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nomena of rock building. Mineral silicates were then supposed to have crystallized from igneous fusion, and the deposition of sediments to have resulted only in amorphous, uncrystallized rocks. The idea that heat and water together may have produced all of the phenomena that have been attributed to the action of either alone has been of slow growth; but may now be said to be pretty generally accepted, although there are those who refer to the action of heat and of pressure alone phenomena that are without doubt properly the resultant of the action of heat and steam under pressure.
3. The discussions that have proceeded along the three lines of geology, chemistry and mineralogy, have been mainly directed to an elucidation of the problems relating to the formation of the crystalline rocks. To determine, therefore, the nature of metamorphic action and the conditions under which it might take place, was the problem to the solution of which Bischof, Hunt, Delesse, Daubrée and several others of the most gifted chemical geologists of this century devoted themselves. These gentlemen first considered the reactions that according to known chemical laws must follow the cooling of a heterogeneous mixture of the elements composing the earth, in a state of gaseous fluidity, and at a temperature that rendered chemical combination impossible; in other words, a state of complete dissociation. It follows that the most infusible elements would first condense and form a solid nucleus around which would float an ocean, in a state of igneous fusion, of more fusible elements and compounds, while over all would hover an atmosphere containing all the nitrogen and oxygen, the free hydrogen, sulphur and allied elements, with the chlorine and other halogens. As the cooling proceeded the silicon would combine with oxygen and bases, forming both acid and basic silicates, which would constitute a solid crust. The hydrogen and haloids combining would form the haloid acids and the sulphur and allied elements would form oxygen acids, all the hydrogen being oxidized into water, which with the acids would be alternately condensed and evaporated, falling as an acid rain upon the surface of silicated rocks, which in turn
i G. Bischof, Chem. and Phys. Geology, Cav. Soc. ed. T. S. Hunt, Chem. and Geolog. Essays. Delesse, “ Essay on Pseudomorphs,” Ann. des Mines, xii, 509; xiii, 393, 415; xvi, 317–392. Mem. Acad. de Scien. de France, xvii. Daubrée, Comptes Rendus de l'Acad., November 16, 1857. Études et experiences synthétique sur le Métamorphisme, Paris, 1859.
would emerge from the ocean of water heavy with dissolved chlorides and sulphates, while an atmosphere dense with carbonic acid
rould help to maintain a temperature that would retard the cooling through vast cycles of geologic time, in the course of which, under conditions entirely different from any now known, vegetable and animal life would appear upon the earth, or, more properly, in the waters that covered the earth.
4. It is very evident that the chemical conditions obtaining in this remote geologic epoch, while not incompatible with the development of life, were, however, very different from those which have prevailed at any time since the advent of any of the higher forms of animals. We have a right to believe that at the dawn of life, of all the elements that enter into the composition of vegetable and animal tissue—carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur-nitrogen alone was wholly free. Carbon and hydrogen existed in combination with oxygen as carbonic acid and water. Phosphorus and sulphur were oxidized, and in combination with basic elements as salts. The excessive proportion of carbonic acid and aqueous vapor in the atmosphere gave to it the property of transcalesence, by which, while readily penetrated by heat from the sun, it refused to transmit this heat when reflected from objects at the earth's surface. This gave to the atmosphere properties similar to those of a greenhouse, by which so high a temperature was maintained during the coal period that semitropical plants flourished at the poles. At an earlier period, before terrestrial vegetation had removed the carbonic acid from the air, and before the surface of the cooling earth had lost its heat by radiation, the palæozoic (dawn of life) ocean and the land gave support to both vegetable and animal life, at a temperature that at the present time would destroy most organic forms.
5. The strata which form that portion of the earth's crust which has been referred to the palæozoic era, are of enormous thickness and are found in different parts of the world, to present aspects strikingly similar. Messrs. Hall, Billings and Dawson, in North America, Salter and Hicks in England, Angelin in Sweden, and Barrande in Bohemia, have shown that the forms of animal life in that early period were very closely related, if not identical, in these widely separated areas; yet, below these formations, which hold the
1 W. H. Brewer, Am. Jour. Sci. (2), xli, 389.
remains of marine animals, in Bohemia and Sweden if not elsewhere, there is a “region of fucoids," of great thickness, carrying back the dawn of vegetable life to a still more remote epoch.' Throughout the last fifty years, successive discoveries of fossils in strata hitherto supposed to be destitute of organic remains, have carried the apparent dawn of life back through successive geological formations, until the azoic (devoid of life) rocks have ceased to be appropriately named, and Mr. Hicks, speaking of the Cambrian fauna of Wales, says, “ Though animal life was restricted to these few types, yet at this early period the representatives of the different orders do not show a very diminutive form, or a markedly imperfect state ; nor is there an increased number of blind species. The earliest known brachiopods are apparently as perfect as those which succeed them ; and the trilobites are of the largest and best developed types. The fact also that trilobites had attained a maximum size at this period, and that forms were present representative of almost every stage of development, .... blind genera along with those having the largest eyes, leads to the conclusion that for these several stages to have taken place numerous previous faunas must have had an existence, and, moreover, that even at this time in the history of our globe an enormous period had elapsed since life first dawned upon it."'3
6. The formations that contain these earliest palæozoic forms of life are now found for the most part in a crystalline condition ; yet, Dr. Hunt affirms, “that the oldest known rocks are stratified deposits of limestone, clay and sands, generally, in a highly altered condition; ... it is, however, quite certain that the advent of life in these oldest fossiliferous strata was subsequent to the period of chemical reactions on a cosmic scale."
The manner in which these geological formations and parts of formations may have been rendered crystalline has been very exhaustively discussed by Dr. Hunt in his chemical and geological essays. He has shown
* James Hall, Paleontology of New York, Vol. iii, Introduction. Billings, Am. Jour. Sci. (2), xxxii, 232. Reports Geological Survey of Canada, v. d. Dawson, Canadian Naturalist, v. d. Reports Geological Survey of Canada, v. d. Salter and Hicks, Proc. Geol. Assoc., Quar. Four. Geolog. Soc., v. d. Angelin, Paleontologica Scandinavica. Barrande, Bul. Soc. Geol, de France (2), xvi, 529-545.
2 Hicks, Quar. Jour. Geol. Sow, May, 1872.
how fully his conclusions, based almost wholly on theoretical considerations, have been confirmed by the experiments of Daubrée, who was led to investigate this subject, from observing that the action of the alkaline, thermal waters of the spring at Plombières, at a temperature of 60°-70° C., had in the course of centuries given rise to the formation of zeolites and other silicated minerals among the bricks and cement of the old Roman baths.' He further shows that at a temperature of 100° C. silicates are produced from a reaction between alkaline silicates and carbonates of lime, magnesia and iron. He says further, “Now the supposed mode of formation of the primitive molten crust of the earth would naturally exclude all combined or intermingled water, while all the sedimentary rocks are necessarily pervaded by this liquid, and are consequently in a condition to be rendered semifluid by the application of heat.
... If now, we admit that all igneous rocks, ancient plutonic masses as well as molten lavas, have their origin in the liquefaction of sedimentary strata we at once explain the diversities of their composition.
The presence of fossil plants in the melting strata would generate carburetted hydrogen gases, whose reducing action would convert the sulphurous acid into suphuretted hydrogen; or the reducing agency of the carbonaceous matter might give rise to sulphuret of calcium, which would be, in its turn, decomposed by the carbonic acid or otherwise. . .... The carburetted hydrogen and bitumen evolved from mud volcanoes, like those of the Crimea and Baku, and the carbonized remains in the moya of Quito, and in the volcanic matters of the island of Ascension, not less than the infusorial remains found by Ehrenberg in the ejected matters of most volcanoes, all go to show that fossili. ferous sediments are very generally implicated in volcanic phenomena. Again, he states, that in a letter to Sir Charles Lyell, dated February 20, 1836, Sir John F. W. Herschel maintains that with the accumulation of sediments the isothermal lines of the earth's crust must rise, so that strata buried deep enough will be crystallized and metamorphosed, and eventually be raised with their included water to the melting point.” Again Dr. Hunt says, “We conceive that the earth's solid crust of anhydrous and primitive rock is everywhere deeply concealed beneath its own ruins, which
1 Études et éxperiences synthétique sur le métamorphisme, par M. A. Daubrée, Paris, 1859, p. 98; Ann. des Mines (5), xiii, 227. ? Essays,
form a great mass of sedimentary strata, permeated by water. As heat invades these sediments, it produces in them that change which constitutes normal metamorphism. These rocks at a sufficient depth are necessarily in a state of igneo-aqueous fusion, and in the event of fracture of the overlying strata may rise among them taking the form of eruptive rocks." He calls the effects produced by such invasion of eruptive masses, local metamorphism. From these extracts from several of Dr. Hunt's essays, it can be easily understood that a struggle has been in progress from the time of the oldest known rocks to the present, between the shrinking and wrinkling crust of a cooling earth and the thickening deposits of sediment accumulating from its erosion.
7. One Sunday in the early summer of 1866, I found myself with Dr. George L. Goodale, now of Harvard University, stranded at a small hostelry, at the San Fernando Pass, near the old Mission of San Fernando, in southern California. The day was very fine and we chose a morning climb to anything the hostelry had to offer; so, mounting our horses, we rode to the eastward over the flood plain of pulverized rock that at some former period had poured out of the great cañon back of where the town of Burbank now stands. We climbed one of the spurs of the San Rafael range to the west of the cañon. We first passed over rounded hillocks of sandy soil which as we ascended became gradually merged into soft fossiliferous sandstone. After a time the effects of heat became manifest. The clam shells and fossil clams, of which there were cart-loads, appeared crystalline, and the iron in the sand was no longer green but red. The sandstones became more dense and the clays were silicated. At length the strata passed into a micaceous gneiss and finally we found the central core of the mountain to be a lightcolored fine-grained granite. About half way up, Dr. Goodale found a vertebra of a whale half buried in the sandstone and still very perfect in form, while I found a fossil pine cone that had evidently received some rough usage on the ancient beach. This cone contained some seeds that showed it to be closely allied to the nut pine of New Mexico. The mountain consisted wholly of Tertiary sediments that had been metamorphosed precisely as Sir J. F. W. Herschel had suggested in his letter to Sir Charles Lyell.
1 Essays, p. 9.
PROC. AMER. PHILOS, soc. XXXVII. 157. H.
PRINTED JUNE 14, 1898.