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Rh one to ten feet. These slight differences in level, however, are found to have a most powerful effect in the direction already mentioned.

It is evident that accurate knowledge of the character and structure of the rock-formations in petroliferous territories is of the greatest importance in enabling the expert to select favourable sites for drilling operations, hence on well-conducted petroleum properties it is now customary to note the character and thickness of the strata perforated by the drill, so that a complete section may be prepared from the recorded data In some cases the depths are stated with reference to sea-level, instead of being taken from the surface, thus greatly facilitating the utilization of the records. Oil and gas are often met with in drilled wells under great pressure, which is highest as a rule in the deepest wells. The closed pressure in the Trenton limestone in Ohio and Indiana is about 200–300 ℔ per sq. in., although a much higher pressure has been registered in many wells. The gas wells of Pennsylvania indicate about double the pressure of those drilled in the Trenton limestone, 600–800 ℔ not being unusual, and even 1000 ℔ having been recorded The extremely high pressure under when oil is met with in wells drilled in some parts of the Russian oil fields is a matter of common knowledge, and a fountain or spouting well resulting therefrom is one of the “sights” of the country. A famous fountain in the Groznyi oil field in the northern Caucasus, which began to flow in August 1895, was estimated to have thrown up during the first three days 1,200,000 poods (over 4,500,000 gallons, or about 18,500 tons) of oil a day It flowed continuously, though in gradually diminishing quantity, for fifteen months; afterwards the flow became intermittent. In April 1897 there was still an occasional outburst of oil and gas.

Three theories have been propounded to account for this pressure.—

1. That it results from the weight of the overlying strata

2. That it is due to water pressure, as in artesian wells (“hydrostatic or “artesian” theory)

3. That it is caused by the compressed condition of the gradually accumulating gas.

Of these tie first has been proved untenable, and while in some instances (e.g. certain wells in Ohio), the second has held good, the third appears to be the most widely applicable.

The conditions of formation and accumulation of petroleum point to the fact that the principal oil fields of the world are merely reservoirs, which will become exhausted in the course of years, as in the case of the decreasing yield of certain of the American fields. But new deposits are continually being exploited, and there may be others as yet unknown, which would entirely alter any view that might be expressed at the present time in regard to the probable duration of the world's supply of oil and gas.

As already stated, every one of the great geological systems appears to have produced some form of bitumen, and in the following table an attempt has been made to classify on this basis the various localities in which petroleum or natural gas has been found in large or small quantities.—

Recent.—Lancashire (Down Holland Moss), Holland, Sweden, Sardinia, Kaluga (Russia), Red Sea, Mediterranean.

Pleistocene.—Schleswig-Holstein, Minnesota, Illinois, Louisiana

Pliocene.—Spain, Italy, Albania, Croatia, Hungary, Hesse, Hanover, Transcaspia, Algeria, Florida, Alabama, California, Mexico, Peru, Victoria, New Zealand

Miocene.—France, Switzerland, Spain, Italy, Sicily, Greece, Rumania, Turkey in-Europe, Styria, Slavonia, Hungary, Transylvania, Galicia, Lower Austria, Wurttemberg, Brandenberg, West Prussia, Crimea, Kuban, Terek, Kutais, Tiflis, Elizabetpol, Siberia, Transcaspia, Mesopotamia, Persia, Assam, Burma, Anam, Japan, Philippine Islands, Borneo, Sumatra, java, Algeria, Egypt, British Columbia, Alaska, Washington, California, Colorado, Texas, Louisiana, Barbados, Trinidad, Venezuela, Peru, South Australia, Victoria, New Zealand

Oligocene.—France, Spain, Greece, Rumania, Hungary, Transylvania, Galicia, Bavaria, Elsass, Rhenish Bavaria, Hesse, Saxony, Crimea, Daghestan, Tiflis, Baku, Alaska, California, Florida.

Eocene.—Devonshire (retinasphalt), France, Spain, Italy, Asia Minor, Montenegro, Bosnia and Herzegovina, Rumania, Dalmatia, Istria, Hungary, Transylvania, Galicia, Moravia, Bavaria, Elsass, Kutais, Armenia, Persia, Baluchistan, Afghanistan, Punjab, Assam, Sumatra, Algeria, Egypt, Maryland, Colorado, Utah, Nevada, California, Louisiana, Texas, Cuba, Colombia, Brazil.

Cretaceous.—Holland, France, Switzerland, Spain, Italy, Sicily, Greece, Hungary, Silesia, Moravia, Westphalia, Brunswick, Hanover, Schleswig Holstein, (German) Silesia, Poland, Kutais, Uralsk, Turkestan, Armenia, Syria, Arabia, Persia, Tunis, Egypt, West Africa, British Columbia, Alberta, Assiniboia, Athabasca, Manitoba, New jersey, South Dakota, Washington, Montana, Oklahoma, Utah, Wyoming, Colorado, California, New Mexico, Arkansas, Texas, Louisiana, Mexico, Hayti, Trinidad, Colombia, Argentina, New Zealand.

Neocomian.—Sussex, France, Switzerland, Spain, Hungary, Transylvania, Bukowina, Galicia, Hesse, Baden, Hanover, Brunswick, California, Texas, Mexico, Bolivia, Argentina

Jurassic.—Yorkshire, Somerset, Buckingham, France, Switzerland, Spain, Italy, Lower Austria, Baden, Elsass, Hesse, Hanover, Brunswick, Sizian, Titlis, Siberia, Persia, Madagascar, Alaska, Wyoming, Colorado, Mexico, Argentina

Triassic.—Yorkshire, Staffordshire, France, Portugal, Spain, Italy, Montenegro, Upper Austria, Tyrol, Bavaria, Wurttemberg, Baden, Elsass, Lothringen, Rhenish Bavaria, Rhenish Prussia, Hanover, Brunswick, Sweden, Spitzbergen, Punjab, China, Transvaal, Cape Colony, Connecticut, New jersey, Virginia, North Carolina, Wyoming, Argentina, New South Wales, Queensland.

Permian.—Yorkshire, Denbigh, Moravia, Bohemia, Baden, Saxony, Vologda, Afa, Kazan, Simbirsk, Samara, Kansas, Wyoming, Oklahoma, Texas (Permo-Carboniferous)

Carboniferous.—Scotland, North of England, and Midlands, Wales, France, Belgium, Carniola, Moravia, Elsass, Saxony, Perm, Sizran, China, Cape Colony, Nova Scotia, Newfoundland, Pennsylvania, West Virginia, Ohio, Michigan, Indiana, Illinois, Iowa, Missouri, Tennessee, Kentucky, Alabama, Kansas, Arkansas Colorado, Oklahoma, Tasmania, Victoria (Permo-Carboniferous), West Australia (Permo-Carboniferous)

Devonian.—Scotland, Devonshire, Spain, Hanover, Archangel, Vitebsk, Athabasca, Mackenzie, Ontario, Quebec, New Brunswick, Newfoundland, New York, Pennsylvania, West Virginia, Ohio, Michigan, Wisconsin, Kentucky.

Silurian.—Shropshire, Vales, Bohemia, Sweden, Esthonia, Manitoba, Ontario, Quebec, Newfoundland, New York, Pennsylvania [?], Ohio, Michigan, Indiana, Illinois, Minnesota, Tennessee, Kentucky, Georgia, Alabama, Oklahoma, New Mexico, New Caledonia

Cambrian.—Shropshire, New York

Archean.—France, Norway, Sweden, Ontario.

In this list, while certain occurrences in rocks of undetermined age in little-known regions have been omitted, many of those included are of merely academic interest, and a still larger number indicate fields supplying at present only local needs. All have been arranged in geographical order without reference to productive capacity or importance. It should be pointed out that the deposits which have been hitherto of chief commercial importance occur in the old rocks (Carboniferous to Silurian) on the one hand, and in the comparatively new Tertiary formations on the other, the intermediate periods yielding but little or at any rate far less abundantly.

Origin.—The question of the origin of petroleum (and natural gas), though for the first half of the 19th century of little more than academic interest, has engaged the attention of naturalists and others for over a hundred years As early as 1804, Humboldt expressed the opinion that petrol cum was produced by distillation from deep-seated strata, and Karl Reichenbach in 1834, suggested that it was derived from the action of heat on the turpentine of pine trees, whilst Brunet, in 1858, adumbrated a similar theory of origin on the ground of certain laboratory experiments The theories propounded may be divided into two groups, namely, those ascribing to petroleum an inorganic origin and those which regard it as the result of the decomposition of organic matter.

M. P. E. Berthelot was the first to suggest, in 1866, after conducting a series of experiments, that mineral oil as produced by purely chemical action, similar to that employed in the manufacture of acetylene. Other theories of a like nature were brought forward by various chemists, Mendeléeff, for example, ascribing the formation of petroleum to the action of water at high temperatures on iron carbide in the interior of the earth.

On the other hand an overwhelming and increasing majority of those who have studied the natural conditions under which petroleum occurs are of opinion that it is of organic origin. The earlier supporters of the organic theory held that it was a product of the natural distillation of coal or carbonaceous matter, but though in a few instances volcanic intrusions appear to have converted coal or allied substances into oil, it seems that terrestrial vegetation does not generally give rise to petroleum Among those who have considered that it is derived from the decomposition of both animal and vegetable marine organisms may be mentioned. P. Lesley, E. Orton and S. F. Peckham, but others have held that it is of exclusively animal origin, a view supported by such occurrences as those in the orthoceratities of the Trenton limestone, and by the experiments of C. Engler, who obtained a liquid like crude petroleum by the distillation of menhaden (fish) oil. Similarly there is a difference of opinion as to the conditions under which the organisms have been mineralized, some holding that the process has taken place at a high temperature and under great pressure; but the lack of practical evidence in nature in support of these views has led many to conclude that petroleum, like coal, has been formed at moderate temperatures, and under pressures varying with the depth of the containing rocks This view is supported by the fact that petroleum is found on the Sardinian and Swedish coasts as a product of the decomposition of seaweed, heated only by the sun, and under atmospheric pressure.

Consideration of the evidence leads us to the conclusion that, at least in commercially valuable deposits, mineral oil has generally been formed by the decomposition of marine organisms, in some cases animal, in others vegetable, in others both, under practically normal conditions of temperature and pressure.