not less than four sandstones roofed with shales that are found at times petroliferous.

From facts like these, it is apparent that the composition and order of arrangement of a series of strata have a vitally important relation to the accumulation of oil and gas that may take place within it. Some geologists count the composition of the series the main element in oilproduction. They regard especially the grain and thickness of the oilsand or reservoir, accounting largely for the difference in production of different fields, or of different parts of the same field by the character of the oil-sand. As already stated, the practical driller also makes great account of these facts.

Mr. John F. Carll, of the Second Pennsylvania Survey, has discussed these questions at length in his invaluable reports on the petroleum fields of western Pennsylvania. He claims that an oil-bearing pebble rock may contain, under favorable conditions, one-tenth, or even oneeighth of its bulk in oil, basing his claim upon the indications of experiments made upon the rock.-(Second Penna. Survey, I, 3, 251). He also shows that the pores of the sandstone would serve as channels for the largest supplies of oil that have yet been found, and that we are under no necessity of resorting to hypothetical "crevices" to account for any of the facts pertaining to the yield of oil-wells. Mr. Carll lays special stress upon the character of the oil-sand, and in this respect Professor Lesley and Mr. Ashburner seem to agree with him, while Professor I. C. White, also of the Second Pennsylvania Survey, urges the paramount importance of another element, especially in gas accumulation. This element will be next considered.

II. EFFECTS OF DISTURBANCES OF STRATA UPON THE ACCUMULATION OF OIL AND GAS.

This subject is one which has been discussed ever since the discovery of petroleum and gas in this country on the large scale. Definite. theories as to the influence of sach disturbances as have occurred in the oil-producing territory were early propounded, some of which have been maintained to the present day. Prominent among them is the anticlinal theory, so-called, which takes account of the low arches or folds that have traversed some portions of the oil-producing districts, and especially of West Virginia and southern Ohio.

a. Earlier Statements of the Anticlinal Theory.

The "oil-break" of West Virginia, in the neighborhood of Burning Springs, furnished, in the early days of the search for petroleum, an

example of the effect of structural disturbance on oil production, that he who runs might read. There is an uplift there at once considerable and conspicuous, viz., the White Oak anticlinal, and the productive oilwells, out of the great number of wells which were drilled in this region, were found to be strictly confined to the region of the anticlinal or axis. These facts were brought out in a very clear manner by the late Professor E. B. Andrews, in a paper published in the American Journal of Science (2, XLIII, 33). The discovery of the axis in its relation to oil-production seems to have been made by General A. J. Warner in connection with Professor Andrews, in 1865. Beyond the structural disturbance shown in the anticlinal, Professor Andrews also claimed the existence of crevices or fissures on the large scale in the rocks from which the oil was derived. To effect the separation of water, oil and gas in the supposed crevices, he invoked the force of gravitation, showing that these substances would necessarily be arranged in the order of their densities in any space which they should occupy in common.

The clue that was thus given as to the location of successful wells was, of course, promptly followed. The anticlinal was traced throughout its entire extent, and test-wells were put down at numerous points, but of these a large percentage failed. To account, if possible, for these failures, Mr. F. W. Minshall, of Parkersburg, West Virginia, undertook, at a later date, a careful determination of the levels of the axis. He found that, instead of either keeping a horizontal plane or of dripping regularly and uniformly, it advanced by a series of pronounced undulations, having domes or summits at some points, and sinks or sags at others. All of the productive oil and gas wells had been located on the domes, and the failures were to be found in the depressions. (10th Census Reports, X.)

Dr. T. S. Hunt, at a still earlier date, viz., in 1863, maintained that the petroleum supply of western Ontario was all derived from the line of a low and broad anticlinal, which runs through the district in a nearly east and west direction. He distinctly taught that the anticlinal structure is a necessary condition for a large production of petroleum, referring its accumulation in such portions of the series, of course, to hydrostatic laws. (Amer. Journ. of Science, March, 1863.)

Dr. Newberry seems also to accept the anticlinal theory, though his statements on this point are less explicit than those already quoted. In speaking of the Canada oil-field, he says:

"This district is in the line of the Cincinnati arch, which here, as in the islands of Lake Erie, shows evidence of disturbance long subsequent to its original upheaval."

In speaking of the Pennsylvania oil-fields, he says:

"These strata have all felt the disturbing influence of the forces which raised the Allegheny mountains. Here, then, we have a peculiar geological substructure such as is especially favorable to the production and accumulation of petroleum, and such as must be, more or less, perfectly paralleled elsewhere to make productive or, at least, flowing wells possible. This structure consists in a great mass of carbonaceous strata below, more or less disturbed and loosened, from which the oil is supplied in a constant and relatively copious flow; above this, strata of porous, jointed sandstone, serving as reservoirs, where the constant product of oil and gas may accumulate for ages; still higher, argillaceous strata, impervious in their texture and not capable of being opened by fissures, forming a tight cover which prevents their escape." (Geol. of Ohio, I, 159.)

Elsewhere, he says:

"The facts I have observed lead me to conclude that the disturbed condition of the strata in certain districts east of Ohio, is the cause of the phenomena which they present. Where the oil and gas-producing rocks, and those overlying them are solid and compact, .. the escape of the resulting hydrocarbons is almost impossible. Where they are more or less shaken up, reservoirs are opened to receive the oil and gas, and fissures are produced which serve for their escape to the surface. Near the Alleghanies, all the rocky strata are more or less disturbed, and here, along certain lines, the liquid and gaseous hydrocarbons are evolved in enormous quantities. As we come westward, we find the rocks more undisturbed and the escape of oil and gas, through natural or artificial orifices, gradually diminished." (Ibid, 183.)

This reasoning, as will be seen, is in harmony with the anticlinal theory. From the statements already quoted, it is shown that distinct theories and claims have been advanced during the last twenty years, connecting the accumulation of oil and gas with anticlinal structure.

b. The Anticlinal Theory as Specially Applied to Gas Wells.

Within the last two years, since natural gas has attained such prominence in Pittsburgh, its sources and the conditions of its occurrence have been studied anew with sharpened inspection, by both geologists and practical men, and some real advance seems to have been made in our search for it. The anticlinal theory has been revived and extended, and has been used successfully in the location of many productive wells. For the new statement, we are indebted to Professor I. C. White, of the University of West Virginia, and recently of the Pennsylvania Geological Survey. Professor White, in turn, gives credit to Mr. W. A. Earseman, an oil operator of many years' experience, who had noticed in 1882-3, "that the principal gas wells then known in western Pennsylvania were situated close to where anticlinal axes were drawn on the geological maps. From this he inferred there must be some connection between the gas-wells and the anticlinals."

Professor White goes on to say:

“After visiting all the great gas-wells that had been struck in western Pennsylvania and West Virginia, and carefully examining the geological surroundings of each, I found that every one of them was situated either directly on or near the crown of an anticlinal axis, while wells that had been bored in the synclines on either side, furnished little or no gas, but in many cases large quantities of salt water. Further observation showed that the gas-wells were confined to a narrow belt, only one-fourth to one mile wide, along the crests of the anticlinal folds. These facts seem to connect gas territory unmistakably with the disturbance in the rocks caused by their upheaval into arches, but the crucial test was yet to be made in the actual location of good gas territory on this theory. During the last two years, I have submitted it to all manner of tests, both in locating and condemning gas-territory, and the general result has been to confirm the anticlinal theory beyond a reasonable doubt.

"But while we can state with confidence that all great gas-wells are found on the anticlinal axes, the converse of this is not true, viz., that great gas-wells may be found on all anticlinals. In a theory of this kind, the limitations become quite as important as, or even more so, than the theory itself; and hence, I have given considerable thought to this side of the question, having formulated them with three or four general rules, which include practically all the limitations known to me up to the present time, that should be placed on the statement that large gas-wells may be obtained on anticlinal folds, viz.:

"(a) The arch in the rocks must be one of considerable magnitude. (b) A coarse or porous sandstone of considerable thickness, or, if a fine-grained rock, one that would have extensive fissures and thus, in either case, rendered capable of acting as a reservoir for the gas, must underlie the surface at a depth of several hundred feet (500 to 2,500 feet). I'robably very few or none of the grand arches along mountain ranges will be found holding gas in large quantity, since in such cases the disturbance of the stratification has been so profound that all the natural gas generated in the past would long ago have escaped into the air through fissures that traverse all the beds. Another limitation might possibly be added, which would confine the area where great gas flows may be obtained to those underlaid by a considerable thickness of bituminous shale.

"Very fair gas-wells may also be obtained for a considerable distance down the slope from the crest of the anticlinals, provided the dip be sufficiently rapid, and especially if it be irregular or interrupted with slight crumples. And even in regions where there are no marked anticlinals, if the dip be somewhat rapid and irregular, rather large gas-wells may occasionally be found, if all other conditions are favorable." (Science, June 26, 1885.)

To some of these statements, Mr. C. A. Ashburner, Geologist, in charge of the Second Pennsylvania Geological Survey, took exceptions, in a letter published in Science, July, 1885. He says:

"Professor White's theory that 'all great gas-wells are found on the anticlinal axes,' cannot be accepted, until he shall limit, by definition, all great gas-wells to exclude all gas-wells, both large and small, comparatively, which produce gas from strata not found either on anticlinal axes or in close proximity to such structural lines. The Kane gas-wells, the Ridgway well, the 'Old Mullin Snorter,' and several Bolivar wells are notable instances among many which might be mentioned, where large gas-wells have been drilled in or near the center of synclines.

"Although it is a fact that many of our largest Pennsylvania gas-wells are located near anticlinal axes, yet the position in which gas may be found, and the amount to be obtained, depend upon (a) the porosity and homogeneousness of the sandstone which serves as a reservoir to hold the gas, (b) the extent to which the strata above

or below the gas-sand are cracked; (c) the dip of the gas-sand and the position of the anticlines and synclines; (d) the relative positions of water, oil and gas, contained in the sand; and (e) the pressure under which gas exists before being tapped by wells."

To these criticisms Professor White makes reply, by urging that subordinate anticlinals often run along the central line of synclines, and that when gas is found in synclines it is at these points, and that when found here, it is seldom free from salt-water by which it is likely to be overpowered. He further urges that all the successful gas companies of western Pennsylvania and West Virginia are getting their gas from the crests of anticlinal axes, while those that have confined their operations to synclines, have met with uniform financial disaster. He points to the brilliant lights along the summits of the eight axes nearest to Pittsburgh, and he has since added a ninth, and also to the darkness that envelops the intervening synclines, in which hundreds of thousands of dollars have been invested without developing a single profitable gaswell.

To the qualifications already made, Professor White would probably add, at this time, one to the effect that gas-wells shall be located on the domes of the axis, rather than its depressions, recognizing the same line of facts in regard to them that Minshall had already established in the case of the White Oak anticlinal of Ohio and West Virginia, to which reference has previously been made.

The facts cited by Professor White as to the gas-supply of Pittsburgh, seem conclusive. Every foot of it comes from anticlinals, but not from them because it has been sought nowhere else, but because, if found in other stations it is speedily overcome and extinguished by salt-water. Where anticlinals of the type here referred to traverse an oil-bearing series, it may be considered demonstrated that they exert a decided effect on the accumulations of oil and gas in this series. So rational is such a conclusion, so directly does it result from the facts already stated, that it is hard to see on what grounds it can be called in question.

While there is no element of the theory, as stated by Professor White, that differs from the theory as heretofore stated, his applications of it are bold, original, and, best of all, successful, and they mark a new period in our study of the geology of oil and gas.

But as has been already shown, anticlinals are of infrequent occurrence in Ohio. A few of the low arches of western Pennsylvania extend across the border, but they soon flatten out and disappear. Even if anticlinals are held to account for the facts of oil and gas accumulation, the theory would have but limited application to our geology.