The fossil resins--often erroneously called gums--amber, kauri, copal, etc., though interestingly related to the hydro-carbons enumerated on the preceding pages, form no essential part of the series, and demand only the briefest notice here.
Amber is the resin which exuded from certain coniferous trees that, in Tertiary times, grew abundantly in northern Europe. The leaves and trunks of these trees have generally perished; but masses of their resin, more enduring, buried in the earth on the shores of the Baltic, have in the lapse of time changed physically and chemically, and have become fitted for the ornamental purposes for which they have been used by all civilized nations.
Kauri is the resin of Dammara australis, a living coniferous tree of New Zealand, and the "gum" is dug from the earth on the sites of forests which have now disappeared.
Copal is a commercial name given to the resins of several different trees, but the most esteemed, and indeed the only true copal, is the product of Trachylobium Mozambicense, a tree which grows along the Zanzibar coast, and has left its resin buried in the sands of old raised beaches which it has abandoned.
The diversity of character which the fossil resins exhibit shows the complexity of the vital processes in operation in the vegetable kingdom, and gives probability to the theory that some of the differences we find in the carbon minerals are due to differences in the plants from which they have been derived.
The variations in the physical and chemical characters of different coals from the same basin, and from different parts of the same stratum, have been sometimes credited to the same cause; but they are probably in greater degree due to the differences in the conditions under which these varieties have been formed.
Cannel coal, as I have shown elsewhere (Amer. Jour. Science, March, 1857), is completely macerated vegetable tissue which was deposited as carbonaceous mud at the bottom of lagoons in the coal-marshes.
Caking coals were probably peat, which accumulated under somewhat uniform conditions, was constantly saturated with moisture, and became a comparatively homogeneous and partially gelatinous carbonaceous mass; while the open-burning coals which show a distinctly laminated structure and consist of layers of pitch-coal, alternating with bands of mineral charcoal or cannel, seem to have been formed in alternating conditions, of more or less moisture, and the bituminous portions are inclosed in cells or are separated by partitions, so that the mass does not melt down, but more or less perfectly holds its form when exposed to heat.
The generalities of the origin and relations of the carbon minerals have now been briefly considered; but a review of the subject would be incomplete without some reference to the theories which have been advanced by others, that are in conflict with the views now presented. There have always been some who denied the organic nature of the mineral hydrocarbons, but it has been regarded as a sufficient answer to their theories, that chemists and geologists are generally agreed in saying that no instances are known of the occurrence in nature of hydrocarbons, solid, liquid, or gaseous, in which the evidence was not satisfactory that they had been derived from animal or vegetable tissue. A few exceptional cases, however, in which chemists and geologists of deserved distinction have claimed the possibility and even probability of the production of marsh gas, petroleum, etc., through inorganic agencies, require notice.
In a paper published in the Annales de Chimie et de Physique, Vol. IX., p.481, M. Berthelot attempts to show that the formation of petroleum and carbureted hydrogen from inorganic substances is possible, if it be true, as suggested by Daubre, that there are vast masses of the alkaline metals--potassium, sodium, etc.--deeply buried in the earth, and at a high temperature, to which carbonic acid should gain access; and he demonstrates that, these premises being granted, the formation of hydrocarbons would necessarily follow.
But it should be said that no satisfactory evidence has ever been offered of the existence of zones or masses of the unoxidized alkaline metals in the earth, and it is not claimed by Berthelot that there are any facts in the occurrence of petroleum and carbureted hydrogen in nature which seem to exemplify the chemical action which he simply claims is theoretically possible. Berthelot also says that, in most cases, there can be no doubt of the organic origin of the hydrocarbons.
Mendeleeff, in the Revue Scientifique, 1877, p. 409, discusses at considerable length the genesis of petroleum, and attempts to sustain the view that it is of inorganic origin. His arguments and illustrations are chiefly drawn from the oil wells of Pennsylvania and Canada, and for the petroleum of these two districts he claims an inorganic origin, because, as he says, there are no accumulations of organic matter below the horizons at which the oils and gases occur. He then goes into a lengthy discussion of the possible and probable source of petroleum, where, as in the instances cited, an organic origin "is not possible." It is a sufficient answer to M. Mendeleeff to say, that beneath the oil bearing strata of western Pennsylvania are sheets of bituminous shale, from one hundred to five hundred feet in thickness, which afford an adequate, and it may be proved the true source, of the petroleum, and that no petroleum has been found below these shales; also that the oil-fields of Canada are all underlain by the Collingwood shales, the equivalent of the Utica carbonaceous shales of New York, and that from the out-crops of these shales petroleum and hydrocarbon gases are constantly escaping.
With a better knowledge of the geology of the districts he refers to, he would have seen that the facts in the cases he cites afford the strongest evidence of the organic origin of petroleum.