Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/792

734 In the peribolos of Zeus on the banks of the Ilyssus at Athens a hole was shown through which the waters of the flood had run off, and where religious rites were regularly performed, but this hole was associated rather with Deu calion than with Ogyges.  OHIO, the third of the States of the American Union in point of wealth and population, is situated between 38 27 and 41 57 N. lat. and between 80 34 and 84 49 W. long., and is bounded on the N. by Michigan and Lake Erie, on the E. by Pennsylvania, on the E. and S. by the Ohio river, which separates it from the States of West Virginia and Kentucky, and on the W. by Indiana. The greatest length from north to south is about 210 miles, the greatest breadth from east to west about 225 miles and the area 40,760 square miles.

Physical Features. The surface consists of an undulat ing plain, generally ranging in elevation between 1550 and 430 feet above sea -level, the portions below 500 feet or above 1400 being comparatively insignificant. The largest connected areas of high land extend from east to Avest across the central and northern central districts. In some limited districts of central Ohio, especially along the ridge of high land just referred to, and also in a few thousand square miles of north-western Ohio, the natural drainage is somewhat sluggish, and, while the land is covered with its original forest growth, it inclines to swampy conditions; but when the forests are removed and the waterways opened most of it becomes arable, and all of it can be made so without excessive outlay by means of open ditches.

The chief feature in the topography of Ohio is the water shed, which extends across the State from north-east to south-west, and divides its surface into two unequal slopes, the northern, which is much the smaller, sending its waters into Lake Erie and the Gulf of St Lawrence, Avhile the drainage of the other is to the Gulf of Mexico by the Ohio river. The average height of the ridge is about 1100 feet, but it is cut by several gaps, in which the elevation is reduced to about 950 feet. The relief of the State is chiefly due to erosive agencies. The entire drain age area of such a river as, for example, the Muskingum or the Scioto, may be conceived as originally a plain, all portions of which were at approximately the same elevation above the sea. Across this area one main furrow has been drawn, deepening and widening as it advances, and a countless number of narrower and shallower valleys are tributary to it. Fragments of the old plain still remain in the isolated &quot; hills &quot; or tablelands that bound the valleys, and which, though often separated by intervals of miles, still answer to each other with perfect correspondence of altitude and stratification. They rise to a maximum height of 600 feet above the river-channels in the main valleys.

Geology. The rocky floor is entirely composed of un altered stratified rocks of Palaeozoic age. Not a single trap dyke or volcanic vent intersects them, and not a trace of igneous metamorphism is shown in any portion of their extent. These strata are disposed in plains so nearly horizontal that the dip is nowhere heavy enough to be safely determined by a clinometer. Not only are sharp flexures wanting, but faults deserving the name are found in but a single corner of a single county. A few low folds, one of which is of preponderating importance, traverse the State and redeem its surface from geological monotony. The only structural irregularity is an occasional case of overlap, but even this is seldom of such a character as to interfere with the easy reading of the record. The aggre gate thickness of the entire series will reach 5000 feet if the maximum of each stratum is taken into the account, but if the average measurements are used the thickness does not exceed 3500 feet. The main elements of the scale, which extends from the Lower Silurian (upper portion) to the Upper Coal-measures inclusive, are given below, and the geological sketch-map shows how the surface of the State is distributed among the principal formations.

Feet. Glacial Drift 0300 Upper barren measures 300 Upper productive Coal-measures 200 I Lower barren measures 500 Carl Lower productive Coal-measures (includ ing Conglomerate group of Penna) ... 500 Maxville Limestone 25 &quot;Vaveiiy group 500 Ohio Black Shale (including Cleveland, Erie, and Huron Shales) 300 (Maximum 1500) Hamilton Shales 25 Corniferous Limestone 75 Lower Helderberg Limestone 50 Niagara Limestone 150 Niagara Shale 50 Clinton Limestone 25 Cincinnati group 800 Lower Silurian.

EB9 Ohio - geological map.jpg

The Cincinnati group, which constitutes the surface rock for about 4000 square miles in the south-western corner of the State, consists of alternating layers of blue limestone and calcareous shale, both of which contain great numbers of admirably-preserved fossils. The group is undoubtedly equivalent in part to the Hudson River group of the general scale, and by some geologists the name is counted a synonym, but it has not been proved that the Lower Silurian deposits of south-western Ohio can be definitely correlated with the subdivisions of the series in New York. The formation everywhere yields a cheap and excellent supply of building stone, which is also burned into a dark lime, valued for its hydraulic properties. The shales contain notable quantities of alkalis and phosphates, and the soils into which they very readily weather are proverbial for their fertility. The water-supply of the formation is poor, or rather wanting, the impervious shales refusing admission to the rainfall.

The Cincinnati axis, a low fold which traverses the State from south-west to north-east, constitutes its most influential geologi cal feature. It entered Ohio from Kentucky at the close of Lower Silurian time, and gradually extended itself to the northward, until it had converted the western half of the State into dry land It has left a clear record of its successive stages in the strata which compose it. It did not advance steadily and equally, and the occa sional relapses which it experienced have given rise to cases of over lap. It lias a low, flat summit, and, on account of the soluble and easily - eroded materials of which it consists, it has suffered more from denuding agencies than the Upper Silurian strata that enclose 