horizon of and surrounded by the dolomite, are best explained on this supposition. In the change which has been undergone, the fossils which the original limestones contained appear to have been for the most part discharged or rendered obscure, as is usual in this metamorphosis. The crystalline character of the dolomite is often very marked, and there results from it a peculiarly open or porous structure. Its storage capacity is much greater than that of ordinary oil sandstones and conglomerates, so far at least as pores visible to the unaided eye are concerned. The change usually extends for ten to thirty feet below the surface of the formation. In some cases, however, sheets of porous dolomite are found as low as fifty feet and very rarely as low as 100 feet below the surface.

The area occupied by this dolomitic phase of the Trenton limestone in Ohio has already been indicated. The eastern and the southern boundaries pass through Lucas, Wood, Hancock, Allen, Auglaize and Mercer counties. It is possible that the line crosses some parts of Ottawa, Wyandot and Hardin counties.

There is good reason to believe that this phase extends far to the northward and westward, outside of the state limits to which it has here been traced. We know that the Trenton limestone is a dolcmite when it pitches rapidly down from the northern boundary of Ohio to make the low-lying floor of the Michigan coal basin, and we also know that it is a dolomite when it rises from under that coal basin as a surface rock of the northern peninsula. In like manner, it is a dolomite when it leaves the western boundary of the state under deep cover, and it is a dolomite when it reaches the surface once more in the Galena district of Illinois and Wisconsin.

South of the line laid down in Ohio, there has not thus far been found a trace of the porous dolomite on which the oil of Lima and the gas of Findlay depend. The change is seen to be taking place in Shelby and Logan counties, but beyond them the Trenton limestone is invariably found with a percentage of more than 75 per cent. of carbonate of lime, and rarely with less than 10 per cent. of silicious impurities. It is this last element, with but little doubt, that has resisted the dolomitization of the stratum throughout the southwestern quarter of the state and in all contiguous territory.

To the eastward of the line laid down in northern Ohio, a less definite boundary is to be looked for. It is quite probable that small areas of porous dolomite will be found beyond the line here recognized as the termination of the Findlay phase of the Trenton limestone. More chemical work is needed in this direction, but thus far there are no exceptions known to the statements made above.

Within the limits named, the limestone of course has a considerable variety of grain and texture, but all of the analyses obtained show the stratum to be in the main a dolomite. As already stated, there are occasional patches or islands of true limestone in this sea of dolomite.

2. A porous rock, buried 1,000 to 2,000 feet below the surface of northwestern Ohio, will not be found empty. Nature abhors a vacuum. With what will its pores be filled? Mainly with salt-water of peculiar composition, possibly representing the brine of the ancient seas in which the limestone was laid down. Ninety-nine hundredths, or perhaps nine hundred and ninety-nine-thousandths of the limestone will be thus occupied. The remaining hundredth or thousandth will be filled with the petroleum and gas which have, in the long course of the ages that have passed, been gathered from a wide and general distribution through the water into certain favored portions of the great limestone sheet.

3. This salt-water will be held under artesian pressure. The porous limestone containing it rises to day in Michigan and Illinois, communicating there with surface waters. The pressure of this head of water will be felt through every portion of the porous rock, and when the stratum is pierced by the drill in the areas that are thus occupied, the salt-water will rise with more or less promptness, depending on the varying degrees of porosity in the rock. The height to which the water will rise will seem to vary in wells, by reason of the different elevations of the locations at which they are drilled, but with reference to sealevel, the water columns will be found to closely agree.

The same artesian pressure accounts for the force with which oil and gas escape when their limited reservoirs in the porous rock are tapped by the drill.

4. The accumulation of oil and gas in the porous rock depends altogether upon the attraction of gravitation. The lighter portions of the contents of the porous rock, viz., oil and gas, are forced by gravitation into the highest levels that are open to them. Everything turns on the relief of the Trenton limestone. The gas and oil are gathered in the arches of the limestone, if such there are. In default of arches, the high-lying terraces are made to serve the same purpose, but the one indispensable element and condition of all accumulation is relief. A uniform and monotonous descent of the strata is fatal to accumulation of oil and gas where everything else is favorable. The sharper the boundaries of the relief, the more efficient does it become. Absolute elevation is not essential; all that is required is a change of level in the porous rock. Each division of the field has its own dead line or salt-water line. Salt water reigns universal in the Findlay field 500 feet below sea-level, except where some minor local wrinkle may give a small and short-lived accumulation of oil or gas. In the Lima field the salt-water line has risen to 400 feet below tide; in the St. Mary's field to 300 feet below tide, and in the Indiana field to 100 feet below tide. These figures stand in every case for the lower limit of production, with the possible minor exceptions already noted. The rockpressure of the gas decreases to the westward in proportion to this decreasing head of water-pressure.

The large accumulations are derived from the large terraces. The Findlay terrace, for example, consists of a very flat-lying tract, ten or twelve miles across in an east and west line, from which the connected areas of the Trenton limestone slope on every side, and to which, therefore, they are necessarily tributary. The gas terrace of Indiana is, by far, the largest of these several subdivisions of the field. The minor elevations of Oak Harbor, Tiffin and Bryan, for example, give rise to the local supplies of gas or oil in these districts respectively.

It is certain that all the considerable areas of productive dolomite, that is, of the porous phase of the Trenton limestone lying relatively high enough to accumulate gas and oil, have already been discovered. The map showing the relief of this formation, page 47, sets this fact in clear light. Nor does there seem room for many minor arches of importance. The productive field as now defined can be well enough extended by a few square miles on this side or on that, but these additions can scarcely change its general poportions.

Some scanty production can be obtained outside the dolomitic limits already laid down, but it will be remembered that there is more or less magnesia in the limestone around the borders of the true field, and is quite possible that this production is in proportion to the amount of dolomite in the rock.

The general structure and arrangement of the Trenton limestone in Ohio have been abundantly indicated in the data already given, but they are still further shown on the appended sheet of sections. The sections were prepared two years since, and though the main features are unchanged, some of the representations do not exactly agree with present knowledge.

CHAPTER IV.

THE BEREA GRIT AS A SOURCE OF OIL AND GAS IN OHIO.

BY EDWARD ORTON.

I. STRATIGRAPHICAL RELATIONS.

The Berea grit holds as central a place in eastern Ohio with reference to oil and gas production as the Trenton limestone does in western Ohio.

As already described in Chapter I, page 35, it is a clean and sharp sandstone of medium grain in its northern exposures. The layers of the formation are sometimes separated by thin carbonaceous films mainly derived from marine vegetation, but occasionally fragments of carbonized tree growths are found in the same position. There is in the formation a thin conglomerate streak that can be followed steadily through several counties, the pebbles of which are mainly quartz, and which range from one inch in diameter downwards.

The sandstone grows steadily finer as it is followed southward from northern Ohio, and at the same time it becomes more impure, but even in the Ohio Valley it does not lose the character of a grindstone grit.

It ranges in thickness from five to one hundred feet in outcrop. A single section is reported in Lorain county, in which 170 feet of uninterrupted sandstone were found by the drill. Under moderate cover in the Mahoning Valley, nearly as great a measurement is found. Though this horizon is subject to frequent and sudden changes, it seldom fails altogether. Its line of junction with the underlying shale is very uneven and irregular, owing to erosion sustained by the formation immediately preceding the deposition of the sandstone. Channels and holes were worn in the shale into which was dropped the sand of which the Berea grit is composed. The sandstone sometimes descends forty or fifty feet in these channels, in less than 200 feet. This irregularity extends, however only to the bottom of the stratum. The upper surface is approximately even and regular.

From the shore of Lake Erie near Cleveland, its outcrop passes southward and westward with a breadth varying between one and ten miles through Cuyahoga, Erie, Huron, Richland, Crawford, Morrow, Delaware, Franklin, Pickaway, Ross, Pike, Highland, Adams and Scioto counties to the Ohio Valley. It is almost everywhere distinct and unmistakable, and gives rise to a large number of quarries small and great along the line.

From Cleveland eastward through Geauga and Trumbull counties, the formation has not yet been identified at all points as clearly and distinctly as elsewhere, but it is safe to say that it extends throughout this territory with characteristics but little changed from those found in the localities mentioned.

It reaches its highest elevation in the Sunfish Hills of Pike and Highland counties, where it is a little more than 1,300 feet above tide. In Adams, Scioto and Ross counties also, it reaches nearly the same elevation. The western outcrop in Franklin county is about 850 feet above the sea, while at Sunbury, in Delaware county, the western outcrop is about 950 feet above the sea; but the stratum rises again to the north*ward on the main water-shed of the state. In Richland and Crawford its western outcrop is about 1,125 feet above tide. At the quarries of Brownhelm and Berlin Heights, it has an altitude of a little more than 800 feet, while at Berea its elevation approximates 780 feet. The upper surface of the formation is counted in these measurements.

From the line of outcrop already given the stratum dips prevailingly to the southeast and south, the descent ranging from fifteen to fifty feet per mile. Its lowest levels are found under deep cover in the Ohio Valley between Marietta and Steubenville. The lowest recorded levels are about 800 feet below tide; its vertical range is thus seen to be at least 2,100 feet.

It is geologically due in fifty counties of Ohio, and it is entirely safe to say that it is present in every one, inasmuch as its presence has been proved in all of those in regard to which a question might most easily be raised. In a number of counties where it lies deepest it has been found in hundreds of drill-holes. Its areas above and below the surface, or in other words, in outcrop and under cover, are shown in the accompanying sketch map. A larger map also accompanies this volume.

The stratigraphical relations of the Berea grit in the Ohio scale are such that it is easy to follow it with entire confidence and certainty, not only through all its outcrops from Lake Erie to the Ohio Valley, but under the deepest cover as well.

Although the marks by which it can be recognized have been repeatedly described, they need to be re-stated in this connection. The Berea grit is the first regular sandstone to be reached in ascending the geological column of Ohio, and by the same token the last regular sand