dip very gently (1°-2°) toward the center of the basin along its southern margin, more steeply around the west end, and at even greater angles on the north side, along the Uinta Mountains. So. far as known the only exception to this general structure is near the head of Hill Creek, where there is evidence of a low dome that causes the outcrop of the oil-shale beds to bend considerably toward the south.

The deformation by which the basin was formed occurred after the deposition of the Green River formation, which contains the oil shale, but before that of the succeeding Bridger formation. There have been other earth movements, however, even since the youngest sedimentary formation (Uinta) of the basin was deposited, as is shown by the presence of great cracks or fissures filled with hydrocarbon material. The origin of these cracks is a disputed question, but the fact that there is neither vertical nor lateral displacement along the fissures and also the fact that their direction bears no consistent relation to the strike of the beds suggest that the cracks are the products of tension, produced perhaps by cooling or drying of the sediments. Where the fissures cut formations that are largely sandstone they present clean-cut walls, but where they cut the beds of oil shale (Pl. VI, B, p. 31), as near Watson, there is no break filled with hydrocarbon, but the movement was apparently taken up by the shale without fractures.

The deformation of the oil-shale beds of the Uinta Basin is not sufficient, except in very small areas, to hinder materially the mining of the beds of shale by methods used in mining horizontal coal beds, and in many places the surface of the land has the same attitude as the underlying beds and the overburden to the rich shale beds is thin, so that steam-shovel mining can be practiced without difficulty.

THE OIL SHALE.

GENERAL CHARACTER.

Good oil shale is black or brownish black except upon weathered surfaces, where it is bluish white or white. It is fine grained, slightly calcareous, and as a rule free from grit. It is tough and in thinbedded deposits remarkably flexible. (See Pl. X, B.) The flexibility of oil shale distinguishes it from ordinary carbonaceous shale, which is brittle. Although oil shale is made up of thin laminae, this is not apparent in some specimens until after the rock has been heated and the oil driven off. Freshly broken oil shale gives off a peculiar odor similar to that of crude petroleum, although the shale contains little oil as such or bituminous matter that is soluble in ether, chloroform, carbon bisulphide, or other solvents of bitumen. Experiments by the writer have shown that from a sample of shale which yielded by distillation oil equivalent to about 14 per cent

4. OIL SHALE NORTHEAST OF WATSON, UTAH.

Shows thin bedding in this part of the Green River formation. Darker bands are richest beds. About 600 feet of rock exposed.

of its weight it is possible to extract with carbon bisulphide oil at the rate of only about 1 per cent of the original shale-in other words, less than 10 per cent of the oil that can be distilled off is extractable with carbon bisulphide. Oil shale contains a large amount of carbonaceous matter (largely remains of lower plants, including the algae), which is probably the ultimate source of the products of distillation. Thin splinters of oil shale will burn with a very sooty flame and give off an asphaltic odor when ignited with a match. Oil shale is heavier than coal, having an average specific gravity of about 1.60,1 and the richer shales are lighter in weight than the leaner ones. Analyses of oil shale of the Green River formation show an average ash content of about 60 per cent; six samples having an average yield of 41 gallons of oil to the ton of shale (8.4 to 86.8 gallons) contained an average of 59.83 per cent of ash (45.73 to 79.0 per cent). As the ash content in good coal is less than 10 per cent, oil shale can not be profitably used directly for fuel. The shale to be used must be mined like coal, then crushed and distilled in huge retos, giving off crude shale oil, ammonia, and fuel gas as the valuable products.

RESULTS OF TESTS.

2

During the field work samples of the shale were taken at numerous places where the Green River formation crops out along the south side of the Uinta Basin in Utah. Most of the samples represent beds of shale including good oil shale having a minable thickness, but several samples taken for study represent thin beds of rich shale and thick beds of lean shale. The distillation tests were made with the crude apparatus described in a previous report, and the results are only approximate, but they afford a rough estimate of the value of the shale as a source of oil. The variations in temperature obtained in the small field apparatus probably affect the quality more than the quantity of the oil. Distillation without the injection of steam probably makes the quantity of ammonium sulphate produced much less than may be obtained in commercial practice, in which steam will be injected into the heated retorts; therefore the results of the ammonium sulphate determinations are much less complete and reliable than the oil determinations. In the field the uncondensed inflammable gas was not measured, but former tests of similar shale show that the distillation of oil shale will produce a large amount of combustible gas that may be used for fuel. According to Cadell

Woodruff, E. G., and Day, D. T., Oil shale of northwestern Colorado and northeastern Utah: U. S. Geol. Survey Bull. 581, p. 3, 1914.

* Winchester, D. E., Oil shale in northwestern Colorado and adjacent areas: U. S. Geol. Survey Bull. 641, pp. 152-153, 161, 1916.

Idem. pp. 148-150.

'Cadell, H. M., Scottish shale industry: Petroleum World, vol. 10, p. 230, 1913.

21544°-19-Bull. 691-4

the gas produced in the distillation of oil shale in Scotland is sufficient to furnish all the heat necessary for the retorting of the shale.

An examination of the following table of results of distillation shows that a minable thickness of good shale is present at nearly every place examined along the south side of the Uinta Basin in Utah: Results of distillation of shale from Uinta Basin, Utah.