VERTICAL SECTION

OF THE ROCKS OF OHIO.

locality, is found to be a dark-blue, almost black limestone, lying in quite massive and even beds, which are often separated by layers of black shale. Both limestone and shale contain excellently preserved fossils of Lower Silurian age. By means of these fossils, and also by its stratigraphical order, the limestone is followed with perfect distinctness from Trenton Falls to every point of the compass. It is changed to some extent, in color and composition as it is traced in different directions, but there is seldom a question possible as to its identity. The Trenton limestone forms several of the largest islands in whole or in part in the northern portion of Lake Huron, as the Manitoulin Islands and Drummond's Island. It dips from this region to the southward, but it is found rising again in outcrop in the valley of the Kentucky river. Its presence underneath the entire states of Ohio and Michigan, and especially under western Ohio, has always been inferred, since the geology of these states was first worked out. But the claim now and here to be made, viz., that the Trenton limestone rises to the surface within the limits of Ohio, appears for the first time on this page in the publications of the Geological Survey. It was shown in Volume I, Geology of Ohio, page 370, that the lowest rocks of the state that appear at the surface are to be found in the quarries of Point Pleasant, which are located in Clermont county, about twenty miles above Cincinnati. These lowest rocks have received the name by which they are distinguished from this locality. A thickness of fifty feet was claimed for the Point Pleasant beds, and they were referred on what seemed good ground to the Cincinnati group. At a later day the claim was made by Mr. S. A. Miller, in a paper read before the Natural History Society, that the Point Pleasant rocks were in reality the Trenton limestone rocks of Kentucky, rising to day at this sole point in Ohio. The claim was afterwards repeated by Mr. W. M. Linney, of the Kentucky Geological Survey. But no occasion for revising the geological column of Ohio, since its first publication, has arisen until now.

It may seem surprising that the discovery of unquestioned Trenton limestone in Ohio should be made in the northern portion of the state, where the stratum lies buried 1,000 to 2,000 feet below the surface, and that by following this stratum southward, by means of the numerous well-records lately made available, it should at last be found rising to day in the quarries of Point Pleasant, confirming the identifications of the rock as quoted above, but this surprise will disappear when the new section is understood. It is the presence of the Utica shale to the northward, in its most characteristic form, that enables us positively to identify the Trenton limestone there, and it is the disappearance of the same shale cover in the Ohio Valley that has left us hitherto uncertain as to the upper boundaries of the Trenton limestone in this region. The facts bearing on this identification will be found on later pages of this report, but for the present it is sufficient to state that there is good reason to accept the identification first made by S. A. Miller, Esq., of the Point Pleasant rocks, as Trenton limestone. This famous stratum thus becomes the foundation of the Ohio geological column, although it has but a single outcrop in the state.

The character of the rock found at Point Pleasant is briefly described in the chapter of Volume I, Geology of Ohio, already quoted. The limestone is a light or grayish blue limestone, quite crystalline in structure, massive in its bedding, and fossiliferous. The solid courses are interrupted to some extent by thinner beds of srale. It breaks under the drill into thin flakes rather than into cubical grains. The analysis of a single sample of the Point Pleasant limestone is as follows: (No. 1). The analysis of the same stratum taken from 300 feet below the surface at Cincinnati is appended, (No. 2), as is also the analysis of the stratum as found at Hamilton at the depth of 550 feet. (No. 3).

The limestone is seen to be in this analysis impure and of a character that by its decay would leave a large residue to the forming soil. In some parts of northwestern Ohio, the Trenton limestone is a magnesian limestone of a high degree of purity. Its composition, as seen in the gas rock of Findlay and the oil rock of Lima, is herewith appended:

Trenton limestone at Findlay, 1,096 feet below surface, 1.

The Galena division of the Trenton limestone of Illinois and Wisconsin is a dolomite, like the oil rock of Northwestern Ohio. In Kentucky, the Trenton limestone has a moderate percentage of magnesia in its composition. In New York it is higher in carbonate of lime, and contains comparatively little magnesia. This whole subject of the chemical composition of the Trenton limestone will be more fully discussed on a later page.

While the Trenton limestone in its uppermost beds is thus seen to make the lowest rocks that rise to the surface in Ohio, it is still true that the drill is revealing to us the composition of the underlying series for many hundred feet below it. The thickness of the Trenton limestone proper can only be judged by a study of the formation as it appears in outcrop to the southward. Mr. W. M. Linney, of the Kentucky Geological Survey, in a paper on the rocks of central Kentucky published in 1882, gives the thickness of the Trenton limestone in Kentucky as 175 feet. The Bird's-eye limestone which directly underlies it, he finds to be 130 feet in thickness, and the Chazy next below 300 feet in thickness; the entire series being thus about 600 feet in thickness. It is altogether probable that these three limestones constitute the solid mass which the drill has so often penetrated in Ohio within the last year or two to a depth five or six hundred feet. The formations which the geologist separates when they rise to the surface are counted by the driller as a single limestone, for which he needs no other name than Trenton. The several divisions, however, are found to vary considerably in grain, in color, and in chemical composition. Below this great limestone a sandstone more or less calcareous is reported in most of our deep wells. This is on the horizon of the Saint Peter's sandstone of the northwest and very likely deserves to be called by this name. It is forty to sixty feet thick, as generally found, and is charged with the rank salt and sulphur water which is known as Blue Lick water, though water of the same grade is sometimes found in or between the limestones above named. Still deeper, impure magnesian limestones again occur that may probably be referred to the Calciferous period. The facts pertaining to these portions of our series will be brought out in connection with the accounts of these deep wells to be subsequently given.