berry, however, states that in the Bay of Marquette, where the shore consists of peat overlying Archean rocks, bubbles of marsh gas arise, together with drops which cover the surface of the water, in spots, with an oily film. The following investigations seem to bear upon the problems suggested by these observations:

In 1899 A. F. Stahl and, independently, G. Krämer and A. Spilker called attention to a possible derivation of petroleum from diatoms, which abound in certain bogs. These organisms, according to Krämer and Spilker, contain drops of oily matter, and from diatomaceous peat a waxy substance, resembling ozokerite, can be extracted. The theory, based upon these data, is briefly as follows: A lake bed becomes filled in time with diatomaceous accumulations, over which a cover of other growths or deposits is formed. By decay of the organic substances, ammonium carbonate is produced, which hydrolyzes the wax, and from the resulting acid carbon dioxide, carbon monoxide, and water are gradually eliminated. Ozokerite is thus formed, which, at moderate temperatures and under pressure, becomes converted into liquid petroleum. With higher temperatures and pressures, in presence of sulphur, heavier oils and asphalt may be generated. In support of this hypothesis the authors describe a lake bed, near Stettin, which is about 23 feet thick and consists chiefly of diatoms. This deposit yields a wax containing over 10 per cent of sulphur, and from it a hydrocarbon, resembling the lekene from ozokerite, was isolated.

Krämer and Spilker's views have not met with very general acceptance, but they seem to contain elements of value. H. Potonie's hypotheses, for example, seem to be a broadening of Krämer and Spilker's. This writer calls attention to the "faulschlamm "sapropel," a slime, rich in organic matter, which is formed from gelatinous algæ, and accumulates at the bottom of stagnant waters. Such a slime, Potonié believes, may be the parent substance from which bitumen, by a process of decay, was probably derived. In this connection, and with reference to the adequacy of the proposed source, it is well to remember the enormous accumulation of "oozes," namely, the radiolarian and globigerina oozes, on the bottom of the sea. The organic matter thus indicated is certainly abundant enough. if it can decay under proper conditions, to form more hydrocarbons than the known deposits of petroleum now contain.'

• Ann. New York Acad. Sci., vol. 2, p. 277, 1882.

Chem. Zeitung, vol. 23, p. 144, 1899. Also note in vol. 30, p. 18, 1906.

Ber. Deutsch. chem. Gesell., vol. 32, p. 2940, 1899; vol. 35, p. 1212, 1902. Criticism by Engler in vol. 33, p. 7, 1900.

d See also C. E. Guignet, Compt. Rend., vol. 91, p. 888, 1880, on wax from peat. Natur. Wochenschr., vol. 20, p. 599, 1905.

These oceanic sediments are especially noticed by Engler in a paper read before the petroleum congress in 1900 (Cong. internat. du pétrole, Paris, 1900, p. 28). In A. Beeby Thompson's monograph, The Oil Fields of Russia, pp. 85-87, London, 1904, a theory is developed to account for the probable formation of bitumens on the sea bottom. Thompson regards fish remains as an important source of supply.

These remarks upon the oceanic sediments at once suggest an intermediate group of hypotheses, which assume a mixed origin for petroleum. Animal matter in some cases, vegetable matter in others, or both together, are supposed to be the initial source of supply. A. Jaccard, for example, argues that the liquid oils are derived from marine plants, while the viscous or solid bitumens may originate from mollusks, radiates, etc. Some oils, again, are supposed to be of mixed origin, and it would seem probable that the last class is the most common. Ideas of this kind have repeatedly been enunciated with reference to American petroleums-that of Pennsylvania being attributed to marine vegetation, that of California to animal remains. The American literature of petroleum is rich in suggestions of this order.

It has long been known that some petroleums are optically active; that is, they are able to rotate a ray of polarized light, sometimes to the right and sometimes to the left. This, according to P. Walden, gives us an important datum toward determining the origin of petroleum. Only the oils derived from organic matter, Walden asserts, can possess this property; the hydrocarbons prepared from inorganic materials, such as metallic carbides, being optically inert. The oils distilled from coal, which is evidently of vegetable origin, are active; and petroleum, which has the same peculiarity, is presumably formed from similar materials. These conclusions are exceedingly important, but need to be more thoroughly tested before they can demand universal acceptance. The presumption, however, is strongly in their favor.

In any attempt to discover the genesis of petroleum the quantitative adequacy of the proposed sources must be taken into account. In such an inquiry superficial observations are deceptive, for one is apt to overrate the visible and productive accumulations which furnish the oils of commerce. These seem large, but they are relatively insignificant. As Orton has said, disseminated petroleum is wellnigh universal; the accumulations are rare. In certain districts the shales and limestones are generally impregnated with traces of

Eclog. Geol. Helvet., vol. 2, p. 87, 1890. See also Arch. sci. phys. nat., 3d ser., vol. 23, p. 501, 1890; vol. 24, p. 106, 1890. Jaccard studied especially the bitumens of the Jura.

In addition to the memoirs already cited, see the reports of the Second Geol. Survey Pennsylvania. Also J. A. Bownocker, Geol. Survey Ohio, 4th ser., Bull. No. 1, 1903; S. S. Gorby, Sixteenth Ann. Rept. Indiana Dept. Geol. and Nat. Hist., 1888; W. S. Blatchley, idem, Twenty-eighth Ann. Rept., 1904; E. Haworth, Univ. Geol. Survey Kansas, vol. 1. p. 232, 1896; H. P. H. Brumell, Geol. Survey Canada, new ser., Ann. Rept. 5, Q, 1893; and W J McGee, Eleventh Ann. Rept. U. S. Geol. Survey, pt. 1, p. 589, 1891. L. Harperath (Bol. Acad. nac. cien. Córdoba (Argentina), vol. 18, p. 153, 1905) has published a long memoir on petroleum and salt.

Chem. Zeitung, vol. 30, pp. 391, 1155, 1168, 1906. Walden cites many examples of this optical activity. See also a criticism by Engler, idem, p. 711, and a paper by Rakusin, p. 1042. Another memoir, by J. Marcusson, appears in vol. 31, p. 419, 1907.

First Ann. Rept. Geol. Survey Ohio, chapter 11, 1890; Geol. Survey Kentucky, Report on Occurrence of Petroleum, etc., 1888-89.

bitumens, which seem at first sight to be insignificant, but which really represent enormous quantities. In the Mississippian ("subCarboniferous") limestones of Kentucky petroleum is generally present. If it amounts to only 0.10 per cent, each square mile of rock, with a thickness of 500 feet, would yield about 2,500,000 barrels of oil. Even more striking are the figures given by T. Sterry Hunt,a who estimates that in the limestone of Chicago, with a thickness of 35 feet, there are 7,743,745 barrels of oil to each square mile of territory. Figures like these, together with the computations, previously cited, made by Szajnocha relative to Galician petroleum, lead to the conviction that the formation of bitumens is a general process and by no means exceptional. Wherever sediments are laid down, inclosing either animal or vegetable matter, there bitumens may be produced. The presence of water, preferably salt, the exclusion of air, and the existence of an impervious protecting stratum of clay seem to be essential conditions toward rendering the transformation possible. Seaweeds, mollusks, crustaceans, fishes, and even microscopic organisms of many kinds may contribute material to the change. In some cases plants may predominate; in others animal remains; and the character of the hydrocarbons produced is likely to vary accordingly, just as petroleum varies in different fields. In one region we find chiefly paraffins, in another naphthenes, and in another nitrogenous or sulphureted oils. Such differences can not be ignored, and they are most easily explained on the supposition that different materials have yielded the different products. On this class of problems the chemist, the geologist, and the paleologist must work together. Physics also is entitled to be heard; for, as D. T. Day has shown, petroleum, by simple filtration through fuller's earth, can be separated into fractions which differ in density and viscosity and are therefore of different composition. Such a filtration, or, more precisely, diffusion, must take place in nature wherever migrating hydrocarbons traverse permeable strata.

By whatever class of reactions petroleum is generated, it doubtless appears first in a state of dissemination. How does it become concentrated? This question does not fall within the domain of chemistry, and can not be properly discussed here. Probably circulating waters have much to do with the process, but whatever that may be the laws governing the motion of liquids must inevitably rule. The oils must gather in proper channels, moved by gravitation, or by hydrostatic pressure of waters behind or below them, or by the pres sure of dissolved and compressed gases, and they accumulate in

• Chemical and Geological Essays, p. 168, 1875.

'Cong. internat. du pétrole, Paris, 1900, p. 53.

For a discussion of this problem, see H. Höfer, Das Erdöl, p. 223, 1906. Also G. I. Adams, Trans. Am. Inst. Min. Eng., vol. 33, p. 340, 1903; and D. T. Day, idem, p. 1053. Orton's reports, previously cited, contain important contributions on this theme.

porous rocks or cavities under layers of impervious material. When the latter are lacking, or when the hydrocarbons enter large areas of porous rocks, they may be either evaporated or rediffused. Pressure, temperature, viscosity, and the character of the surrounding rocks must all be taken into account, and each productive area needs to be studied independently with reference to its local conditions.

In conclusion, I may be allowed to suggest that nearly all of the proposed theories to account for the origin of petroleum embody some elements of truth. Sokoloff's cosmic hypothesis is sustained by the fact that hydrocarbons are found in meteorites. The volcanic hypothesis is sustained by the fact that hydrocarbons occur among volcanic emanations. The organic origin of petroleum, however, seems to be best supported by the geologic relations of the hydrocarbons, which are found in large quantities only in rocks of sedimentary character. Any organic substance which becomes inclosed within the sediments may be a source of petroleum, and when the latter happens to be rich in nitrogen, animal matter was probably the initial material. There is no evidence to show that any important oil field derived its hydrocarbons from inorganic sources.

14399-Bull. 330-08-41

CHAPTER XVII.

COAL.

ORIGIN OF COAL.

Although doubts may exist as to the origin of petroleum, there are none whatever as to the essential origin of coal. It is obviously derived from vegetable matter, by a series of changes which are plainly traceable, even though their mechanism is not fully understood. Vegetation, peat, lignite, soft coal, anthracite, and some graphitic minerals form a series of substances which grade one into another in an unbroken line, reaching from complex organic, oxidized compounds at one end to nearly but not quite pure carbon at the other. All of these bodies, except perhaps the last, are indefinite mixtures which vary in composition, and it is therefore impracticable to write chemical equations that shall properly represent their transformations. Such equations, to be sure, have been suggested and written, but they embody fallacies which are easily exposed. They start from the assumption that the principal initial compound contained in vegetation is cellulose, a definite carbohydrate of the formula CHO, which gradually loses carbon dioxide, marsh gas, and water. and so yields the series of products represented by the different kinds of coal. This assumption, like most other assumptions of its class, is partly true and partly false. Cellulose is an important constituent of vegetable matter, but it stands by no means alone. When it decays, it loses the substances named above and it also undergoes other changes which are difficult to measure. In every swamp or peat bog the waters are charged, more or less heavily, with soluble organic matter of which the written reactions take no account. This soluble matter is found in the waters of all bogs and streams, and it is just as much a factor in the real reactions as are the gaseous products or the solid carbonaceous residues.

If, instead of the composition of cellulose, we begin with the composition of wood, we shall have a better starting point for our series of derivatives. Wood or woody fiber is by no means the only substance to be considered, but it is the most important one, and its ultimate composition has been well determined. Its proximate composition is not so clearly known, but certain available facts are per

a The formula C6H10Os represents only the empirical composition of cellulose, and not its true molecular weight. According to A. Nastukoff (Ber. Deutsch. chem. Gesell., vol. 33, p. 2237, 1900), the true formula is probably 40CH1005, or C240H4000200- This may be an exaggeration, but the molecular weight of cellulose is certainly high.