2. Discontinuous inclined Porous Beds.-Where a porous band thins out, becomes fine-grained, or otherwise loses its porosity before reaching outcrop, gas and oil may accumulate at the upper limit (oil at the lower limit if the rock be dry). Examples are known in the Oklahoma and Coalinga Oilfields (Fig. 13), and probably are very common.

3. Pools resulting from differences in Capillarity and Porosity.-Over large tracts in the United States where oil is mined from strata situated at great depth and almost undisturbed, e.g. parts of the Appalachian Oilfields, oil is distributed sporadically in isolated pools. The pools are discovered and their limits proved by trial; generally they occur in the most porous parts of highly porous rocks. Probably

FIG. 13. Oil in inclined discontinuous porous beds, as in Coalinga Field. Length of section, say 1 mile.

many of these pools of oil have been driven into their present position' by the complicated movements of waters circulating under the guidance of capillary attraction-a force which would vary greatly in different beds, and from place to place in each bed, according to texture, cement, current-bedding, etc.

PART III. SURFACE PHENOMENA OF DECARBONIZATION PROCESS. The visible indications of this natural process will now be briefly enumerated.

Oil-sands at Outcrop.-At outcrop oil-sands, though they have a pitchy smell, are seldom visibly moist with oil, for on evaporation oxidized residues fill up the pores of the rock. Most common are the oils having an asphaltic base, in which case the bituminous residue is very dark, giving the rock a deep-brown or black appearance. Although an outcropping oil-rock is generally dark and dry, it is often possible with the pick to break far enough into it to find it oily and of a lighter colour. In coast-sections oil-rocks washed by the tides often present this appearance, and sometimes are so soft and sticky that drops of oil may be squeezed out by hand. In strata containing light oil with a paraffin base, however, evaporation may be so complete as to leave little or no indication of petroleum at the outcrop. If, however, the outcropping oil-sands be broken with the hammer, the oil often may be detected by smell, or if water be present, by the oil film which spreads over its surface.

1 M. J. Munn, "Theories of Gas and Oil Accumulation," Economic Geology, 1909, p. 509; also "Anticlinal and Hydraulic Theories", Petroleum Review, January, 1910.

Oily Clays.-The exposed clays of a petroliferous formation do not so readily lose their hydrocarbons, and often they are found to smell of oil quite strongly, even where exposed at the surface.

Burnt Clays.-Occasionally, as in Trinidad and California, there occur beds of clay-shale, sometimes 10 feet or more in thickness, which have been naturally burned (at depth) into hard red rock like brick by the combustion of the bituminous material contained in them.

Oil Seepages.-Oil exuding from outcropping oil-sands, or coming up faults and joint-cracks, frequently seeps through the surface soil, forming pools which are sometimes of considerable size.

Likewise films of oil are often seen on the surface of water in swamps, wells, and springs. The surface of the sea is sometimes similarly overspread for miles.

Pitch.-Pitch is one of the commonest indications of oil. Extensive deposits occur in Mexico, California, Venezuela, Trinidad, West Africa, Borneo, Russia, etc. The composition of the pitch depends on that of the oil it is derived from, and also varies according to the extent to which evaporation and oxidation have advanced. Many oil-sands near the surface contain sticky pitch, and spreads of pitch mixed with sand often cover the outcrops and occupy the bottoms of ravines into which they have flowed. Generally the pitch appears on the surface as numerous cones, isolated or in groups. Usually the cones are only a few feet or less in height, but in some places there are pitch hills of 50 feet or more. Single cones are symmetrical, and on the outside may be hard and dry, but at the apex is a little crater from which flow takes place. When cut open it is seen that the cone is built upon the surface soil above some crevice, up which soft sticky pitch and thick heavy oil is slowly ascending through the ground, and thence up the central pipe of the cone to the apex.

In country covered with thick vegetation the outcrops of oil-sands can be traced by the occurrence of pitch and the approximate course of an anticline may be followed for miles. Pitch also occurs at submarine outcrops, for soft fresh pats of pitch are washed up ashore abundantly on the coast of petroliferous country. The extensive deposits of pitch at present on the earth's surface bear witness to the immense quantities of petroleum which have escaped into the atmosphere in recent times.

The best known individual and uninterrupted pitch spread in the world is the Trinidad Pitch Lake with an area of about a quarter of a square mile, but there are more extensive areas covered with pitch in Venezuela. Estimating the total contents of the Pitch Lake at ten million tons, it follows that some forty million tons of oil must have evaporated at that one site alone.

Bitumen in Fissures. Fault fissures, joint-cracks, and cavities among rocks above or associated with petroliferous strata, are sometimes filled with bitumen derived from the oil. The bitumens derived from asphaltic oils differ from the pitch which is formed from the same oil at the surface, being hard, black, lustrous substances which break with conchoidal fracture. One of the best known of the many different kinds is manjak, which is mined in Trinidad and Barbadoes.

Limestones, sandstones, and conglomerates are often heavily charged with asphaltic bitumen.

The bitumens derived from paraffin oils are soft waxy substances of pale colour, the most important being ozokerite, which is mined in the strata overlying the oil rocks at Boryslav in the Galician Oilfield, and also occurs in quantity on the island of Cheleken. This ozokerite occurs in fissures sometimes as lumps and cakes weighing several pounds, but the main production is obtained on treating the clay shales with hot water.

Gas.-Exudations of gas are numerous in most oilfields, ascending through the surface soil from outcrops and fractures. Generally they can be detected by the smell, and often also they can be ignited and may burn continuously. In wet ground the noise of the gas bubbling up through pools of water assists detection. Occasionally violent outbursts have been known to blow out cavities in the rocks.

Sulphur.-Sulphuretted hydrogen and sulphur-dioxide frequently accompany the escaping gas, and deposits of sulphur in cracks in the rocks or on the surface are not uncommon indications of hydrocarbons below.

Sandstones in Peru, and thick limestones in Texas and Louisiana, are impregnated with sulphur to such an extent that when they are heated melted sulphur runs out. Hot sulphurous springs of water are of frequent occurrence in oil territory, and often when a well approaches exhaustion sulphur water ascends.

Salt. The complete relationship between salt and petroleum is not understood, but it is a conspicuous fact that salt is nearly always in some way associated with petroleum either as crystalline masses of rock-salt, or salt water, or saliferous strata. It is true that the conditions favourable for the preservation and storage of hydrocarbons are also those favourable for the presence of bodies of imprisoned sedimentation water, but the salt water which accompanies petroleum is usually much more saline than the water normally present at depth in sedimentary rocks, and suggests some further relationship.

Mud Volcanoes.-These occur in almost all parts of the world where oil-rocks approach the surface. They are built up over springs which exude mud, gas, and hot water often with a little oil. Sometimes

mud volcanoes form hills a hundred feet or even several hundred feet high, but most numerous are small cones rising only a few feet. Common also where much water accompanies the mud are volcanoes of larger area and very slight elevation. A typical example observed by the writer in Trinidad had a soft mud crater of 35 yards diameter, in the middle of which was a salt pool 12 yards across.

The noise of the great gas bubbles, which burst up every minute or two, could be heard two or three hundred yards away. The soft mud was surrounded by harder mud on which one might cautiously walk, the whole volcano forming a nearly flat, bare mud patch of 100 yards diameter, in the midst of thick forest. The volcano was surrounded by a little moat into which streams flowed from the crater. Outside this, sandy ground rose 5 or 10 feet, but was cut through in one place by a miniature gorge where the muddy salt water found egress from the moat as the source of a stream.

Sometimes mud volcanoes act with such violence as to throw up tons of clay and rock 20 or 30 feet into the air, and in two known instances small islands, hundreds of feet across, have been built up off the coast by submarine mud volcanoes situated on well-known anticlines. Mud volcanoes occur chiefly along the crests of anticlines or along fault lines.

CONCLUSION.

Petroleum or other bituminous matter is common in small quantity in sedimentary rocks of all ages, and in certain cases where the structure is favourable it is present in large amounts.

Some idea of the quantities of oil found imprisoned in the earth's crust is obtained from a study of oilfields' statistics. The world's annual output during the last few years has exceeded 40 million tons, whilst from only 6 square miles of country near Baku 11 million tons of oil were extracted in one year, and it is estimated that the oil which has been obtained from one 27 acre property there would fill a tank of that area to a depth of 270 feet.2 In a number of instances in various parts of the world the amount of oil yielded in one year by a single well has exceeded 100,000 tons. At the present time immense quantities of petroleum oil and gas are escaping naturally at the earth's surface almost wherever Tertiary strata have been upraised and exposed to denudation. This is a natural process of decarbonization; shallow-water sediments which probably have formed the main bulk of the marine deposits of every age originally contain a great amount of carbonaceous material disseminated in their muds, and this carbon is capable of undergoing chemical change into volatile hydrocarbons, which must be eliminated ere the composition of the rocks is stable.

Thus we do not now commonly find oil in the older rocks, except where they have been deeply buried and sealed up by newer formations practically undisturbed. Under such circumstances occur vast stores of oil in the Silurian, Ordovician, Devonian, and Carboniferous formations in the Appalachian fields of the United States. But in the majority of oilfields the oil-rocks are of Tertiary age.

The decarbonization of the freshly upraised Tertiary sediments is taking place in the earliest stage of their denudation, and long ere the next marine transgression has overlaid the area with new permanent deposits, it is likely that further denudation will have entirely obliterated all records of the process, leaving in the Tertiary as little indication of petroleum as in the old formations now. It is therefore not improbable that the older formations were as petroliferous as the Tertiary, and that the decarbonization process at present witnessed on the surface of the newly upraised sediments has been usual on similar occasions in the past; or, in other words, many of the old planes of discordance have been the scenes of phenomena similar to those described above.

1 A. Beeby Thompson, Petroleum Mining, 1910, pp. 102, 103.
2 Ibid., p. 86.

III. THE TACHYLITE OF THE CLEVELAND DYKE.

By Miss M. K. HESLOP, M.Sc., and R. C. BURTON, B.Sc., F.G.S.

THE

(PLATE IV.)

HE Cleveland Dyke is well exposed low down on the left bank of the River Tees, near the junction of this river and the Lune, being washed at flood-times by the water. It trends in a direction north of west, and can be traced for a distance of several hundred yards: its thickness is difficult to estimate as the north edge is covered by drift. The tachylite variety of the rock is only exposed for a few yards, and has only been found at this point-about 100 yards west of the junction of the Lune and Tees. The dyke here appears to occur in sheets very like successive lava-flows, and during cooling a columnar structure has been developed and the bases of the columns, mostly hexagons, face the river. The Cleveland Dyke at this exposure is represented by three varieties of rock

1. Ordinary porphyritic rock.

2. Amygdaloidal porphyritic rock.

3. Stony rock associated with tachylite.

It is with the third variety that this paper is concerned. The stony rock occurs as a layer 14 feet thick, dipping 10° E.S.E. at a point just opposite the junction of the two rivers, while a few feet higher up the Tees the dip changes to 10° W.N.W.; above and underneath it the ordinary variety of the dyke occurs, and the junctions are quite as sharp as between successive lava-flows.

The true tachylite occurs as a selvedge -inch thick, covering the stony rock on its southern face for a distance of several yards, and also seems to occur as veins in this variety. The latter mode of occurrence is remarkable, and suggests that just before consolidation part of the magma on the extreme edge of the dyke was injected into cracks in the very viscous layer adjacent to it, the temperature of which was low enough not to interfere with the almost immediate consolidation of the injected material as tachylite. This explanation, however, is put forward with a considerable amount of hesitation. In some parts of the exposure the stony rock forms an inner selvedge about 4 inches thick, while the tachylite covers this as a thin outer selvedge.

A few yards higher up the river there is an overflow of the dyke to the south. A black shale is found lying horizontally underneath the dyke for a distance of 11 yards; we could not determine whether the shale was greatly baked or not as the river water has softened the rock and altered it; the junction of the shale and the dyke is also difficult of access. This fragmentary section is, however, sufficient to point to the existence of a small overflow to the south, and this conclusion is supported by the occurrence of the igneous rock in sheetlike form, where the tachylite is found; lower down the river the ordinary dyke-form is resumed. The actual extent of the overflow, as seen, is 11 yards, but it is probably exposed over a greater distance. Its existence is interesting as it seems to prove that, for some reason possibly connected with the hade of the dyke, the injection of the molten magma has taken place with particular force to the south;