veh, and Herodotus has described the occurrence of oil springs in the island of Zachynthus, now Zante. In Roman times petroleum was obtained from Sicily and burned in lamps. The first mention of petroleum in America (about 1635) is in a letter written by the Franciscan missionary Joseph de la Roche d'Allion, who refers therein to springs found in the region of what is now southwestern New York or northwestern Pennsylvania. The early settlers of Pennsylvania obtained small quantities of oil by digging wells and scooping out the liquid which seeped in from the surrounding rocks. The drilling of brine wells on the western slopes of the Alleghanies in the early part of the nineteenth century led to the discovery of petroleum at greater depths. A well sunk near Burkesville, Ky., in 1829, yielded great quantities of oil, which flowed to the surface and was drained into the Cumberland River, where at one time it was set on fire. The most important application of petroleum in the early days was in medicine; it was utilized as an illuminant only to a small extent, owing to its offensive odor. In the year 1853 Dr. Brewer suggested the use of petroleum for lubricating and illumination purposes, and set to work devising means for purifying the crude product. The Pennsylvania Rock-Oil Company was organized in 1854 to drill for oil; although its first well yielded from 400 to 1000 gallons a day, the company was not successful in its business ventures. Five years later, however, Col. E. L. Drake put down a productive well on land leased from this company, and the successful outcome of this undertaking may be said to mark the beginning of the oil industry in the United States. The news of the discovery was followed by a rush of adventurers from all parts of the country, so that by 1860 more than 100 square miles of territory in the vicinity of Oil Creek had been shown to be productive. Much of the oil which reached the surface was allowed to escape, owing to the lack of storage and transportation facilities. As the explorations were extended new fields were opened along the Allegheny River in Pennsylvania, also in Ohio and West Virginia. The Lima field of Ohio and Indiana was first developed in 1885, while the California fields have become large producers only in the last few years. The discovery made in the Beaumont region of Texas early in 1901, which has been followed by extraordinary development, is the most important event in the recent history of petroleum.
Among foreign countries, Russia is the largest producer of oil and the strongest competitor of the United States in supplying the world's markets. Operations have been conducted in this country since 1873. The largest fields are located on the Apsheron Peninsula, Baku being the chief centre of the industry. The distillation of petroleum from shales was first undertaken in France in 1834, and was successfully introduced into Scotland in 1850. The importation of shale oil into America led to the use of cannel coal for distilling; this industry gained considerable importance in the United States previous to 1860, but quickly succumbed when the first wells became productive.
Origin and Geological Occurence. The geological history of petroleum and natural gas are closely connected, so that what is said of one practically holds true of the other. Petroleum is always found in sedimentary rocks. For many years it was known only in sandstones or shales, and the term oil-sand was applied to the containing strata. Subsequently oil was struck in limestone in Ohio, thus forming a new type of occurrence, although one which has since proved to be rather unique.
Petroleum is considered by most geologists to have been derived by the destructive distillation of either animal or vegetable matter contained in the rocks. The products of this distillation have in some cases accumulated in the strata in which they were formed, while in others they have escaped upward into the overlying beds, in some instances even reaching the surface.
Petroleum occurs in all geological formations, from the Lower Silurian or Ordovician up to the Tertiary; it is chiefly of importance in the Silurian, Devonian, and Tertiary rocks. The relation of the distribution of oil to geological structure was not recognized until as late a date as about 1880, at which period geologists began to find the cause of oil accumulation and pressure. Prof. Edward Orton, of Ohio, was the most prominent investigator in this field. In all regions where petroleum occurs the strata are not only disturbed, but they are bent into anticlinal or arch-shaped folds. If the rocks are porous the gas tends to collect at the summit of the anticlinal fold or arch, while the oil collects in the flanks of the fold. Salt water is usually associated with the gas and oil, and, being heavier, accumulates in the flanks of the anticlinal or in the neighboring synclinal folds. There is little use of searching for oil in regions where the strata are flat; and it is rarely found in highly folded regions, for where the flanks of the fold have a dip of more than 10° the bending of the rocks is often sufficient to create cracks through which the oil or associated gas will escape to the surface. The rock in which the oil is found is spoken of as the reservoir, or oil-sand, and it is essential that this rock should be porous. The degree of porosity not only influences the quantity of oil which the rock can hold, but it may also influence the rate of flow of the well. Some wells may yield as little as 15 barrels per day; others may reach a production of 50,000 or 60,000 barrels per day. The porosity of the oil-bearing formation may also change from place to place and would account for the location of a profitable well at one point and a barren one a short distance from it. In order to prevent the escape of oil from the containing stratum it should also be overlain by a rock of more or less impervious nature. In many wells the petroleum flows to the surface under pressure. Professor Orton believcd that the oil was under hydrostatic pressure; according to his theory, the pressure in different wells of the same basin or pool ought to be nearly constant. In any region, however, the pressure usually diminishes with time. While Orton's theory may be true for Ohio, it seems doubtful whether hydrostatic pressure will account for the great oil and gas pressure found in some regions. The quantity of oil which a given territory can yield is often very great, since some sand will hold as much as one-eighth of its bulk in oil under pressure. This means that there is 1.5 inches of oil to every vertical foot of oil-sand, or about 5000 cubic feet per acre.
Character and Composition. Petroleum is a liquid of varying color, being black, brown, red, amber, or straw, and by reflected light often ap-
