wells near together interfere in the way of drawing the one from the other. "It is," says Prof. Evans, one chance out of many to strike oil at all, even in the neighborhoods where it exists in abundance. The drill, as it enters the cavity, sinks variously from four or five inches to as many feet, sometimes sticking fast, as if between the oblique sides of a narrow fissure. But there are facts connected with the history of oil wells, particularly in their intermittent action, and their interference with one another, which serve to show the existence, in many cases, of systems of cavities connected together by channels of communication more or less free, running sometimes along the strata, and sometimes across them. The productiveness of a well depends on its entering either one of the main reservoirs, or some one of its important connections."

Speaking generally, the contents of oil-containing fissures or other cavities, and whether single or communicating, will be, water at the bottom, oil floating upon the water, and gas filling the space above the oil, the three standing thus in the order of their specific gravities. By natural outlets to the surface, from one or other of these parts, there may result either a gas or an oil spring, or one of mixed water and oil. In sinking wells, a more or less vertical fissure may be struck at any point, the well entering the portion filled either with gas, oil, or water. In the first case, gas is at first discharged, and, if it has been subjected to great pressure, with proportionate violence; while, if water from communicating crevices below then rapidly enters the fissure, the oil may next be forced into the lower end of the tubing and be pumped out; and later, oil and water mixed will be obtained. If the well enters the part of the fissure holding oil, this, by the pressure of the gas above it, may be raised toward the mouth of the well, or projected into the air, and sometimes to a height of many feet; but this action of the gas after a time becomes exhausted; and the flow is then, of course, no longer maintained. After the flow of oil from such a well has ceased, and the pump is resorted to, it is likely sooner or later to bring up a mixture of oil and water. If the well directly enters the water, unless the gas has sufficient tension to force out the water, and then oil, the latter can only be obtained by pumping out so much water as, if possible, to lower the water-surface to the point at which the boring enters the fissure, when, usually, oil and water mixed will be obtained. It is probable the wells may be intermittent from several causes, connected with the gradual accumulation of gas, oil, or even water, in the fissures pierced; but with pumping wells the most common cause must be supposed to be the time required for oil to collect from surrounding porous rock, or small crevices.

Any pressure, as of gas, air, or a head of water, will raise oil to a height as much greater than that to which water would be raised, as

the specific gravity of the former is less; i. e in round numbers to a height comparing with that of a water column under like circumstances nearly as 5 to 4. While the oil wells thus far vary in depth from 50 to near 800 feet, it is stated that the deepest are as likely to raise water to the surface as the shallowest. By the occurrence of rains, the production of some wells is increased, while that of others, with less gas, is impaired. The very large quantities of oil in some cases delivered, the fact that borings near together may reach oil at very different depths, and other facts, go to show that the somewhat popular idea of an "oil rock" to be reached is a mistaken one; and, although the supply of oil is sometimes, and in some districts to a greater extent than in others, contained in pores, or very small cavities in the rock, it is more commonly doubtless found in cavities of larger size, single or communicating.

Not only is the supply of oil, geologically speaking, higher in West Virginia and southeastern Ohio than in the northwestern Pennsylvania region; but thus far, at least, most of the wells in the former region go to much less depth than in the latter. The wells in Ritchie County are said to be usually from 150 to 200 feet deep; those of Bull Creek, 75 to 250 feet; and particular ones on and near Horse Neck Run, from 160 to 380 feet in depth. It has been supposed that a gain in production might be secured by sinking wells in parts of West Virginia to from 350 to 500 feet. In southeastern Ohio the wells are usually no more than from 150 to 300 feet deep: but the fact that two wells lately sunk, on Duck Creek and Cow Run, to depths of more than 700 feet, have each struck a good quality of oil, has led to the opinion that in this region deep as well as surface supplies of oil exist.

The transition from the region last considered to that of northwestern Pennsylvania and northeastern Ohio is, in a geological point of view, a transition only from the carboniferous strata to some of the higher members of the devonian, which lie next below. The Catskill group of sandstones may be said to be confined to more eastern parts of the State of New York; and in eastern Ohio and northwestern Pennsylvania, both these and the strata known as carboniferous limestone are wanting; so that, in the parts of the country last named, the conglomerate of the coal measures comes very nearly, if not in fact directly, over the Chemung and Portage groups of sandstones. Some beds of coal from the great Pennsylvania coal-fields come out on the hill-tops some miles to the southward of parts of the Alleghany River along which there are now productive oil wells, and some of more limited extent are found on hills even in the oil region. In the higher country of this region, in fact, the surface rocks are still mostly carboniferous; and on the hills near to some of the oil-producing valleys, the conglomerate floor of the coal meas

ures lies in some cases 100 feet thick. The rocks of the valleys here, however, are devonian, and are those of the Chemung and Portage groups. The beds of these rocks lying along and beneath the course of Oil Creek dip at a small angle to southward, but so that they pass 500 to 600 feet below the coal measures. In fact, the outcrop of strata from under the great Pennsylvania coal-basin spreads out toward the west and north somewhat in fan shape; and the eastern or inner line of the outcrop of the Chemung and Portage sandstones (known in the Ohio reports as the Waverley sandstones) is traced in a general way from Portsmouth, on the Ohio, by Cuyahoga Falls, whence, taking a more easterly course, it passes through the southern part of Crawford County, Pa. Thus, northeastern Ohio and northwestern Pennsylvania, north of this line, and a portion of southwestern and southern New York, in some parts as far north as the southern border of Genesee County, have altogether or mainly the Chemung and Portage sandstones as their surface rocks.

The strata now in question have been described as 66 compact, close-grained white and gray flaggy sandstones, alternating with red and olive-colored argillaceous shales." Drillers have for convenience numbered the beds of sandstones 1, 2, 3, &c., beginning in the bottom-land on Oil Creek, below Titusville. Prof. Ridgway estimates the total mass of oil-bearing strata as 1,200 feet in thickness, extending from the overlying conglomerate down to the Genesee slates. It has already been seen that the outcrop of these rocks to northward extends over all the southern counties of western New York, and to the northwest beyond Trumbull County, Ohio. The Mahoning Valley region, lying to the southward of the limits already named, is in fact within the outcrop of the coal measures; though here the oil is found also in the underlying Chemung and Portage sandstones. The lowest oil-bearing sand-rock comes to the surface in the town of Waterford, Erie County, Pa. Unlike those of the Ohio River region, these rocks have been but little disturbed from their nearly horizontal position; though, according to Prof. Ridgway, some miles north of Titusville the inclination becomes as great as 10° to the south, while under that town occurs a synclinal trough, farther on near the Stackpole farm, 2 miles down, an anticlinal roll, and from this to the mouth of Oil Creek a series of small undulations and crimps in the strata.

In the borings along Oil Creek, and generally where the surface strata are similar, the "first sand-rock," a whitish sandstone, is met with at a depth of from 70 to 200 feet; and it is stated that the wells penetrating only this bed yield generally a thick oil. From 100 to 200 feet below this, after passing through soap-stone and perhaps slate, lies the "second sand-rock," of similar aspect to the first; and in or beneath this oil is again obtained, also heavy, though

less so than the former. From this rock proceed nearly all the wells of the Alleghany River; while, with few exceptions, the wells of French Creek descend only to the first rock. From 100 to 200 feet again below the second, there is a "third sand-rock;" and wells penetrating this more commonly yield a light and purer oil. To this rock descend most of the flowing wells; and in fact, most of the wells more recently sunk along Oil Creek. Though the wells have thus far been sunk chiefly in the valleys, there appears no reason why oil should not also be reached through the higher land, by boring proportionally deeper. So far as the oil supply of wells is that which slowly collects from porous rocks, or those filled with fine cavities, it should certainly be found under the hills as well as the valleys, and so far as it exists in large fissures or chasms, these too must probably to some extent underlie the hills. To the foregoing statements, and which substantially agree with those of a notice in the "American Journal of Science" for Jan., 1865, it should perhaps be added, that to the extent to which fissures of irregular position and height, and penetrating one or several successive layers of rock, serve as the reservoirs of petroleum, it cannot of course be intended to assert that entire invariableness of the quality of the oil at different levels, and of the depths at which it is found, which the statements as made would appear to imply. Further, so far as the wells of the northwestern Pennsylvania region receive their supply from such fissures, remarks similar to those already made respect ing the wells of West Virginia will apply to them.

In whichever sand-rock the borings terminate, the southern dip of the strata requires that the wells shall go deeper in the same direction. The depth of the early wells of Mecca, Ohio, was from 30 to 200 feet; while along Oil Creek, from near Shaeffer's to its mouth, wells reaching the third sand-rock are of depths increasing from about 500 feet to 700 feet or more; and the depth of wells on the Alleghany below Franklin are also quite various, ranging from 360 to 750 feet.

As, proceeding northward, the Chemung and Portage groups of sandstones run out, the Hamilton group, next below, comes to or near the surface. The black inflammable shales of the devonian series in western Canada, and which over some parts of it appear highest, are considered as belonging to the base of the former, while the argillaceous shales and limestones underlying belong to the Hamilton group. The low broad anticlinal axis running nearly east and west through the peninsuls north of Lake Erie, brings these shales and limestones in the township of Enniskillen nearly to the surface. At the base of the group occur the Marcellus slates or shales, which, as well as those at its summit, contain petroleum inclosed in very small cavities. Immediately below the group in question is the cor

niferous limestone, also specially named "devonian limestone." The wells of the Enniskillen region, which reach oil at the depth of 120 feet and more, would appear to pass below the Hamilton group toward or into the devonian limestone, in which latter, in such case, the supply of oil in this region must be considered to be found. The devonian or corniferous limestones, in fact, at points where they come to the surface, both in Canada and New York, are found to be very generally impregnated with petroleum, or to contain it in small cavities (as is the case at "Black Rock," on the Niagara River), and sometimes also, it appears, to be charged with bituminous matter in the solid state. To reach these limestones, or the equivalent slates or shales, south of Lake Erie and in northwestern Pennsylvania, supposing such strata to be there continued, and to be as at many other points charged with petroleum, would of course require wells of far greater depth than any yet attempted in the lastnamed region, though probably not greater than that of some artesian wells already in existence. It is questionable, however, whether the corniferous limestone is continued beneath the regions mentioned; and if it be so, or not, whether in that part of its distribution also petroleum would still be found. The producing wells of the Bothwell (Canada) district are of a depth of 270 feet and upward; and some that were in process of being sunk in 1864 had already reached depths of from 350 to 500 feet.

In the cliff limestone (devonian) of Ohio, as well as in the corniferous limestone at Black Rock, petroleum occurs in cavities, generally in the cells of fossil corals. The Niagara limestone (silurian) in Monroe County, New York, contains bitumen. Still farther down in the series, almost all the lower silurian rocks are found, where exposed, to contain petroleum. The oil is either infiltrated in small quantity through the rock, or fills small cavities in it, as is seen in parts of Canada; or it makes its appearance at certain points in springs, as in case of the long known petroleum spring of the town of Guilderland, near Albany, and that on Great Manitoulin Island.

In respect to the origin of petroleum, space will here allow of nothing further than the statement that, undoubtedly, like all the varieties of coal, the oil has in all cases had its source in the bituminous transformation of collections of preexisting organic matters-these, in case of the oil, having usually been of vegetable, but sometimes, perhaps, of animal character; while the direct production of the oil has probably taken place sometimes from the resulting bitumen as appearing in coal-beds, and sometimes from the allied bitumen infiltrated through or collected in certain rock strata.

Indications of Petroleum.-As the most important of the natural indications or "signs," in any section, of the existence of rock oil beneath the surface-should be named the following: gas-springs; oil-springs, the oil either flowing

pure or mixed with water; appearance of oil on the surface of the ground, of rocks, or of pools, creeks, etc.; surface collections of bitumen; the so-called "show of oil," from strata through which wells are being sunk, or on the water raised from them; salt-water wells or springs, in some cases perhaps, and especially where oil or gas also appears; and in a geological point of view, the fact of lines of disturbance in strata not far below the surface, affording increased probability of numerous cavities within the rocks, as well as also, in certain instances, the nature of the strata that come next below the surface-as, whether they are such as have elsewhere yielded oil, or such as are in immediate proximity to bituminous strata, which appear at some other points to have been the source of oil in large quantity. Among the indications named, the least fallacious of all, it would appear, should be those of free-yielding oil-springs and the surface bitumen which they sometimes deposit, as the former was shown in the early history of the western New York and northwestern Pennsylvania regions, and as both have appeared in those of Canada and California.

One among the surest signs of subterranean collections of oil, nevertheless, is afforded by gas-springs or jets, the gas coming up from an opening in the surface, or through the water of a spring, or escaping from fissures in rocks. Springs of uninflammable gas, however, as carbonic acid-and Mr. Richardson states that such are numerous in the boggy bottom-lands which he examined in West Virginia-could so seldom be supposed to have their origin in connection with supplies of oil, if even they ever did so, that they would be of no real value as indications. An inflammable gas will doubtless invariably consist of or contain marsh-gas; and this, from its intimate relation in composition and in origin with petroleum (as previously shown), is the valuable criterion. Prof. Andrews states that, in our bituminous coalmeasures, a gas-spring without doubt indicates petroleum, and he adds, "in the rocks below; but Mr. Richardson argues that gas may sometimes travel to a considerable distance laterally, along horizontal or irregular fissures. When it does so, the gas-spring can show no more than that oil is somewhere in the vicinity. Hence, instead of sinking a well directly at or near the vent, the explorer should endeavor first to ascertain the nature, position and peculiarities of the subjacent and surrounding rocks, so as to determine the probable place of the oil fissures or basins. The greater buoyancy of the gas, however, and its elasticity and consequent upward pressure, while the oil can only flow at its level, render the former the less likely of the two to wander very far laterally before finding an exit to the surface. But again, Prof. Hunt, speaking of the Enniskillen wells, says: "The inflammable gas which issues from the wells is not necessarily connected with the petroleum, inasmuch as it is an almost constant

product of the decomposition of organic matters, and is copiously evolved from rocks which are destitute of bitumen. It is similar to the gas of marshes and to the fire-damp of coal mines." Accepting this view, even an inflammable gas would not show beyond question the existence of petroleum; and especially in marshy places such a gas may arise only from the bottom of the pools in which it appears. Still further, a well may penetrate a fissure and release from it a large volume of gas, when the oil is contained in a lower one to one side, and is not reached by the same well. But while it is thus always necessary to examine into the character and source of gassprings, in order to determine their value as a sign of oil, it is well known that in some of the great oil regions of both hemispheres where inflammable gas-springs are most abundant, there petroleum is likewise so. To the examples of the Alleghany region, and those of West Virginia and Canada, in this country, it seems that we should add that of western New York. As to the last-named, however, since the surface oil was there long since mainly exhausted, while some of the gas-vents have been kept supplied to a recent date or to the present, the inference would seem warranted that there are supplies of oil in strata deeper than those yet penetrated in or near that region—perhaps entirely beneath the Chemung and Portage sandstones. Finally, to quote again from Prof. Andrews: "The great majority of these gassprings are unknown, since they are seldom discovered, except when they appear in streams; and," he continues, "probably the same may be true of oil-springs, since the soil would absorb the oil, and in only a few cases would it be detected."

Little further need be said in regard to oilsprings, with or without water, and the surface bitumen which shows the former or present place of such springs. These are, indeed, unquestionable signs of the past or present existence, as the case may be, of accumulations of oil somewhere beneath the surface, and usually not very far removed from the place of outflow. The important questions in these cases are, the direction, and the probable depth, from which the supply may come. A little oil on the surface of a spring may prove a deceptive sign; for the water feeding the spring may have borne the oil to a great distance from its hidden reservoir; or again, such oil may be the last drainage of long standing oil-cavities. But continued gas-jets, on the other hand, if connected with oil supplies, quite certainly indicate that the quantity of oil is large at the time, though it has been suggested that they may show only a greater depth of the oil.

Surface oil, exuding upon rocks, or in the soil, or on pools or streams, is a sign quite generally regarded as trustworthy, and one that is much sought. It was one of the marked signs in all the great oil regions of this continent that have, so far, been developed; as in the

Oil Creek and surrounding districts, in Enniskillen, on Hughes River, and elsewhere; and it is said to be now abundantly met with, at least at some seasons, on the waters of parts of the Great Kanawha and Elk Rivers, and of some of their tributaries. As to the exact promise, however, of particular spots or locations, this sign may prove wholly fallacious. Thus, the oil on pools or streams, where it may appear either as an oily scum or as a thin film of oil, and often reflecting the prismatic colors, may be the slow washings from alluvium or cavities in which little of it finally remains. The Rev. Mr. Eaton states that some of the most unsuccessful operations in the whole Oil Creek valley have been in the midst of the ancient pits there found, though undoubtedly these were at some past time dug with profit for the oil that then filled the very surface earth where they were made. Again, petroleum on the surface or in crevices of rocks may, as previously seen, indicate no more than its presence in small cavities within the rock, or even simply that the rock is itself bituminous. An instance of the former kind is the oozing of petroleum from broken surfaces of the darkcolored limestone at Black Rock; and examples of the latter kind, it appears, are found in its appearance upon sandstones quarried in Erie County, Pa., and limestones in Monroe County, N. Y., and in Canada.

The "show of oil" sometimes obtained in the course of wells going down, is not so much as formerly considered decisive as to a successful result. A good show of oil near the surface or still lower may disappoint, since the oil so found may be from small fissures or veins that are not with any certainty in the course to, or connected with, the large collections of it; and on the other hand, productive wells have been obtained with no intermediate "show." The value of this show as a sign, however, according to Prof. Evans, increases with the depth at which it occurs.

Salt water, in springs or wells, taken alone, has little value as a sign of petroleum; since the salt may be from seams or deposits of that msterial, and, as in springs and wells, generally, it may emerge or be struck at some distance from its source. One of the earliest facts in the course of discovery of rock oil in this country was its very frequent appearance on the water of wells bored for salt, and often to so great an extent as to be regarded as a nuisance, or even to lead to the abandonment of such borings. It was thus found on the water of salt wells, on the Great Kanawha, and on numerous tributaries of the Alleghany and Ohio Rivers within and near the limits of the present oil regions. And in any case, salt water becomes a more important sign when it occurs in the vicinity of gas-springs or surface oil.

Finally, it appears that no infallible criteria or rules as to whether petroleum is or is not present in remunerative quantities in the strata beneath any district, can yet be laid down.

Evidently, the prospecter will be aided by a knowledge of the geology of, and the history of explorations in, the various oil regions, as well as by experience. Almost any of the signs already named may be considered as affording, where met with, a presumption, more or less strong, of the existence of oil deposits; and the supposition thence arising requires then to be tested by examination with reference to the other indications.

The Sinking of Oil Wells.-The processes involved in the sinking of wells for oil, and which, with the exception usually of some feet depth of superficial alluvium, must penetrate mainly or altogether in the way of drilling through rock, are substantially those resorted to in the case of artesian wells generally. The place for a well having been fixed upon, a derrick of some thirty to sixty feet height-its frame being chiefly four strong upright posts, standing some ten feet apart below and about five feet above-is erected over the spot. The distance from the surface to the rock is at the present time usually traversed by driving down into the earth a strong cast-iron pipe, of an internal diameter of about five to seven inches. This is forced down in the manner of pile-driving. The power for this purpose, and for the subsequent operations, including those of drilling and pumping, is now commonly that furnished by a steam-engine of some eight to twelve horse-power. The cast-iron pipe is in sections of a few feet length, which are successively jointed on above as required, till the rock is reached. Drilling tools may be used to loosen up the earth within the pipe; and the water entering it from above converts this mass, as it penetrates it, into a paste or mud: a "sandpump"-a three to five-inch copper tube, six feet long, with a simple valve in the lower end, opening upward-is then let down into the muddy water, agitated until it fills, and raised; and these operations are repeated until the whole depth is cleared.

Of the complete drilling apparatus, consisting essentially of the rope socket, of two heavy intermediate links, or "jars," the purpose of which is to jar the tools loose when they might otherwise stick fast in the rock, and of a "sinker bar" and "auger stem," to the lower end of which last is finally attached the "centre bit," for cutting, or the slightly larger "reamer," which is blunt and used for rounding and finishing the opening made by the former, the total weight is commonly not less than from seven hundred to nine hundred pounds. By the socket above the apparatus is attached to the rope, which again is now commonly made to receive its motion from that of one end of a working-beam suitably erected near the spot, having a stroke of thirty to thirty-six inches, and actuated by the steam engine. The centrebit has an edge of about three to three and a half inches, and the reamer somewhat larger, brings the boring to near the calibre of the pipe above; the drilling is with both performed sim

ply by the force with which the tools are let fall. As often as the boring has been carried down a few inches, so that the débris would clog the tools, these are withdrawn, and a somewhat smaller sand-pump is used to clear out the comminuted rock by aid of the water which is always present in the boring. It is said that in gravelly or shaly formations a progress of from three to ten feet a day is made; in rock, of from one to three feet. Judicious operators commonly keep a record of the strata passed through, their thickness, the place of veins of water or oil, etc. Among the difficulties in the way of sinking the iron pipe, are the meeting of boulders, which then usually have to be drilled through, the fracturing of the pipe, etc.; and in the way of drilling, the breaking of the bits, or rarely, their sticking so that they cannot be removed, etc.; and from causes of this sort the borings, even when far advanced, must sometimes be abandoned altogether. And again, wells which have consumed months of labor, may in the end prove unsuccessful.

In ordinary wells, as soon as the boring has struck oil, and in flowing wells, so soon as the flow so far remits as to allow of the process, the tubing of the well is to be performed. Sections of a copper or iron tube, two to three inches in diameter, are screwed together to the required length, a pump-barrel containing two valves always forming the lowermost joint; and the tube so formed is lowered within the larger pipe and boring, so as to enter the liquid in the subterranean cavity. A long jointed pumprod is inserted within the tube, having at its lower end a metallic valve-box, with its valve opening upward. The pumping of the oil or water from the well is effected, subject to the control of the operator, by connecting the rod with the working-beam or other device for imparting motion to it. In order to arrest the downward flow around the tubing, and into the oil cavity, of water from veins pierced in boring the well, a sort of packing is made use of, in form of what is known as a "seed-bag." This, a leathern tube some eighteen inches long, partly filled with flaxseed, and closed at the ends, is, before sinking the tubing, bound about it near the lower end: once in place, the water soon swells the flaxseed so as to close the space about the tube, and thus the further access of water from above is prevented. The upper end of the coiled seedbag being more slightly tied, this readily ruptures whenever an effort is made, from any cause requiring it, to raise the tubing. When two or more wells belonging to different owners enter the same oil cavity, or are in any way connected below, the drawing of the tubing of one of the wells, allowing as it does the water to flow freely down into the oil reservoirs, floods the connected wells also, usually rendering them quite worthless for the time. In this way the proprietors of one well have, in some instances, compelled those of others near, and more profitably yielding, to come to their terms,