PALEOZOIC COLUMN IN ILLINOIS.
Accompanying this section, figure 128, we give the Pennsylvania and New York equivalents, with the respective thickness of each. It will be noticed that few of the slates and shales of New York and Pennsylvania are found in Illinois, but that the limestones are continuous; and while the Chester or Carboniferous limestone is not represented in the anthracite regions of Pennsylvania, the Galena limestone of Illinois has its counter part in the Auroral limestone, or No. II. The Galena limestone is only 300 feet thick, while the Auroral is often 5000 feet thick in Pennsylvania.
The Umbra' red shale, or No. XI., entirely disappears, and the Vesper tine, No. X., unites with the conglomerate, or millstone grit, forming, when united, 300 feet in Illinois, where the conglomerate proper is a thin plate of from 10 to 20 feet in thickness. Following the millstone grit, in Illinois and all the bituminous coal-fields of the West, is the Carboniferous limestone, which has a general thickness of 1000 feet.
The Old Red Sandstone of the English, Catskill of New York, or No. IX. of Pennsylvania, may be represented by the sandstone following the Chester limestone in Illinois, which is 100 feet thick against 5000 feet in Pennsylvania. The St. Louis, Keokuk, and Burlington limestones, or Mountain limestones, are included in the Chemung and Portage groups; and the Oil, Black Slates, and Hamilton find their counterpart in the Cadent and Post-Meridian of Pennsylvania, or the Hamilton, Marcellus, &c. of New York. The Oriskany sandstone cannot be mistaken in either while the Niagara lime and the Hudson River group are distinctly named in each. The Galena lime is the equivalent of the Auroral in Penn sylvania, and the Chazy or Calciferous in New York; while the Potsdam sandstone is conspicuous as the base of the Palaeozoic column in Penn sylvania and New York. In the West it is claimed that a lower fossi liferous sandstone exists on the Gneissic or Azoic; but there seems to be no great distinction between it and the Potsdam, or Primal, and both may be referred to the same formation.
The limestones predominate in the West, and the sandstones in the East; but some of the great sandstones have their horizon coextensive with the area of the ancient sea, yet invariably much thinner in the West than the East. They depreciate from massive formations several thousand feet in thickness to mere knife-edges, and from heavy, coarse-grained sandstones or conglomerates to fine-grained flaggy or slaty sandstones. This depre ciation, particularly in the coal measures, is manifested in the same or greater proportions, considering the distance, in the Northwest as in the West. The coal measures in Michigan, containing the Lower coal series, which in Pennsylvania are from 500 to 700 feet thick, are only from 30 to 40 feet in thickness ; which corresponds nearly with the measures on the western edge of Missouri.
The whole evidence goes to establish the propositions set forth in Chapters II. and III., describing the North American Continent as it formerly existed, or that portion of it which lies between the Rocky and Alleghany Mountains. We can conceive how these formations (the Pale ozoic) arose, and why they were thicker at the East than in the West, why limestone in the interior of the great basin, and sandstone at its eastern edges, only on the basis of the theory set forth. A great inland sea stretching from the high, granite mountains in the East to those in the West; fierce and almost continuous volcanic action duriug the Gneissic period, while the metamorphic or crystalline sedimentary strata were de posited; violent but intermittent volcanic action during the Paleozoic ages, or while the fossiliferous strata arose in the waters of the ancient sea; a constant depression of the Eastern mountain-ranges, whence most of the material forming those vast lithographical structures came, and where all ,the volcanic vents existed to the East during the Paleozoic period.
The consequence of this natural process is that which we are now dis cussing. The deep basins of the East, formed by the depression resulting from long-continued volcanic eruption, received the largest amount of the vented volcanic material, and naturally the coarser matter settled in the nearest and deepest basins, while the finer and more limited amount floated away, on wind and tide and waves, to the centre and western parts of the ancient sea.
Limestones only formed in the East during periods of quiet; but they formed rapidly, since the carbonic acid and salts of magnesium and calcium existed in greater quantities where the heat was greatest; but they formed more constantly in the West, where comparative quiet almost constantly existed.
As the Eastern strata were depressed and the bounding mountain-ranges in that direction became lower in consequence, so the sea receded from the Western mountains and gradually exposed its western margins to the day, or left their extensive and shallow reaches in bog and swamp to form those vast prairies in the manner, perhaps, so ingeniously and scientifically described by Prof. Lesquereux, of Columbus.
We have adverted to this subject in this connection because, with the proof so abundantly furnished as we proceed, the subject becomes more clear to the mind, and the facts as they arise can be better appreciated and applied.