In the paper on the Oil and Gas Fields of Ohio and Indiana the discovery of the supply of those materials, the great value of which was only realized in 1884 and afterward, is spoken of as being more surprising and anomalous than any similar discovery that had preceded it, and as a development which experts were hardly more prepared for than others. The oil and gas derived from the Trenton limestone in certain parts of these States were found to differ from the oil and gas in the Pennsylvania wells in chemical composition and physical properties, in the horizons from which they were obtained, in the structural features of the rocks associated with their production, and, most of all, in the kind of rock that produced them. "No facts more unexpected have ever been brought to light in connection with the geology of this country than those with which we are now becoming acquainted." Professor Orton's paper, which fills one hundred and eighty of the large pages of the report of the Geological Survey, includes a sketch of the history of the discovery to July, 1887, when it was prepared; a designation of what was known in regard to the geological scale and geological structure of the regions within which the new fields are embraced, and the tracing of the chief factors that influence or control the productiveness of the oil rock, with the description of the special features and boundaries of the several fields and the setting forth of the leading facts and present development of these lately found sources of power. Two principal conditions under which the new oil rock had proved petroliferous on a large scale were found to be porosity, connected with and apparently dependent on the chemical transformation of the upper portion of the limestone, for a number of feet in thickness, into a highly crystalline dolomite; and a relief resulting from slight warping of the strata, whereby the common contents of the porous portions of the Trenton limestone had been differentiated by gravity, the gas and oil seeking the highest levels, and the salt water maintaining a lower but definite elevation in every field. Professor Orton found nothing in the new experience to make it safe to count the Trenton limestone an oil rock or a gas rock in any locality, unless it could be shown to have undergone the dolomitic replacement by which its porosity was assured; and even in case it had suffered this transformation it would not be found a reservoir of gas or oil in an important sense unless some parts of it had acquired the relief essential to the due separation of its liquid and gaseous contents.