The rock formation of the country appears to be simple and uniform. At the lowest depths observed in valleys, there is a crystalline sandstone, which often consists of transparent quartzose grains, adhering by the force of aggregation. The lead-bearing limestone reposes upon this. Both formations are deposited in perfectly horizontal strata. Valleys which carry streams have been worn down into this formation, presenting this order of arrangement very satisfactorily. A stratum of red, marly clay, spreads over the limestone. Above this, constituting the top layer, or surface soil, rests a bed of diluvial materials, filled with broken-down fragments of rock, masses of radiated quartz, and chips of hornstone. Vegetable matter and black sand form a covering over such parts of this diluvial deposit as constitute valleys and agricultural plains. The Mississippi river lays open this formation along its western banks, from the influx of the Missouri to Cape Girardeau.
Beneath this metalliferous column lie the primitive rocks. The most striking feature of this kind is found in the occurrence of a primitive formation at the sources of the river St. Francis. My attention was arrested by this fact, soon after I began to examine the mine country. This formation consists of sienite, rather than granite; the mica being generally replaced or represented by hornblende. The feldspar, which constitutes three-fourths of the mass, is of a dull red hue. The rock in connection is greenstone trap, which is sometimes porphyritic. I observed small masses of sulphuret of iron in some parts of this rock. The upheaval of this formation appears to have been of the most ancient era of geological action; for the stratified limestones and sandstones, which lie upon or in juxtaposition to these elevations, have not been disturbed in their horizontality. The altitude of this primitive tract does not probably exceed one thousand feet above the waters of the St. Francis river. Vast blocks of the red sienite have been detached, and scattered southwardly over the secondary rocks, apparently by the force of some antique deluge, setting from the north. The whole series of formations may be judged of by the following diagram:
The general aspect of the country is sterile, though not mountainous. The lands lie rolling, like a body of water in gentle agitation. In some places they rise into abrupt cliffs, where the rock formations appear. Generally, they present the form of diluvial ridges, sparingly covered with forest, and bearing a growth of prairie-grass and herbage. The western banks of the Mississippi, between St. Genevieve and Herculaneum, present a mural front to this district, in a series of elevated perpendicular cliffs of compact limestone. The whole coast extending to St. Louis, appears to be sufficiently elevated to have served as a former barrier to waters covering the low grounds of Illinois. The strata exhibit ancient water-marks of a diluvial character. They are broken through, from the west, by small streams draining the mine country.
No indications of lead-ore have been found in these cliffs. The mines are situated at considerable distances west of them; and when the observer has arrived at their localities, he finds the ore often lying in the unconsolidated soil. This soil is a stiff, reddish-colored clay, filled with fragments of cherty stones, quartz, and small gravel, clearly attesting its diluvial character. This soil extends to the depth of from ten to twenty feet, or more, and is based on limestone rock. It is so firm, in some places, as almost to resist the pick-axe; in others, it partakes more fully of marl, and is readily penetrated. The ore lies in this marly clay, and is often accompanied by sulphate of barytes and calcareous spar. The country is particularly characterized by radiated quartz, which is strewn in detached pieces over the ground, and is also found imbedded in the soil at all depths. This substance is here called blossom of lead, or mineral blossom. Pyrites, and some other ores of iron, are also found in detached masses upon the surface, and, very rarely, lead-ore.
Such is the general character of the mineral lands, which are covered with a stunted growth of oaks, denominated post-oaks. Walnut is found in some instances out of the valleys. A ridge of yellow pine extends west of the mines, between the St. Francis and Maramec, and is more decidedly barren than the grounds covered with oak. All the open, elevated tracts, are clothed with herbage, which hides their flinty aspect, and gives the country a picturesque appearance. The minor slopes and ravines are often rendered almost impassable by hazel, vines, and other bramble, which appear to be indicative of a better, or rather a deeper soil. The whole area of upland soil, which rests as a mantle over the rocks, is a diluvium, which must, we think, be referred to an early period of diluvial action.
The only true alluvium of the mines appears to be confined to the valleys or plains, which are, consequently, the principal seats of cultivation, and thus derive an additional value from their contiguity to the barren tracts. This alluvium rests on the red marl-clay, or mineral diluvium; the latter of which is uniformly found on penetrating it. Some of the mines exist in, and have been pursued beneath, this top alluvion, across the valleys. Others are seated beneath an arable soil, bearing a forest. Many of the most barren and stony parts of the elevated lands are, on the contrary, destitute of mines. The depth of the mineral soil varies exceedingly. It barely conceals the rock formations in many of the more elevated positions, and frequently does not conceal them. It is deepest in the plains and depressed grounds, being accumulated much in the manner we should expect, on the supposition of a general diluvial submersion.
The principal objection to a general diluvial action, involving the whole Mississippi valley, appears to arise from the admission of the limestone rock's being the true locality of the ore. But we think there are too many facts in support of this opinion, to leave any reasonable grounds for questioning it. Several of the mines in the mineral soil have been traced down into the rock, and have been pursued through apertures, closing and expanding in the manner of true veins. In the numerous cases where the rock has put a stop to further mining, and it has exhibited no signs of ore, it may be supposed that the ore has been moved, by diluvial force, from the original position of the mine, and been finally deposited, with the soil, upon unmetalliferous portions of the rock. And could we with certainty determine the course of diluvial action, the principles of mining might be, in some respects, employed in searching for the original vein. It is evident, from the unscratched and unbroken surface of much of the ore and its spars, that it could not have been transported far; while the portions of it called gravel ore, which evince its diluvial character, are manifest proofs of a change, more or less extensive, in the general position of the ore.
With respect to the character of the limestone, we have been perplexed with its protean character, and, to avoid apparent contradictions, were led, at first, to adopt distinctions of strata, which we very soon saw were untenable. It is evidently the American equivalent for the metalliferous limestone of England, and, as a formation, is of the transition era. In a specimen of this rock, now before us, taken from a fresh excavation at Potosi, forty feet below the surface of the soil, and thirty-one feet below the original surface of the rock, the structure is in part compact, and in part granular; the compact portions having minute shining crystalline points, and the granular being without any appearance of crystallization, but changing, in the width of about forty lines, from compact granular to a dull arenaceous structure, quite friable between the fingers. Part of the mass is vesicular, and the vesicles are studded over with minute crystals of white opaque quartz. The two extremes of this specimen have the appearance of totally different formations, yet are both calcareous. By experiment, I found a portion of the lower arenaceous part almost completely soluble, in the cold, in nitro-sulphuric acid; and the actual residuum was, in part, owing to a defect in trituration.
Most of the limestone rock disclosed by excavation in the mines, is of the granulated kind; while the structure of the rock above the surface, where the strata are exposed to the weather, as in cliffs and hill-sides, is of the solid, glistening, pseudo-compact variety. Both these varieties, as shown in the specimen, are geologically identical, notwithstanding their striking differences in hardness, structure, colour, and particularly in crystalline lustre. This lustre is, however, as shown by examination with the magnet, owing almost exclusively to minute facets of calcareous crystals, which render it rather sparry than crystalline.