Under proper conditions below the earth's surface the derived oil accumulates in porous or fractured rocks. There must, of course, be a source from which the petroleum is derived or distilled; a porous or fractured rock formation to take it up; a cap rock or impervious layer to hold it in; and a proper geologic structure to favor accumulation. The most common porous (containing) rock is sandstone, and the most common cap rock is shale. Oil is rarely found without gas, and saline water is likewise often present. If the containing strata are horizontal, the oil and gas are usually irregularly scattered, but if tilted or folded, and the beds porous throughout, they appear to collect at the highest point possible. It was the result of observations along this line that led I. C. White to develop what is known as the "anticlinal theory." According to this theory, in folded areas the gas collects at the summit of the fold (anticline), with the oil immediately below, on either side, followed by the water. It is, of course, necessary that the oil-bearing stratum shall be capped by a practically impervious one. [Illustration: Fig. 78.--Map showing the principal petroleum and natural gas fields of the United States. (After U. S. Geological Survey.)] [Illustration: Fig. 79.--A vertical (structure) section showing a very common type of oil-bearing structure. In this anticline, water, oil, and gas arranged in order of their specific gravities. Removal of the gas would allow the oil and water to rise higher toward the apex of the porous layer. (After Indiana Geological Survey.)] [Illustration: Plate 19.--General View in the Appalachian Mountains Along New River, Virginia. This is a typical portion of the great area which, during Mesozoic time, was reduced by erosion to the condition of a low-lying plain ("peneplain"). Since early Cenozoic time the peneplain has been upraised and New River has carved out its V-shaped valley to its present depth, while tributary streams have carved out a series of valleys along belts of weak rocks nearly at right angles to the main valley. The remarkably even sky line marks approximately the old peneplain surface. (_Photo by Hillers, U. S. Geological Survey._)] [Illustration: Plate 20.--(_a_) A Big Glacial Bowlder of Plutonic Igneous Rock Carried Miles from Its Parent Ledge by the Ice Sheet Which Passed Over the Adirondack Mountains During the Ice Age. (_Photo by the author._)] [Illustration: Plate 20.--(_b_) A Long, Winding Ridge of Sand and Gravel (Called an "Esker") Deposited by a Stream in a Channel in the Ice Near the Margin of the Great Glacier During Its Retreat from the Adirondack Mountains. (_Photo by the author._)]