Imagens das páginas
PDF
ePub

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 oc. cur 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.—(dee 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 diagrain 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 met. alliferous 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 imbed. ded fossils, the presence of which constitutes the most decisive evidence of the identity of geological strata.

by recent aled up in varion States, genera

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 lowa 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 clif 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 de. tected 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 en. gross the entire mountain limestone group; and the coal-measures, where tound, (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 lowa, 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 series. It belongs, further, to a division of this class of rocks described in Europe as the mountain limestone, or sometimes as the carboniferous of metalliferous, or encrinital limestove. 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 some what uncommon feature in the geology of western America presents itsell. 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 chiedy observable in the bluffs on both sides of the Wiskonsin river ; whence (! 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 bide limestone can be seen emerging from beneath it to the surface ; and, secondly, as apparently supplying an example of those alternations of neigh boring strata, to which I have already alluded as being partial exceptors 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 Time stone group, where it has been observed east of the Mississippi, there occulos and shows itself in the Wiskonsin bluffs, a stratum of sandstone, in sollic places of a deep red, and in others of a white color, resembling loal-sugana and thence called, in Dr. Locke's diagrams exhibiting the sections on Wiskonsin river, saccharoid (or sugar-like) sandstone.

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

* It differs, however, from the cliff rock in several particulars, when examine is almost destitute of fossils; its lower members have sometimes a greenish lint; " siliceous masses are more rugged and quartzose than those in the clifi rock.. DE tallized specimens of rose-colored quartz are of frequent occurrence in its DEO of an oölitic structure (made up of minute egg-shaped grains, like the roe abundant,

118, when examined in situ. It

greenish lint; the imbedded to the cliff rock. Beautiful crys.

Trence in its beds; and chert haped grains, like the roe of a fish) is also

[ocr errors][ocr errors]

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.f

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

[ocr errors][ocr errors][ocr errors]

* 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 ihe 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 il to be what geologists term primitive limestone."--page 108. It is true that fossils are rare in this limestone, (yel they do occur occasionally in it,) and that its stratification is but indistinctly marked; but it is associated and conformable with fossiiliferous rocks: is also almost precisely similar to the fossil bearing cliff limestone above; and thereiore is evidently a member of ihe secondary serius, not a primilive limestone.

the Blue Mounds to the Wiskonsin river, at the northeast corner of the ! tract; exhibiting the order, dip, and thickness of the strata, as they suess1. sively sink from the surface, and gradually disappear beneath the beds si

the streams. A horizontal geological chart, similar to a part of chart 1, 01 cut off so as to correspond with the above vertical section, is added; 1 further illustrates the manner in which the successive strata come to the surface, and extend over a larger or smaller portion of country in proportion to their dip and thickness.

The vignette on this chart is a view on the Mississippi river, abcut fre miles above the mouth of Rock river, exhibiting the interesting geolocica? spectacle of the coal-measures resting immediately on the cliff limestone

The coloring on this chart corresponds with that on chart No.l. ? In this chart, the dip and heights, compared to the horizontal distress, are necessarily exaggerated. The actual average dip of the rocks throaghout the district, according to the observations made by Dr. Locke, is imm nine to ten feet per mile, but it is occasionally much greater; for example,

from the mouth of Turkey river to Prairie du Chien, the blue limestone "rises at an average rate of seventeen and a half feet per mile. The diz bor. : 'ever, is subject to undulations; for instance, at Dubuque, the blue limestone Sudoes not show itself above low-water mark; at Eagle point, a me and ?

half up the Mississippi, it rises ten feet above low water; at the mouth of the Little Mekoqueta, four miles farther up, its height above low-wate? mark is forty feet; at the mouth of Turkey river, twenty miles farther of:

it disappears again beneath the waters of the Mississippi ; a few miles de . yond this point, it emerges again to the surface; and, finally, at Prairie e

Chien, twenty miles above Turkey river, its upper surface has already "attained an elevation of more than four hundred feet above the level of in

Mississippi. The line of greatest general dip is about south, ten to iweaty degrees west.*

The importance of these observations on the dip of the rocks, forming as ; they do the materials to calculate the thickness of each stratum at any greu

spot, is very great. Indeed, such observations are indispensable, bedcre accurate estimate can be formed of the value and extent of a mineral tratto They indicate, with much fidelity, the depth to which, at different perille, a productive vein of ore is likely to extend.

The diagrams attached to Dr. Locke's report, which exhibit vertical set 'ntions of the strata (specifying the thickness of each) at Dubuque, ale

Chien, and the Blue Mounds, and also his diagram of relative heights, sup ply much useful and accurate information on the subject.

I now proceed to describe more accurately the prevailing and lead bcas 2 ing rock of the explored district, namely:

o portion of the distritt. Sometimes

Dally to the porthwest, as if I al-measures bad ocurred, 314 13 vest and souib, ha a les car MICH

creek, botilvast uilere

* There are, however, several local tills in the southern portion of the distrit the dip there is to the northeast and north, and even occasionally to the per dislocation of the cliff torination at its junction with the coal-measures had southern margın had been slightly elevated towards the southwest and soud," than that which raised the whole mass to the northeast. On Crow creek, noriu port, the northern dip can be distinctly seen.

Indeed, there are appearances of a slight disposition in the whole strata to Latincelice. cewire of the lead region, (the vicinity of ibe Platte Mounds :) for the underlyina" which appears on the sw lace where the cliff limesione runs oui, may be tract somewhat semi circular land, circumscribing the lead region from Turkey point where the Peccatonnica enters the State of Illinois ; in the southern porta however, only visible in the cuts of the streams.

t I regret that the severe indisposition of Dr. Locke at Mineral Point and an the coinpletion of several observations in connexion with this importail suo

the underlir bone line.de

may be traced in 3 m

| Turkos tiver Learly 11.30 le saubern porti o of that way

ineral Point and Galepa presented

« AnteriorContinuar »