It is, however, evident that oil has often moved from the formations in which it was made to other formations, generally loose or porous, which have served as natural reservoirs for storing the oil in the earth. It is probable that in most instances the migration took place by filtration or flowing through fissures or openings from one formation to another, while in some cases it is evident that a distillation took place and the migration probably was made in the form of vapor, which was ultimately condensed in a cooler formation and there stored. Generally speaking, however, it reveals itself in commercial quantities chiefly in the Devonian and carboniferous formations which are comparatively old; or in the Tertiary rocks, aeons younger in geological evolution. The geographical distribution is as diverse as the geological; the deposits in many instances occur along well-defined lines and in association with mountain ranges, though this condition is by no means axiomatic. It is assumed that in the elevatory processes which obviously occurred while the earth’s crust was attaining its present characteristics, certain folds were formed which arrested and collected the oil in productive belts. Early misapprehensions with regard to the origin of petroleum are indicated by the familiar word “coal-oil,” now used to signify one of the most popular products of crude petroleum; but originally derived from the fact that what we now know as kerosene or lamp oil was produced from the distillation of coal before petroleum became an important source from which the lamp oil was obtained. Over a century ago miners in Shropshire, England, observed oil trickling from fissures in coal veins and assumed that coal was the source of the liquid. This belief was intensified by the fact that the earliest discoveries in Pennsylvania, which resulted in the creation of the great modern petroleum industry of the United States, were in the vicinity of vast deposits of bituminous coal. Shortly afterward this belief was disproven by the discovery of valuable oil fields in the western part of the province of Ontario, Canada, where no coal exists; and other discoveries on this continent and elsewhere have furnished abundant proof that oil may exist in large volumes independently of coal.