the oil which accumulates in (5) the sandstones of the Portage and Chemung groups in (E) Northwestern Pennsylvania and contiguous parts of Ohio.

It affords also the oil which accumulates in (6) the sandstones of the Waverly (Marshall) group, in (F) Central Ohio. It affords also that which accumulates in (7) the mountain limestone of the Glasgow region of Kentucky and contiguous parts of Tennessee, as also some of that which is found in the drift gravel of the Ontario region.

IV. The shaly coals of the false Coal-measures, aided, perhaps, by the Genesee and Marcellus shales, seem to af ford the oil which assembles in (8) the coal conglomerate as worked in (G) Southwestern Pennsylvania, (H) West Virginia, (I) Southern Ohio, and the contiguous but comparatively barren region of Paint Creek, in Kentucky.

V. The Coal-measures may perhaps be regarded as affording a questionable amount of oil, which may have been found within the limits of (9) the Coal-measures in the West Virginia and neighboring regions.

From this exhibit it appears that the principal supplies of petroleum east of the Rocky Mountains have been generated in four different formations, accumulated in nine different formations, and worked in nine different districts.

Fig. 90. View of the Salt Works at Mason City, West Virginia.

CHAPTER XXVI.

SOMETHING ABOUT ROCK-SALT AND GYPSUM.

COMM

OMMON salt, upon which the chemist has imposed the more dignified title of chloride of sodium, is a mineral almost universally distributed through the stratified portion of the earth's crust. Like those other substances of universal utility to man-petroleum, coal, iron, water, and lime it is supplied by Nature to every habitable region. of the terrestrial surface. Like lime, which is the chief constituent of the bones and teeth of man and the other vertebrates, the shells of molluscous animals, and the mountains of coral accumulations reared in the bottom of the sea, common salt also subserves the necessities not only of man, but of the quadrupeds and various other terrestrial animals, including insects, and is the characteristic constituent of sea-water, the home of two thirds of all the animals now existing, and a much larger proportion of the animals of former geological ages.

The salt of the rocks is the residuum of the once universal ocean. The reader will remember that reference has already been made to the origin of salt lakes, like those of Utah and the Caspian and Aral Seas. Such lakes are but remnants of the last oceanic inundation. They occupy depressions in the terrestrial surface from which there is no outlet. If, like Lake Superior, they had been drained to the sea, the original saline waters would long since have been replaced by fresh waters from the clouds.

In consequence of the changed condition of the earth, the amount of evaporation from the surfaces of these inland seas has generally exceeded the contributions of fresh water from the clouds. Their saltness has therefore been intensified, and, in many cases, a deposit of crystallized salt has been formed upon the bottom and around the shores. Indeed, there have been salt lakes that are now extinct, in consequence of the exhalation of their waters; and in the place of each remains a salt plain, the surface of which is composed of salt and the other mineral constituents of the ancient sea-water, variously intermingled with argillaceous matter washed in from the surrounding country. The interior of the American continent furnishes abundant phenomena of this kind, stretching from Utah, through the Great American Desert, to Mexico. Such products are the residua of salt lakes which have evaporated since the surface received its existing configuration. It will be remembered, however, that extensive salt-beds exist in Nevada, which are derived from the leachings of the saliferous strata of the mountains; and it may be that some of the ancient salt lakes of the region were supplied with salt to some extent by contributions from similar sources. This, nevertheless, would not prove that all salt lakes have been similarly fed. Besides, if it should appear that they are, we have still to account for the existence of strata of salt

packed away among the solid rocks; and there is no explanation so natural and so consonant with what we know of the history of the world as the doctrine of evaporated sea-waters. How the waters of the sea came into possession of their saltness is a question of primeval chemistry to which allusion has heretofore been made. It was the resultant of the chemical actions which took place between the fire-born rocks and the atmospheric acids washed down by the primeval rains, and gathered with "the gathering together of the waters."

Salt lakes, or detached outliers of the great ocean, have existed in all ages since the continents began to shed the ocean's waters from their backs. In the age just preceding the last, an inland sea occupied the region of the upper waters of the Missouri River; and, a little earlier, the same sea extended a few hundred miles farther south, over the country of the "Bad Lands" of Dakotah. In the middle ages of the world's history, the evaporation of salt lakes or bays more or less shut off from the ocean, and the bedding of their saline constituents, was a phenomenon of so frequent occurrence as to constitute the most prominent feature of an entire group of strata. This group has consequently been styled the "Saliferous system." The saliferous beds of this group are extensively worked for rock-salt over a territory stretching along both sides of the Carpathians, embracing the mines of Wallachia, Transylvania, Galicia, Upper Hungary, Upper Austria, Styria, Salzberg, and the Tyrol. In England they are mined in the counties of Cheshire and Worcestershire. In the United States we find saliferous beds of the same age extensively distributed over the region between the Mississippi River and the Rocky Mountains.

Descending in the series of American strata, we find the Coal-measures in certain regions-or rather the conglomer