This stratum has proved to be very persistent and wide-spread. It is sparingly fossiliferous, but several of the forms that it contains are characteristic, that is, they have thus far been found in no other stratum. The first of the deep wells that was drilled in 1884 in Findlay revealed to our surprise, at a depth of 800 feet, a stratum of black shale containing the most characteristic fossil of the Utica shale, viz., Leptobolus insignis, Hall, and it was thus positively identified with the last-named formation. This bed of shale has the normal thickness of the Utica shale in New York, viz., 300 feet, and with the other elements involved, it extended and continued the New York series into northern Ohio in a most unexpected, and at the same time, in a most satisfactory way.

The Utica shale, thus discovered and defined, is a constant element in the deep wells of north-western Ohio. Its upper boundary is not always distinct, as the Hudson River shale that overlies it sometimes graduates into it in color and appearance; but as a rule the driller, without any geological prepossessions whatever, will divide the well section in his record so as to show about 300 feet of black shale at the bottom of the column or immediately overlying the Trenton limestone. This stratum holds its own as far as the southern central counties. In the wells of Springfield, Urbana and Piqua, it is found in undiminished thickness, but apparently somewhat more calareous in composition. From these points southward, the black shale thins rapidly. It is apparently replaced by dark-colored limestone bands known as pepper and salt rock by the driller. No great falling off in black shale appears in the Dayton well, but at Middletown the driller reported a sharp boundary between gray shale 310 feet thick, and black shale 100 feet thick-the latter directly overlying the Trenton limestone. At Hamilton the same driller reported the boundary still distinct, but the black shale was here reduced to 37 feet, according to his record.

From these and similar facts it appears that the Utica shale is much reduced and altered as it approaches the Ohio Valley, and is finally lost by overlap of the Hudson River shale in this portion of the state and to the southward. Considerable discussion has been indulged in over the question, what part of the series exposed at Cincinnati and its vicinity is the Utica shale?

The answers have all agreed thus far at least, that if any part is to be counted Utica, the 50 to 100 feet of greenish blue shale overlying the Point Pleasant or Trenton limestone must be so considered. In this division, certain fossils occur that are also found in the Utica shale of New York, but that are not characteristic of it in the sense of being limited to it. Of these fossils the trilobite Triarthrus Beckii is an example. It is found in the Trenton, Utica and Hudson River formations of New York, though most abundantly in the Utica. Certain new species of shells belonging to the genus Leptobolus, which contains the most characteristic Utica forms, are also found in the lower beds at Cincinnati. But associated with these semi-characteristic fossils were many of the ordinary fossils of the Hudson River series. Until the new facts from northern Ohio came to light the arguments for recognizing the beds in question as Utica shale, were fairly strong, but at present it does not seem probable that these greenish blue shales were formed contemporaneously with the black Utica shale of northern Ohio. The Cincinnati axis was already begun, and by its advancing border in southern Ohio the growth of the Utica shale was checked in this direction, and the record became defective in the Ohio Valley by the more or less complete exclusion of this element from the scale. The discussion of these questions will be taken up more fully in the account to be given of the Cincinnati wells.

3. THE HUDSON RIVER GROUP.

The very important and interesting series now to be described appears in all the previous reports of the Geological Survey under another name, viz., the Cincinnati group. It is unnecessary to review here the long discussions pertaining to the age of this series, or the grounds on which the changes in the name by which it is known have been based. The return to the older name here proposed, is necessitated by the discoveries recently made in our underground geology, to which reference has already been made. So long as the Utica shale was held to be included in the section at Cincinnati but without distinct or recognizable boundary, so long could the maintainance of the name Cincinnati group be justified. It was held to cover two divisions of geological history which were practically inseparable, and therefore the name of either would be inapplicable to the compound series; but since it has been demonstrated or at least made highly probable, that the Utica shale forms either no part, or but a very small part, of the section at or near Cincinnati, there is no longer any reason for continuing the name Cincinnati group. It becomes a synonym and must be rejected as unnecessary and indefensible.

The Hudson River group in southwestern Ohio consists of alternating beds of limestone and shale, the latter of which is commonly known as blue clay. The proportion of lime and shale vary greatly in different parts of the series. The largest percentage of shale occurs in the 250 feet of the series that begin 50 or 75 feet above low water at Cincinnati. The entire thickness of the series in southwestern Ohio is about 750 feet. The division of the series into lower and upper is natural and serviceable. The lower is known as the Cincinnati division and the upper as the Lebanon division. The Cincinnati division has a thickness of 425 to 450 feet, and the Lebanon division a thickness of about 300 feet. The divisions are separated on both paleontological and stratigraphical grounds. Both divisions abound in exquisitely preserved fossils of Lower Silurian time; and in fact the hills of Cincinnati and its vicinity have become classical grounds to the geologist on this account.

As the series takes cover to the northward and eastward it retains for a time the same characteristics already described, but as it is followed further it rapidly becomes less calcareous. The limestone courses are thinner and fewer, and inasmuch as they resist or delay the drill but little in its descent, the entire series comes to be counted shale. One other fact needs to be mentioned. The shale at certain points, and especially on the western border, often grows dark in color so that the boundary between this and the underlying Utica division is somewhat obscure.

The entire interval in such circumstances may pass with the driller as black shale. The shale of this series are thinnest in this part of the state, the entire measure running as low as 300 feet, or even less. To the eastward the greenish blue element already named, is always found. The Hudson river shales are fossiliferous, as the fragments of corals and shells brought up in the drillings abundantly testify. A few of the fossils are identifiable.

The Hudson River group occupies in its outcrop about 4,000 square miles in southwestern Ohio, but it is doubtless coextensive with the limits of the state. The shales of the series contain in outcrop large quantities of phosphates and alkalies, and the soils to which they give rise are proverbial for their fertility.

The presence of these fine-grained and impervious shales in so many separate beds forbids the descent of water through the formation. In its outcrop the formation has no water supply, and, as found by the driller, it is always dry. It gives rise to frequent "blowers" or shortlived accumulations of high-pressure gas when struck by the drill, as has been found in the experience in many towns of western Ohio within the last two years, and it also yields considerable amounts of low-pressure shale gas, which proves fairly durable. In a single instance it has given rise to high-pressure gas, but the supply was not of long duration.

4. THE MEDINA SHALE.

A stratum of non-fossiliferous shale, often red or yellow in color and having a thickness of ten to forty feet, directly overlies the uppermost beds of the Hudson River group at many points in southwestern Ohio. In the Report of Progress for 1869, Geology of Ohio, page 148, the suggestion was made that this red band represents the Medina epoch, the position of the stratum and its color being the grounds of reference. The same view was further urged by Newberry in Geology of Ohio, Vol. 1, page 126, but no additional reasons for assigning the stratum to this position have been found until within the last year. The occurrence of 50 to 150 feet of red shale in most of the recent deep borings in northwestern Ohio at exactly the place in the general column where the Medina should be, and so much nearer to the known outcrops of the formation that its continuity with these was hardly to be questioned, this fact, taken in connection with the occurrence of like beds of red shale holding the same relative position in several deep borings in the central portions of the state, serves to confirm the halting reference of 1869, and to give warrant for counting the Medina epoch duly represented in the outcropping strata of southwestern Ohio. It occurs here only in included sections, its thin and easily eroded beds never being found as surface formations for extensive areas. There is good reason to believe that the Medina formation is coextensive with the limits of the state, except in the regions from which it has already been removed.

The red color of the shales is quite persistent, but there are many well-records in which this color does not appear. This is especially true in Allen county, and to the westward and northwestward from Lima. Blue shales alternate with the red in the eastern sections. In the western they replace the latter. Thin beds of sandstone are found in the Medina, especially to the eastward. Small pebbles occur in some of these beds.

5. THE CLINTON LIMESTONE.

The Clinton group of New York appears as a surface formation in Ohio only in the area already named. It forms a fringe or margin of the Cincinnati group through eight or ten counties, rising above the soft and easily eroded rocks of this series, and of the previously named Medina shale in a conspicuous terrace. It is everywhere a wellcharacterized limestone stratum. It is highly crystalline in structure, and is susceptible of a good polish. In some localities it is known as a marble. A considerable part of it, and especially the upper beds, are almost wholly made up of crinoidal fragments. In thickness, it ranges between ten and fifty feet. Its prevailing colors are white, pink, red, yellow, gray and blue. At a few points it is replaced by the hematite ore that is elsewhere so characteristic of the formation. The ore is generally too lean and uncertain to possess economic value, but it was once worked for a short time and in a very small way in a furnace near Wilmington, Clinton county.

The limestone contains a notable quantity of indigenous petroleum throughout most of its outcrop, but no very valuable accumulations of oil or gas have been found in it thus far. It is the source of the lowpressure gas of Fremont (upper vein), and also of the gas at Lancaster from 1,962 feet below the surface, and at Newark from 2,100 feet below the surface. In fact, a small but fairly persistent flow is maintained from this horizon in several of the gas-producing districts of Northern Ohio. In a single instance in Wood connty it is proving itself an oil rock. A well near Trombley, drilled to this horizon, has been flowing twenty to thirty barrels of oil for a number of months, the oil being referable to this formation.

In outcrop the stratum is quite porous as a rule, and the water that falls upon its uncovered portions sinks rapidly through them to the underlying shale (Medina), by which it is turned out in a well-marked line of springs.

In composition, the limestone, in its outcrops in southern Ohio, is fairly constant. All of its most characteristic portions contain eighty to eighty-five per cent. of carbonate of lime, and ten to fifteen per cent. of carbonate of magnesia. At a few points, however, it is found as the purest carbonate of lime in the state. Under cover, to the northward, it is much more magnesian in composition, being indistinguishable from the Niagara. It also becomes shaly and changeable in character at many points. As it becomes shaly the thickness is much increased.

It is everywhere uneven in its bedding, being in striking contrast in this respect with the formations below it and also above it. The beds are all lenticular in shape, and they extend but a few feet in any direction. They seldom rise to one foot in thickness.

The uneven bedding, the crystalline and crinoidal characters, the high colors, and particularly the red bands and the chemical composition, combine to make the Clinton limestone an exceedingly well-marked stratum throughout southwestern Ohio, and from the hints yielded by the drill in northwestern Ohio, it seems to have something of the same character there, especially so far as color is concerned. It becomes more shaly and much thicker to the eastward. It carries bands of red shale