may be locally restricted, the impervious stratum must. present the form of a dome or roof. The underlying strata may, and generally do, conform in position to the roofing strata. We have here the requisite conditions for accumulation. Some portion of the oil and gas may filter through to the surface, or it may not. Obviously, if the outlet be large, the product must escape as fast as elaborated. If the reservoir be nearly closed, it may hold the products of the slow distillation of thousands of years. When one of these store-houses is exhausted it will be filled again, but perhaps not before the millennium.

I said that the oil and gas would displace the water previously occupying the spaces beneath the roof. It is plain that these substances must be hard pressed by the surrounding waters, re-enforced as they are on all sides by a virtual column reaching to the surface of the earth, which may be a hundred or five hundred feet above. The lateral pressure of a column of water five hundred feet high is enormous. All this the forming oil and gases must resist. No wonder that when given vent from above they sometimes burst forth with tremendous violence. At a well which I visited in Knox County, Ohio, the pressure of the confined gas was 180 pounds to the square inch, in addition to the pressure of a column of water 600 feet high. It escaped from the mouth with a roaring sound which could be heard at the distance of a mile. The supply was sufficient to illuminate a large city, and it continued to escape for several months.* When conducted horizontally through a pipe to the outside of the building and ignited, it formed a ragged and spiteful stream of fire of the diameter of a hogshead, which roared like a conflagration, and caused an illumination which was seen at the distance of sixteen

* This was in May, 1866. A letter from Peter Neff, Esq., of Gambier, dated June, 1868, states that this well is still "blowing."

miles. We can form but little conception of the circumstances under which such an enormous volume of gas can be confined at the depth of six hundred feet beneath the surface.

The escape of oil at the surface of a well is caused sometimes by mere hydrostatic pressure, as water rises in common Artesian wells. More frequently, perhaps, the oil is forced up by the elastic reaction of confined gases. An open cavity, or a porous portion of rock bounded on all sides by impervious walls-which constitutes a virtual cavity-may be partly filled with oil, while gases occupy the higher portions of the cavity. Such a cavity, whether actual or virtual, may possess any form or extent-or may consist of a number of cavities connected by narrow passages or mere fissures. In nearly all cases, more or less gas accompanies the oil, and subsists under a very high degree of pressure. The pressure in such cases is not the hydrostatic pressure of water, but a consequence of the continued generation of gas and oil long after the cavity had been filled. If a boring happens to penetrate the higher portion of such a cavity (Fig. 89), the gas at once rushes forth with greater or less violence and persistence. As soon, however, as the tension is relieved, the escape ceases. No oil will be obtained in such a case without applying suction, since there is no hydrostatic pressure exerted from behind, and the reaction of the gas tends rather to confine the oil in the lowest ramifications of the cavity.

Suppose, however, on boring a hole for oil, we happen to penetrate some of the lower portions of the cavity occupied by the oil (Fig. 89, b). The elastic pressure of the confined gas above will at once force the oil up, and produce a spouting or blowing well. The flow must necessarily subside by degrees as the confined gas, by the escape of the oil, acquires more space for its accommodation. It may con

at

tinue, however, until the cavity is exhausted of its oil, after which pumping will be of no avail. If the confined gas tains its equilibrium before the oil has been completely forced from the cavity, it is evident that the remainder must be obtained by pumping. There is no cavity so large, however, as not to be destined to ultimate exhaustion. Every oil well, of whatever class, is destined to abandonment. It is true that Nature is constantly at work replenishing the exhausted reservoirs, but her accumulations are slow. Her working days are centuries.

Intermittent wells appear to act in some cases precisely after the manner of intermittent springs. More frequently, however, it is manifest that the combined action of gas and oil produces the phenomenon. In boring a well, suppose a stream of gas is struck one hundred feet from the surface of the rock, and a small stream of oil twenty feet below the gas. The entrance of oil fills twen

ty feet of the hole, and begins to submerge the fissure at which the gas is escaping. The gas forces its way through the oil with a sputtering sound, bubble after bubble rising to the surface. As the oil ascends, the gas makes louder and louder complaints, till finally, summoning all its accumulated energies, it hoists the superincumbent column of oil to the surface, and pours it out in a stream of a few seconds' duration. The flow then ceases, and the same operation begins to be repeated. After a minute or more of renewed grumbling and sputtering, the pent-up gas again relieves itself, and thus the work continues. The same results would ensue if oil and gas found entrance at the same fissure, or even if the gas were admitted at any distance beneath the entrance of a small supply of oil.

The amount of oil that has been ejected from certain wells is marvelous to relate. Though Western Pennsylvania has produced numerous flowing wells of wonderful capacity, there is no quarter of the world where the production has attained such prodigious dimensions as in 1862 upon Oil Creek, in the township of Enniskillen, Ontario. The first flowing well was struck there January 11, 1862, and before October not less than thirty-five wells had commenced to drain a store-house which provident Nature had occupied untold thousands of years in filling for the usesnot for the amusement-of man. There was no use for the oil at that time. The price had fallen to ten cents per barrel. The unsophisticated settlers of that wild and wooded region seemed inspired by an infatuation. Without an object save the gratification of their curiosity at the unwonted sight of a combustible fluid pouring out of the bosom of the earth, they seemed to vie with each other in plying their hastily and rudely erected "spring-poles" to work the drill that was almost sure to burst, at the depth of a hundred feet, into a prison of petroleum. Some of these wells

flowed three hundred and six hundred barrels per day. Others flowed a thousand, two thousand, and three thousand barrels per day. Three flowed severally six thousand barrels per day; and the "Black & Mathewson" well flowed seven thousand five hundred barrels per day! Three years later, that oil would have brought ten dollars per barrel in gold. Now its escape was the mere pastime of full-grown boys. It floated on the water of Black Creek to the depth of six inches, and formed a film upon the surface of Lake Erie. At length the stream of oil became ignited, and the column of flame raged down the windings of the creek in a style of such fearful grandeur as to admonish the Canadian squatter of the danger, no less than the inutility and wastefulness, of his oleaginous pastimes. From detailed determinations, I have ascertained that, during the spring and summer of 1862, no less than five millions of barrels of oil floated off upon the water of Black Creek—a national fortune totally wasted, as inherited fortunes are apt to be wasted, by those not educated to an understanding of the amount of labor and time consumed in the accumulation of such fortunes. [See Appendix, Note VIII.]

The general conditions of oil-accumulation may be thus epitomized:

1. A formation containing the material for the production of oil by slow spontaneous distillation.

2. A porous formation or reservoir above the mother rock, or within it, in which the oil may be accumulated.

3. An overlying impervious formation, which shall prevent the escape of the product to the surface of the earth.

4. A dome-shaped conformation of the impervious roof, which shall prevent the lateral escape of the oil, or its dissemination through spaces too extensive.

The failure of either one of these requisites will convert all the other indications into illusory and seductive temp