In all cases where sandstones contain petroleum in quantity, I think it will be found that there are sheets of carbonaceous matter below, from which carbureted hydrogen and petroleum are constantly issuing. A more probable explanation of the occurrence of petrolem in the sandstones is that they have, from their porosity, become convenient receptacles for that which flowed from some organic stratum below. Dr. T. Sterry Hunt has regarded limestones, and especially the Niagara and corniferous, as the principal sources of our petroleum; but, as I have elsewhere suggested, no considerable flow of petroleum has ever been obtained from the Niagara limestone, though at Chicago and Niagara Falls it contains a large quantity of bituminous matter; also, that the corniferous limestone which Dr. Hunt has regarded as the source of the oil of Canada and Pennsylvania is too thin, and too barren of petroleum, or the material out of which it is made, to justify the inference. The corniferous limestone is never more than fifty or sixty feet thick, and does not contain even one per cent. of hydrocarbons; and in southern Kentucky, where oil is produced in large quantity, this limestone does not exist. That many limestones are more or less charged with petroleum is well known; and in addition to those mentioned above, the Silurian limestone at Collingwood, Canada, may be cited as an example. As I have elsewhere shown, we have reason to believe that the petroleum here is indigenous, and has been derived, in part, at least, from animal organisms; but the limestones are generally compact, and if cellular, their cavities are closed, and the amount of petroleum which, under any circumstances, flows from or can be extracted from limestone rock is small. On the other hand, the bituminous shales which underlie the different oil regions afford an abundant source of supply, holding the proper relations with the reservoirs that contain the oil, and are spontaneously and constantly evolving gas and oil, as may be observed in a great number of localities. For this reason, while confessing the occurrence of petroleum and asphaltum in many limestones, I am thoroughly convinced that little or none of the petroleum of commerce is derived from them.