To adopt the words of Dr. Thomas Cooper:—
"There appears to be a series of strata, or, as Werner calls them, formations, that may be considered as surrounding the nucleus of the earth. The first formed, or lowest series, always preserve the same situation to each other, except where occasional eruptions, or circumstances not of a general nature, make a variety in their situations. These strata are not only the deepest, but they are also the highest that are observable in the crust of the earth; forming the tops of the highest mountains. They are characterized by an appearance of crystallization, and by containing no remains of organic matter, animal or vegetable. The strata or formations that in general constitute this first, deepest, highest, and crystallized series, are granite, gneiss, mica-slate, clay-slate, primitive greenstone, granular limestone, serpentine, porphyry, and sienite. These formations are so generally found, and in the same situations as incumbent upon or subtending each other relatively, that they may be considered as universal. Their crystallized appearance shows that their particles have either been dissolved or very finely suspended in water, so that the attraction of crystallization has been free to operate; that this water has been deep, so that the lowermost parts of it have not been much agitated during the crystallization, which would otherwise have been more confused than it is; and, indeed, the oldest formations are the best crystallized. A part of the water covering the nucleus must have been taken up, as water of crystallization, in the primitive formations. When these were deposited, there were no vegetables formed; of course, no animals; nay, even the sea was unpeopled, for there is no trace of any organic remains in these strata. Even the belemnites, the asteriæ, the echini, the entrochi, the most simple forms of oceanic animal life, do not occur until the transition strata appear. Hence the propriety of denominating these formations primitive.
"By processes of nature, besides the consumption of water by the new crystallized masses, to us unknown, the waters appear to have diminished. The highest parts of the primitive formations became the shores to the water superincumbent on their bases and middle regions; the simplest forms of oceanic animals came into existence; the mosses and lichens of high latitude would generally occupy the surface of the primitive strata, gradually decomposed by the alternate action of air and water after many ages. During this period, while the strata were in a state of transition from the chaotic to the habitable state, other deposits would gradually be made from the waters, now decreased in quantity, and take their place below the summits of the primitive range. Those summits being exposed to the action of the atmosphere, of rains, of frost probably, and to the action also of the waters with their contents still incumbent on the earliest strata, would furnish masses and particles washed away, which would mingle with the deposits of the transition series. This series, therefore, will exhibit appearances of mechanical and chemical intermixture of earths and stones, such as are found in the silicious porphyries, the graywackes, the silicious and argillaceous hornblende rocks, the elder red sandstone, &c. During the period when these transition formations were deposited, there would be no land animals, for there would be no vegetables for them to feed upon. There would be no vegetables unless some few lichens, mosses, or ericas, that would find foothold upon the slight decomposition that, after the lapse of some ages, would take place on the surface of the primitive rocks. The sea only would be peopled, and that but sparingly; for, in that mass of muddy water, none but the lowest and most inferior grades of animal life, and such as do not inhabit deep water, could exist. Hence, we find the transition formations contain in their substances some belemnites, asteriæ, entrochi, echini, &c., but no organized vegetable substance except, very rarely, in the latest rocks of this series, and no remains whatever of terrestrial animals. Indeed, in the high latitudes of the outgoings or summits of the primitive strata, very few vegetables, even at the present day, can live. No vegetation fit for animal life could take place until the transition, and most of the next series of secondary or floetz formations had subsided. These would occupy lower and lower situations, till a rich soil, from every kind of intermixture of earth mechanically deposited, would afford a proper temperature of region, and an easily decomposed soil, wherein vegetables could grow.
"Next to the transition series, come the secondary, or, as the German mineralogists call them, the floetz rocks; so called, because they appear to be more floated or horizontal, though I confess the appellation does not appear to me peculiarly appropriate. These strata consist principally of sandstone, limestone—sometimes fetid from bituminous impregnations, sometimes shelly—secondary greenstone, graphite, coal, gypsum, rock salt. I have observed that the Alpine heights of the primitive mountains could at no time furnish much food. The same remark, but in a less degree, will apply to the transition range; the low and kindly climates occupied by the secondary series. The soft and decomposable nature of these depositions would furnish the true theatre of vegetable life, and, until these regions were filled with vegetables, the race of animals could not have been produced; for on what could they subsist? Graminivorous animals, therefore, must have succeeded the various forms of vegetable existence; and carnivorous, the graminivorous. The vegetable matter imbedded in the substance of the secondary strata will consist of the remains of vegetables that grow in the transition strata; and the animal remains will consist chiefly of such animals as were produced in the early stages of animal existence, particularly the smaller aquatic animals; and, of these, chiefly shell-fish, as shells are not so soon decomposed as mere animal substance."
It is to the latter class of depositions—to the secondary series—that we must refer the sandstone of the River Des Plaines, in which we find a walnut, of mature growth, enveloped by, and imbedded in the rock, in the most complete state of mineralization; and, since all geological writers who subscribe to the Neptunian theory are constrained to employ the agency of oceanic depositions of different eras, in explaining the structure of the earth's surface, it is one of the most obvious and important conclusions, to be drawn from the fact that such submersions and depositions of rock matter have taken place subsequent to the existence of forests of mature growth, and that the rock strata and beds composing the exterior of the earth are the result of different geological epochs, and of successive subsidences of chaotic matter—positions which have been so severely attacked and so often denied, particularly by the disciples of the Huttonian school, that it is not without a feeling of lively interest, I communicate a discovery which appears so conclusive on the subject.
Considerations arising from the frontier position of the country, and the infrequency of the communication, have also induced me to draw from incidental sources, a corroboration of the facts advanced.
In a letter to Governor Cass, of Michigan, dated September 17, 1821, I made the following observations on the subject under review:—
"I consider the petrified tree discovered during our recent journey up the Illinois, so extraordinary an object in the natural history of the country, and calculated to lead to conclusions so important to the science of geology, that I am anxious to avail myself of your concurrent testimony as to the fact of the existence of the tree in a mineralized state, and the natural appearances of the spot where it lies imbedded. I feel the more solicitude on this subject, as I am aware that any description of this phenomenon which I may be induced to communicate to the public, will be received with a degree of caution and scrutiny which it is the province of the naturalist to exercise whenever any discovery is announced affecting the existing theories of the natural sciences, or tending to increase the volume of facts upon which their advancement and perfection depend. I am aware, also, that whatever degree of caution and vigilance it may be proper to exercise to prevent errors from mingling with the sound doctrines of the physical and other sciences, still more care and circumspection is requisite in examining facts which affect the progress of geology."
I quote an extract from Governor Cass's reply on the subject:—
"The appearance of the wood and bark indicates that it was a black walnut, the juglans nigra of our forests. We computed its original diameter, at the place where it is concealed in the earth, to have been three feet, and at the other end eighteen inches. The texture of the wood, and the bark and knots, are nearly as distinct as in the living subject, and the process of decay had not commenced previous to the commencement of this wonderful conversion. Every part of the mass which we could examine is solid stone, and readily yields fire by the collision with steel.