Зображення сторінки

nothing but decomposition products are found in the distillate, while coke remains in the still. These decomposition products are very varied. Those that are geologically old yield paraffine, while those that are recent do not.

Prof. Mabery has remarked that all petroleums contain the same proximate principles in different proportion. While this statement may be absolutely true, it is not so relatively. The palæozoic bitumens have been most carefully studied and they consist mainly of paraffines. The Tertiary bitumens have been less carefully studied, and they consist principally of benzoles and their derivatives in great variety. Mingled with these are the olefines and other series of hydrocarbons in small proportion, with an immense number of oxygen, sulphur and nitrogen derivatives and substitution compounds, the existence of which has been only recently suspected.

It can, therefore, be asserted that the natural bitumens and the substances resembling them that are obtained by the destructive distillation of mineral and organic substances, are strikingly similar. The palæozoic bitumens bear a resemblance to the simple distillates produced in the presence of steam, at low temperatures, when nitrogen is practically absent. The Tertiary bitumens resemble the distillates obtained at higher temperatures and when the raw material is rich in animal remains. There are, however, a large number of bitumens that have been too little investigated to admit of any generalizations concerning them. In illustration of this statement I would call attention to the valuable papers of Prof. Henry Wurtz, in which he shows that many so-called native paraffines are probably olefines. I would suggest that some of them may be the higher naphtenes, that have the same percentage composition as olefines. The solution of these problems awaits a vast amount of research.

14. In the preceding pages I have given an outline of the views generally held by chemical and physical geologists concerning the chemical phenomena attending the cooling of the earth and its shrinking and contracting crust. To these I have added a résumé of the technical and chemical knowlege we possess concerning bitu

I shall now proceed to discuss, in the light of these facts, 1S. F. Peckham and L. A. Linton, Amer. Jour. Sci. (4), i, 193. S. F. and H. E. Peckham, Jour. Soc. Chem. Industry, xvi, 424; H. Endemann, ibid. xv, 871; xvi, 121.

2 H. Wurtz, Eng. and Min. Jour., xlviii, 25, 114; li, 326, 376.



This is a pure

the occurrence of bitumens and the relation of such occurrence to their probable origin.

Leaving the problems of orography to the physical geologist for solution, there are a few suggestions to be made relating to these problems that I have not seen anywhere mentioned. If we regard the dizzy heights of the Andes and Himalayas, or the profound abysmal depths of the Pacific as isolated phenomena, they appear on a scale of oppressive grandeur and immensity; yet these irregularities in the earth's crust reach a maximum of only about ten miles in vertical height, which is only one twenty-five hundredth or four hundredths per cent. of the circumference of the earth at the equator. The local foldings of a few hundreds of feet in disturbed strata are microscopic when compared with the earth's diameter; and yet we are accustomed to regard these plications of strata as the result of sudden movements in the earth's crust. assumption. The period of time through which critical observations of geological phenomena have been made when compared with the time that has elapsed since life dawned upon the earth is also microscopic; it is a smaller fraction than four hundredths per cent.

The element of time in geological phenomena is only just beginning to be appreciated. We have learned from a few years of observation that some continental masses are rising and others falling with reference to the sea level ; yet no one has observed these movements through many centuries, nor have these vertical movements of the coasts of the world been corelated and the laws that govern such movements been determined. We do not know whether a continent has emerged from an ocean maintaining a constant level, or whether the ocean has receded as the contracting mass has rendered the ocean depths more profound, or, as is more probable, the shrinking of the crust has changed the distance of the ocean surface from the centre of the earth, rendering the elevations apparently greater.

It is not material to this question that we should know. Nor is it of importance to consider whether the continued operation of forces at present active through countless centuries, or the repeated interjection of cataclysms of world disaster, has brought the earth to its present condition. Volcanic eruptions, earthquakes and floods, separately and unitedly, change the face of nature within our own generation; it is reasonable to suppose that they have acted from the earliest period of the earth's history to the present time with constantly lessening vio

lence. It is true that the local effects of such phenomena as the earthquakes at Lisbon and Java and the Red River fault appear cataclysmic; yet these effects are microscopic when compared with the dimensions of the eartii, and may have been, nay, probably were the culmination of a series of movements that had been in progress for immense intervals of time. I therefore believe that in stating the causes of those changes that have taken place at the surface of the earth as we now know it, one of the most important considerations is the unlimited periods of time through which the pressure due to accumulation of sediments and the consequent development of heat has acted upon those sediments, which in many instances were filled with water charged with mineral matter in solution. From the combined action of pressure, heat and steam, through unlimited periods of time, the constituent elements of sediments have been brought into every possible state of combination, from obsidian and pumice, which have been completely fused, through lavas, granites, gneisses, etc., to sediments in which there has been no change at all. As Dr. Hunt has fully shown, the action of thermal waters, which have been largely instrumental in producing these changes, has been often extremely localized both laterally and vertically, and may be greatly varied by the constituents of the sediments themselves.

15. If, then, we accept the hypothesis that all of the rocks as we now know them are sediments, whatever may be their present condition, we are forced to the conclusion that life first appeared upon the planet at a date too remote to be determined even in geologic time, and that the remains of organic forms have practically been a constant constituent of sediments from that time to the present. As might be expected, we find organic remains in every possible condition, from crystallized graphite to unaltered cellulose. Vegetable and animal remains are found in every conceivable condition of replacement and alteration. We find pseudomorphism in the strictest sense as well as metamorphic action developed in every possible degree. Nor can we assert that any of the older strata are free from such action, for metamorphism is, as the word signifies, a change of form, and no limits can be assigned to such change in either time, place or degree that are not arbitrary. There can be no question that as sediments have accumulated slowly so these changes have progressed slowly.

Nevertheless, following upon long periods of quiet, there appear to have been periods of cataclysmic violence, as when the vast lava

sheets that form the table mountain of the Sierra Nevada were poured out, not from a single peak, but from a whole range of peaks; when the whole of southern Colorado and northern New Mexico and Arizona were covered with lava sheets thousands of square miles in extent; or when the valleys of West Virginia were upheaved, the Oil Break formed and the mass of plastic grahamite forced into the fracture; or when the basic rocks that form the mounds of iron porphyry in Cumberland and Foster, R. I., were thrust up from the deeps; and the trap dykes along the whole eastern borders of the Alleghanies were poured into fractures of local extent. But these convulsions that have brought basic porphyrys, basalts, trap dykes and local metamorphism to the surface, have in the physical and chemical operations of nature produced anthracites and anthracitic residues and not bitumens. Bitumens are not the product of the violence of volcanic or cataclysmic action, but of the gentler action of normal metamorphism exerted through long periods, during which the volatile bitumen has been distilled from sediments containing organic matter, and at the lowest possible temperature, without regard to time, as the sediments were pressed down to an isothermal that admitted first of their distillation and then of the conversion of the carbon residues into graphite.

16. Dr. Hunt has left hundreds of pages in which he has shown that the crystalline and eruptive rocks, as we know them, are altered sediments. His argument is conclusive that the carbon that they contain is derived from organic forms. When discussing bitumens he shows, first, that the pyroschists do not, except in rare instances, contain bitumen, and are not in the proper sense of the word bituminous. Secondly, he shows that the pyroschists do not, “whether exposed at the surface or brought up by boring from depths of many hundred feet, present any evidence of having been submitted to the temperature required for the generation of volatile hydrocarbons." Thirdly, he shows that as the oil occurs in the limestone it could not have been distilled. He further shows that the Utica slate that is beneath the lower Devonian limestones is unaltered, and adds, “More than this, the Trenton limestone, which on Lake Huron and elsewhere has yielded considerable quantities of petroleum, has no pyroschists beneath it, but on Lake Huron rests on ancient crystalline rock with the intervention only of a sandstone devoid of organic or carbonaceous matter.

1 T. S. Hunt, Essays, p. 169, ed. 1875.

I have already shown ($ 6) that sediments become crystalline at very low temperatures and that the crytalline schists below the lowest stratified rocks contain abundant evidences of organic forms. Are we to suppose that there was no intermediate zone in which normal metamorphism died out and faded into unaltered sediments ? We ought to expect to find the pyroschists in their normal condition. We ought to expect to find the coal altered or unaltered, according to its proximity to the heated area. We should not expect to find the carbonized remains of organic forms in rocks containing bitumen; for we cannot suppose that those beds that yielded the bitumen by distillation were suddenly plunged into a condition of igneo-aqueous fusion by which the organic constituents were instantly converted into anthracite and gas. As a general rule the process of conversion must have been as gradual as the progress of deposition. We cannot assume that in every instance the anthracite is the residue from a distillation of which the distillate was completely lost. Moreover, the example cited in $ 7 is a complete demonstration, occurring as it does in a region rich in bitumen, that the change from sediments to crystalline schists is progressive and involves the organic as well as mineral constituents of the strata.

17. If a traveler should leave Boston, Mass., and travel in a generally southwest direction toward San Diego, in southern California, he would encounter along his route a series of object lessons that would lead to but one conclusion. Whatever the age of the crystalline rocks of New England may be, they are certainly for the most part older than the Carboniferous. The small basin around Mansfield, Mass., extending into Rhode Island, which contains the anthracites of that region, is surrounded by crystalline rocks, and, indeed, the anthracite beds themselves are, as already stated, altered to a substance nearer graphite than coal. The coal slates contain only impressions of coal plants, and fossils of any description are extremely rare in the vicinity. Intrusions of trap are frequent, and cones of highly basic porphyrys are thrust up through all of the crystalline sediments at several points. The change of form has been very complete in respect to every constituent of the sediments.

Westward around New Haven, Conn., the bedding of the sediments has not been so completely obliterated, but the change in the organic constituents has been quite as general. In the gneissoid

« НазадПродовжити »