By the 24th of October, the exploration of the Dubuque district was completed, and the special reports of all the townships therein were despatched to your office, and to the office of the register at Dubuque. On the 14th of November, the survey of the Mineral Point district was in a similar manner brought to a close; and by the 24th of November, our labors finally terminated at Stephenson, in Illinois; the examinations of all the lands comprehended in my instructions having been completed in two months and six days from the date of our actual commencement in the field. Also several thousand specimens, some of rare beauty and interest, were collected, arranged, and labelled.

The weather was favorable, and the winter did not set in with severity until about a fortnight later than is usual in that latitude; yet, the same day on which the survey was completed, a severe snow-storm occurred, a gale blew up from the northwest, the thermometer fell to 12 or 14 degrees below zero, and the expedition could not have continued its operations in the field a single day longer.

The details of this exploration, exhibiting results of an interest and importance far beyond my anticipations, and equally, perhaps, beyond those which the department may have formed, are submitted in the following

GENERAL REPORT.

SITUATION AND EXTENT OF THE COUNTRY EXPLORED.

The district of territory which has been explored lies nearly in equal portions on both sides of the Mississippi river, between latitude 41 and 43 degrees; commencing at the mouth of Rock river, and extending thence north, upwards of 100 miles, to the Wiskonsin river, which discharges itself into the Mississippi immediately below Prairie du Chien.

The average width of this body of land exceeds 100 miles. It comprehends about 11,000 square miles, equalling in extent the State of Maryland.

ITS GEOLOGICAL CHARACTER.

To a correct understanding of the geological formations of this district, without which its probable value as a mineral region cannot be correctly appreciated, a few words of general explanation may be required.

A casual observer may imagine that the various rocks which compose the crust of our globe are thrown together in indiscriminate confusion; but those who have given even a superficial attention to the science of geology know that the order of superposition among these rocks is constant. The various geological strata overlie each other in a succession which is invariable, with the exception of trifling alternations sometimes occurring at the junction of two formations.

Beneath, at the greatest depths, are found granite and other crystalline rocks of a similar character.

Lying above these, are the primary fossilliferous rocks, such as occur throughout America, immediately beneath, and in connexion with, anthracite coal.

Above these lie the secondary rocks, including the mountain limestone, the bituminous coal-measures, the salt-producing rocks of Europe, and the chalk.

Above these secondary rocks occurs the tertiary series-a succession of marine and fresh-water deposites, such as are found in the United States, along the Atlantic seaboard, and on the Mississippi river, as high as Vicksburg.

Last, and resting upon these, are found the recent deposites, such as occur in river bottoms, and throughout the richest lands in the western States.

This order of succession is never inverted, though occasionally certain classes of rocks are in whole, or more frequently in part, deficient. Thus, the recent deposites exist (if they exist at all) universally above all the other classes of rocks; the tertiary above all except the recent deposites; and so of the rest.

This invariable order of succession supplies the geologist's most trustworthy guide in his researches after mineral riches; for certain minerals are found almost exclusively in certain formations. The geologist is thus enabled to predict, previously to any examination in detail, where gold, where iron, where lead, and where other valuable mineral products are likely to occur, and where it would be in vain to look for these.

The several layers or classes of rocks above enumerated are imagined to have been originally deposited in a horizontal position, thus.-(See diagram No. 1.)

If the conjecture be accurate, and if they had thus remained, we could have known but little of any, except the superior strata. But it will be readily perceived that some great convulsions of nature, heaving up the lower strata, and causing them to burst through and displace the upper and superincumbent layers, might produce an arrangement similar to this. -(See diagram No. 2.)

And thus the crystalline and other inferior classes of rocks might become the highest, and be found occupying the summits of the lofty and rugged mountain ranges; while the others would slant up in succession to the surface, flanking the mountain sides, and extending over the inferior ridges and plains; the superior strata being commonly found the most remote from the primitive and crystalline rocks.

This, in effect, seems to have occurred, with various modifications, throughout the known world. We find each group of rocks appearing in regular succession along the surface of the earth, dipping at various angles, and running out as each approaches the next overlying stratum, which, in its turn, disappears beneath a superior series. And as we ascend the highest mountains, we frequently find those strata, which, by a horizontal arrangement, would be the deepest seated, heaved up into the loftiest positions. Such are the granite peaks of the Alps and Alleganies, and the masses of porphyry which occur at the highest altitudes among the Andes.

But for some such arrangement as this, many of our valuable mineral deposites would be inaccessible, for most of the metallic ores are confined to inferior strata. As it is, all the formations are presented to the geologist in different portions of our globe; and thus, as each class of rocks has its peculiar ores and minerals, these are distributed over the earth wherever metalliferous strata come to the surface. Thus, too, important practical results are obtained, by a careful examination of the extent and localities of the various formations, and, as a consequent, by the study of the imbedded fossils, the presence of which constitutes the most decisive evidence of the identity of geological strata.

Throughout the western States, generally, the secondary formation prevails, covered up in various locations, sometimes to a considerable depth, by recent alluvial and diluvial deposites.

This secondary series of rocks comprehends (as do also all the others which have been enumerated) various subdivisions of distinct character and invariable succession, which, in their turn, have been again subdivided.

As one of these subdivisions predominates throughout the whole district of country upon which it is my present duty to report, it is important to take note of them; and, accordingly, they are here represented. (See diagram No. 3.)

These are the chief groups composing the secondary series, represented in the order in which they succeed to, and rest upon, each other.

Of these groups, the mountain limestone particularly claims our attention, as almost all the rocks of Iowa and Wiskonsin are referrible to that subdivision.

In the States of Ohio, Indiana, Kentucky, and Tennessee, where the members of this group are complete and all distinctly marked, the subdivisions observable are these.-(See diagram No. 4.)

In the western States, above mentioned, these subdivisions generally vary in thickness from one hundred to one thousand feet, with the exception of the cliff limestone,† which, in some districts, is hardly distinguishable, and in general does not exceed one hundred feet in thickness.

Now this cliff limestone, so sparingly developed elsewhere, swells, in the Wiskonsin lead region, into the most remarkable, most important, and most bulky member of the group. It becomes, as it were, the Aaron's rod, swallowing up all the rest. It attains to a thickness of upward of five hundred and fifty feet; while the underlying blue limestone, which in Ohio has usually from eight hundred to one thousand feet of thickness, shrinks in many places to less than one hundred feet, and in others seems wholly wanting; while, at the same time, the black slate, commonly found above the cliff limestone, seems also deficient; and it is doubtful whether the finegrained sandstone, or the oölitic limestone, or the conglomerate, can be detected at all throughout the entire tract of country which has been subjected to exploration.

In a word, in the region now under consideration, the cliff limestone, with a variable and usually thin substratum of blue limestone, seems to engross the entire mountain limestone group; and the coal-measures, where found, (namely, in the extreme southern boundary of the tract,) occur in immediate contact with it, instead of being separated, as usual in Ohio and the neighboring States, by three distinct members, occupying about one thousand feet in thickness.

In Wiskonsin and Iowa, then, instead of the various subdivisions of the mountain limestone group, as given in the last diagram, the geologist finds but these. (See diagram No. 5.)

This enormous development of one of the members of the mountain limestone group, and the almost complete obliteration of the rest, (with the single exception of the blue limestone, upon which also it much encroach

* With the exception of occasional and slight alternations, where the formations come into

contact.

The origin of this term, and my reasons for adopting it, are given at length in another part of this report.

es,) is peculiar, so far as my observations in the western States extend, to the district of country which is the object of the present report. In the north of this district, the cliff limestone appears to run out, the blue limestone and underlying sandstones coming to the surface. South, it disappears beneath the coal-measures. East, it seems to be chiefly covered up by recent deposites; extending, probably, in an east or southeasterly direction beneath these, across the States of Illinois and Indiana, into the State of Ohio. And west, so far as our examinations went, it is also chiefly covered up by recent deposites; occurring, however, occasionally in the beds of the streams, and projecting, at first in cliffs, and at last only in low ledges, from their banks. The general geological character of the country explored may, then, be thus briefly summed up. It belongs to that class of rocks called by recent geologists secondary, and by others occasionally included in the transition It belongs, further, to a division of this class of rocks described in Europe as the mountain limestone, or sometimes as the carboniferous or metalliferous, or encrinital limestone. And it belongs, yet more especially, to a subdivision of this group known popularly, where it occurs in the west, as the cliff limestone, and described under that name by the geologists of Ohio.

This last is the rock formation in which the lead, copper, iron, and zinc of the region under consideration, are almost exclusively found; and its unusual development doubtless much conduces to the extraordinary mineral riches of this favored region. It therefore demands, and shall hereafter receive, particular analysis and attention.

In the northern portion of the district surveyed, an interesting and somewhat uncommon feature in the geology of western America presents itself. I refer to the strata (of considerable depth) which crop out along a narrow strip of the northern boundary-line of this district, and which are chiefly observable in the bluffs on both sides of the Wiskonsin river; whence (if we may rely on the representations of Schoolcraft and others) they extend north, even to the falls of St. Anthony.

These strata are interesting, first, as being the only instance known to me, in the valley of the Mississippi, in which the rocks underlying the blue limestone can be seen emerging from beneath it to the surface; and, secondly, as apparently supplying an example of those alternations of neighboring strata, to which I have already alluded as being partial exceptions to the invariable order of geological superposition.

Immediately below the substratum of blue limestone, which constitutes (as in a previous diagram shown) the lowest member of the mountain limestone group, where it has been observed east of the Mississippi, there occurs, and shows itself in the Wiskonsin bluffs, a stratum of sandstone, in some places of a deep red, and in others of a white color, resembling loaf-sugar; and thence called, in Dr. Locke's diagrams exhibiting the sections on the Wiskonsin river, saccharoid (or sugar-like) sandstone.

Immediately beneath this, succeeds a magnesian limestone, so similar to the cliff limestone, both in external appearance and chemical composition, as not to be distinguishable from it in hand-specimens, alternating with other layers of sandstone similar to that above described.

It differs, however, from the cliff rock in several particulars, when examined in situ. li is almost destitute of fossils; its lower members have sometimes a greenish tint; the imbedded siliceous masses are more rugged and quartzose than those in the cliff rock. Beautiful crys tallized specimens of rose-colored quartz are of frequent occurrence in its beds; and chert of an oolitic structure (made up of minute egg-shaped grains, like the roe of a fish) is also abundant.

So that the mountain limestone group of this district, exhibited in connexion with these underlying strata, appears thus: (See diagram No. 6.)

The above example of the blendings of adjacent strata constitutes probably an alternation of the old red sandstone with one of the lower members of the mountain limestone group, at the junction of these two formations.* This lower magnesian limestone, and associated sandstones, closely resemble, in structure, position, and deficiency of fossils, the rocks prevailing throughout the lead region of Missouri, and are probably their equivalents. If so, we are furnished with a clew to the true geological position of that region; a point heretofore undecided.†

If, in connexion with this brief outline, the colored charts numbered 1 and 2 be inspected, it will be easy to form a general idea of the relative position, extent, range, bearings, succession, dip, and thickness of the geological strata which occur in the district I was instructed to explore.

Chart No. 1 exhibits the superficial extent, succession, and bearings of these strata, as they come fairly to the surface. The brown color, (burnt umber,) along a strip in the extreme south, designates the bituminous coalmeasures as they overlap, and often come into actual contact with, the cliff limestone. This is the extreme northwestern margin of an immense coal basin which occupies the greater part of Illinois, about one third of Indiana, a northwestern strip of Kentucky, and, occasionally encroaching beyond the Mississippi, extends a short distance into the State of Missouri, and into the Burlington district of Iowa. Chart No. 3 exhibits, with general accuracy, the form and extent of this gigantic coal-field, the superficial area of which equals that of the entire island of Great Britain.

In chart No. 1, the light yellow color, (Indian yellow,) occupying nearly six-sevenths of the whole district, represents the cliff limestone, covering a surface of upwards of 9,000 square miles. Within this boundary, all the productive lodes of lead ore yet discovered are to be found, as in another part of this report will be more particularly shown. The blue color (indigo) designates the blue limestone, forming a belt which runs with undulations east and west, broadest near the mouth of the Wiskonsin river, and very narrow in the vicinity of the Blue Mounds.

The buff stripe represents a narrow belt of buff-colored limestone of little importance; it was not detected west of the Mississippi.

The red color (lake) and deep yellow (dark Indian yellow) designate, the first, the red and white sandstone; and the second, the lower magnesian limestone, which alternate with each other.

Chart No. 2 contains a vertical section running obliquely through the district, nearly in a line with the greatest dip, commencing at Rockingham, immediately below the mouth of Rock river, and running thence through

An examination prosecuted north of Wiskonsin river would decide this point. No characteristic fossils, or trustworthy indications, were discovered south of that river, in the sandstone; so that its character cannot positively be pronounced upon.

Schoolcraft, in his "View of the Lead-mines of Missouri," speaking of the lead-bearing rock of that region, says: "As no remains or impressions of shells, animalcula, or other traces of animal life, are to be found in it, I conclude it to be what geologists term primitive limestone."-page 108. It is true that fossils are rare in this limestone, (yet they do occur occasionally in it,) and that its stratification is but indistinctly marked; but it is associated and conformable with fossilliferous rocks: is also almost precisely similar to the fossil bearing cliff limestone above; and thereiore is evidently a member of the secondary series, not a primitive limestone.