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The entire area of Illinois seems at one period to have been a level plain, or ocean bed, which has not since been disturbed by any considerable upheaval. The present irregularities of the surface are clearly traceable to the washing out and carrying away of the earth which once filled the spaces occupied by our valley. The Illinois River has washed out a valley about 250 feet deep, and from 14 to 6 miles wide. The perfect regularity of the beds of mountain limestone, sandstone, and coal, as they are found protruding out of the bluffs on each side of this valley, on the same levels, is pretty conclusive evidence, that the valley itself owes its existence to the long-continued action of the water. The lower bed of the coal as at present worked, which is 30 feet above the river, is found along the banks of Kickapoo Creek for 15 miles from its mouth at nearly the same elevation from the water. The upper bed of coal is 65 feet above the lower, and 95 feet above the bed of the river. 'The mountain limestone is 65 feet above the upper bed of coal, and 160 feet above the river. It is supposed that there is another, or third workable bed of coal, below the bed of the river. The limestone and the three uppermost beds of coal are identical in character at La Salle and in Peoria County. This lowermost bed of coal, as found at La Salle, is quite different in its quality from the other two, and is quite free from sulphur.

Among the valuable natural products noted up to this time, may be mentioned the ores of iron, lead, and zinc; coal, porcelain earth, fireclay, potter's clay, fuller's earth, marble, oolitic marble, limestone, grit-stones, flags, &c. The value of the salt-springs in the southern portion of the State, cannot yet be estimated. . Notwithstanding they bave been worked from the earliest settlement of the country, nothing sufficient seems to bave been developed, upon which an estimate of their true value could be based. The investigations made in the southern coal region, have led to some conclusions, which will, ultimately, be of great service to the public in preventing the loss of capital by vain explorations for that mineral in sections where it does not exist, and also by pointing out the special conditions under which labor and capital may be employed with a prospect of success.

Marble, lime, and sandstone are found, either the one or the other, in each county; secondary sandstone forms the basis of the rocks in the whole northern part of the State. Near Athens, in Du Page County, fine, milk-white limestone quarries have been found; the stone is of a marble-like appearance, and susceptible of receiving an excellent polish. Near Chicago is found quite a peculiar variety of stone, of a dark grey color, a variety of marble, of a granulous cleavage, from which a bituminous matter constantly oozes.

If lime should ever be largely used in farming concerns in Illinois, or if it should be deemed worth while to export it, many of the counties would be able to supply large quantities of it.

Sandstone, which when dug out, hardens through the influence of the air, is preferred to lime. In Randolph County are the finest marble quarries. Quartz crystals are found in Gallatin, and the adjacent counties; gypsum in St. Clair County. In general, however, metallic ores are considered to be of a higher value; and though in this branch Illinois cannot boast of gold and silver mines, it is in possession of other ores which are of a far greater importance. There are in the State two hilly districts, one in the north-east of the State, north of Galena, which derived its name from galena (lead ore), and one in the south of Illinois, in the counties of Union, Johnson, Pope, Hardin, Gallatin, and Williamson, which latter seems to be a continuation of the hilly regions which are encompassed by the Cumberland and Tennessee Rivers. These two districts form the metallic region. The southern metallic districts have only been worked for a few years.

In prospecting and sinking shafts for the lead mineral, or galena, after penetrating the earth from 40 to 70, and even 100 feet, the miner sometimes finds himself in caverns of different dimensions, varying in size from about three to six rods. It will sometimes happen that he hits on a crevice, which hardly affords space enough to crowd the body through. A great many of these subterranean apartments present scenes of grand and brilliant splendor, from the various crystallizations found in them. Calcareous spar, in great diversity and beauty of shape, is often found in considerable quantities, in some of the richest of these mineral-bearing grottoes.

In some of the caves, more particularly in the vicinity of the copper mines, the sulphates of lime are to be found in different forms, such as opaque plaster and gypsum; and sometimes in beautiful crys. tallized forms, as selenites and alabastrites, which are generally of a pure, sparkling white. The richest and most abundant lead ore is generally found in caves, beneath an earth whose drippings are fruitful with these beautiful specimens of spar; it is in most cases a clay or marl soil, in which aluminum constitutes a large ingredient, and where soap-clay is found in abundance. It can be easily cut or modelled into various forms and images, and hardens when dried—but shakes into fragments when exposed to the air.

Iron is one of the most considerable productions of the State.

In the year 1850, the pig iron produced in this State amounted in value to $65,000, for which iron 5500 tons of ore were required. Of cast iron, 4477 tons were manufactured of pig iron, and 50 tons of old iron. The entire capital invested in the iron manufacture, amounted to $325,400; the cost of the ore, expenses, &c., to $197,830; wages, $153,264; and the total value of the manufactured article, to $511,385.

Copper has been found in large quantities, in the northern counties of the State, especially at the mouth of Plum Creek, and other little creeks. It is also found in small quantities in Jackson County, on Muddy River, and back of Harrisonville, in the bluffs of rivers in Munroe County, to some small extent.

Zinc exists in considerable quantities in several districts of the State.

Silver has been found in rather small quantities in St. Clair County, two miles from Rock Spring; whence Silver Creek has derived its name. It is said that in early times, the French sunk a shaft here, and tradition tells us that considerable quantities of the metal were then obtained ; and it is even asserted that in the southern part of the State, several sections of land were reserved from sale, owing to the silver ore which they were supposed to contain.

Before commencing to speak of coal mines in this State, it will not be improper to give a few more hints with regard to the geological formation of this vast State, which may serve as an addition to what was already mentioned, concerning geology, at the beginning of this chapter.

The profile of the country, in fact, does not present one uniform, dead level, but a succession of gentle updulations, which have very forcibly been compared to the swells of the ocean. The highest, or culminating points, attain an elevation of not more than 800 feet above the Ohio River, and about 300 feet above the level of Lake Michigan. The valleys cut through the superfical deposits, and occasionally expose the rocky strata beneath ; while from the main channels start numerous ravines.

The Illinois Central Railroad passes over all those systems of rocks which are included between the Upper Carboniferous, and the Lower Silurian. The greater portion of the underlying rocks consists of sandstone, shale, and limestone. The question has, as yet, not been solved, whether they form an uninterrupted assemblage of strata, dipping towards a common centre, or are arranged like the Appalachian coal-field, in a series of undulations.

The continuity of the coal-bed has been found in one or two instances, to be interrupted by older rocks, interfering between them; but if the supposition be made that the coal-bearing strata had once been arranged in a series of waves, or corrugations, and that in the progress of time their crests had been abraded, so as to expose the subjacent rocks, all those phenomena would be exhibited which one attempts to explain by a resort to limited basins.

Geological Structure of the Southern Division. The strata contiguous to the above mentioned railroad, may best be divided into three groups, which are in the ascending order, thus :

Group I.-Alternation of blue compact limestone, black slate, and fine-grained sandstone, with traces of hydrated brown iron ores.

Group II.-Small pebbles, mill-stone and grindstone grit, and sandstone of various colors, or variegated sandstone.

Group III.- Alternations of shale with vegetable impressions, fireclays, impure limestone, and sandstone, with seams of coal, and traces of iron carbonate.

Geological Structure of the Northern Division. In the northern, or north-eastern part of the State, adjacent to the Central Railroad, is found the La Salle coal-field, the northern margin of which extends but a few miles north of the Chicago and Rock Island Road. The intermediate space between its outcrop and galena, is occupied by groups of strata below the Carboniferous.

The buff-colored magnesian-limestone also belongs to this series ; in its texture it is compact, and close-grained, and may be worked easily by the chisel, into any form, and is from that reason very well adapted for building, the more so, since it is but little more expensive than brick. It is found in layers, the under surfaces of which are covered with water-marks. Such rocks as contain a large number of ingredients, not chemically combined, are apt to crumble; but this objection does not apply in this case.

Near La Salle, Dr. Norwood, the State Geologist, discovered one of the most interesting facts in the history of the Carboniferous period of America, viz: the existence of the coal seams upon the upturned strata of the Lower Silurian series.

Visiting the “Split Rock,” about three miles east of La Salle, he observed a coal-seam with a thin intervention of shale, occasionally wanting this, reposing, though only at intervals, upon strata which contain fossils belonging to some of the earliest forms of organic life.

The vast series of rocks that are so conspicuously displayed in the slate of New York and Pennsylvania, interposed between the Lower Silurian, and the productive coal strata, as well as the carboniferous limestones and sandstones, which form so prominent a feature in the geology of Southern Illinois, are here entirely wanting. Examining further, it will be perceived that the sandstone here comes out in bold scarps, and is surmounted by limestone containing fossils, and reposes upon a magnesian limestone which contains traces of early animal life. This sandstone stratum is about 100 feet in thickness.

The limestone reposing upon it, according to Dr. Norwood's measurement, is about 250 feet thick.

The coal-field at La Salle occupies the trough-like depression thus created. Its lowest seam sometimes reposes upon the older rocks. Its thickness is from three to four feet. Between the lower seam and the middle one, there is an interval of 176 feet, consisting of alternations of shale and limestone, with thin bands of sandstone.

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