Other internal chemical activities are at work, producing changes which are still the cause of debate and earnest study. The origin of petroleum must undoubtedly be traced to chemical transformations of a complicated character, if we may judge by the number of experimental methods which yield petroleum as a product. All opinions agree that the mineral oil is derived by some form of chemical action, though whether it arises from the decomposition of organic remains or whether it be of inorganic origin is still matter of dispute. Geological students have on the whole ranged themselves on the side of the first-named view, pointing out that the petroleum fields are all associated with sedimentary strata, whether sands or limestones. The inorganic view has been held as tenaciously by a number of men experienced in the search for oil, and it is capable of argument that sulphur dioxide and sulphuretted hydrogen, if being produced simultaneously, may result in the alteration of limestone to gypsum, free sulphur and petroleum being also obtained in the reaction. In this connection it is interesting to note that gypsum, sulphur and petroleum are associated at Jemsa, on the Gulf of Suez. One of the most interesting features in connection with petroleum is the phenomenon presented in most oil-fields of oil-wells separated perhaps by only thirty metres emitting oil under pressure at the same time; also the great pressures indicated by the remarkable fountain flows which are of constant occurrence in the principal petroleum fields.[8] VI.—THE FORMATION OF IGNEOUS ROCKS. The external and internal transforming agents are therefore of the deepest interest and of the highest economic importance, and the further study of the deep-seated changes leads directly to a consideration of the formation and sustained activity of the molten materials which find their main present-day expression in the phenomena of volcanoes. Their extension in the past is also revealed by the wearing hand of time in the wide distribution of coarsely crystalline granites and other igneous rocks, once deep-seated, but now exposed in regions which are either the cores of ancient continents or centres of exceptional deformation.