Imagens das páginas

Riding of Yorkshire, and Durham meet, as it were, in a central point, and from which they radiate.” “The mines in this part of England have yielded, of late, on an average, about twenty-five thousand tons of lead annualiy, which is more than one-half of the whole produce of Great Britain."

It appears, then, that the north of England lead district produces more than ONE-THIRD of all the lead obtained in Europe. It is, confessedly, the richest lead region in the world, unless the Wiskonsin lead region may rival and surpass it. I have, for this reason, sought up with care the materials, and here submitted them, for a comparison between the geological formation of that favored mineral region in the old world, and that not less favored, perhaps, to which, in this western portion of the new world, my instructions have directed my attention.

The facts yet collected are not, to my mind, of sufficient number and force to authorize a decision that the lead bearing rock formation of northern England is identical with ihat of Wiskonsin in geological position, and in mineralogical character ; but it certainly supplies proof that the resem. blance, in both respects, but especially in the former, is close and striking.

Materials for further comparison are furnished in the following additional details regarding the mineralogical structure, external appearance, specific gravity, and organic remains, of the cliff limestone of Wiskonsin.

The external features of this rock have been correctly described by Keating, and correspond, also, to the description by Shepard of the limestone in northern Illinois. When newly fractured, the true cliff rock is usually of a light grayish yellow, passing occasionally into a brownish or reddish yellow, especially when it has been exposed, in cliffs, to the action of the weather. In this latter case, the texture, though it often appears san. dy and granular to the naked eye, yet is found, under the microscope, to be made up of minute rhomboidal crystals disseminated over its surface. The fresh fracture exhibits, in addition, a glistening lustre and sub-crystalline aspect. Though sometimes, from exposure, liable to crumble into a very fine powder, it is yet, throughout its mass, a solid, hard, compact rock.

Its fracture is sometimes flat, but usually conchoidal.

In many specimens, innumerable minute crystalline facets of calcareous spar may be found distributed throughout the entire mass, and giving tothe rock, when held up so as to reflect the light, a brilliant appearance.

The specific gravity of this rock varies from 2.65 to 2.70.
The cliff limestone is distinguished into-

Upper beds. More regularly stratified, and less frequently vertically fissured, than the middle and lower; also, more rich in siliceous fossils; containing layers of chert; and, indeed, passing sometimes wholly intomasses of flinty rocks. Containing also good iron ore, and much crystallized carbonate of lime; but lead rarely, and in unprofitable quantities.

Middle beds. Aspect more arenaceous, though it contains but a small percentage of sand. Cherty masses are rare. Stratification imperfect, with numerous vertical fissures. Rich in ores of lead and zinc, associated with iron in small quantities.

Lower beds. Also of arenaceous appearance; rather more distinctly stratified than the middle beds; and imbedding, more frequently than these, siliceous cherty masses. They contain the same ores as the middle beds, with the addition of copper ore; and sulphuret of zinc (black-jack) is very common.

These several beds are also distinguishable by their fossils.

The most characteristic fossils of the cliff limestone of Iowa and Wis· konsin are ::

Casts (often siliceous) of several species of terebratulæ. (See sketch No. 5.) Some of them, probably, of new species. These are chiefly confined to the upper beds. They are numerous and very perfect.

Several species of catenipora, (chain coral,) in greater abundance and in more perfect preservation than I have ever seen them elsewhere: among them, the catenipora escharoides of Lamarck; the catenipora labyrinthica of Goldfuss; and another species not described by Goldfuss, nor elsewhere that I have seen-probably new. I name it the catenipora verrucosa. (See sketch No. 6.) The tubes are smaller than those of the labyrinthica, closer together, anastamose more frequently, and are furnished internally with minute transverse partitions similar to those in the tubes of the sarcinula organum of Lamarck. Externally, the parietes are thickly studded with irregular wart-like incrustations. These catenipora are very characteristic of the upper beds of the cliff limestone. They do not occur in the rich lead-bearing strata.

A coscinopora, (the sulcata ? of Goldfuss,) the only coralline discovered in the middle and lower beds, and therefore characteristic of the true leadbearing rocks. A drawing of this fossil will be found under the head “ lead-mines."

Several species of calamopora, columnaria, tupipora, aulopora, sarcinula, (costata ?) astrea, cyathophylla, and caryophyllia. These are found, with the chain coral, in the upper beds.

Several casts of spiral univalves; of a cirrus resembling the perspectivus ; of an ampullaria, resembling the canaliculata ; imperfect impressions of a long spiral univalve resembling the genus vivipara ; all taken from the walls of a lead-bearing fissure near Dubuque; a univalve of the genus trochus, (found in the underlying blue limestone ;) and another resembling a delphinula, found chiefly in the building rock.

I also found, both in the cliff rock and in the blue limestone, (but chiefly in the latter, of which it is the characteristic fossil,) several species of the strophomena of Goldfuss. * (See sketch No. 7.)

Likewise, both in the cliff rock and in the blue limestone, several species of orthoceratites. At Eagle point, on the Mississippi, several gigantic specimens of this fossil were discovered ; one measuring in diameter five inches and a quarter, and another four feet in length, were obtained by Mr. Bolton,t one of my sub-agents.

A variety of other less important fossils were collected in the course of the expedition, which time does not permit me to examine or describe. This portion of my report is also imperfect, because here in the west I have no access to any public library, and my own contains but few of the voluminous works of reference, which, in treating of fossil remains, one ought to have an opportunity of consulting.

* A species of this strophomena is stated by Shepard in his “Geology of Upper Illinois," to abound in the magnesian limestone near Navarino, at the head of Green bay. It is drawn and described by him, in that work, as a new species of producia.

+ I am also indebied to Mr. Bolion tor many interesting fossils and minerals collected in the neighborhood of Dubuque and Mineral Poini.

LEAD-MINES. The extent and position of the lead region may be seen by referring to chart No. 1.

The boundary is represented by a red line; and within it are included, as well all the productive diggings, as the entire region wherein, from the observations made during this survey, there is reason to believe pro- . ductive lodes of lead may readily be found.

This lead region lies, as will be remarked, chiefly in Wiskonsin, including, however, a strip of about eight townships of land in Iowa, along the western bank of the Mississippi, the greatest width of which strip is on the Little Mequoketa, about twelve miles from east to west, and including also about ten townships in the northwestern corner of Illinois. The portion of this lead region in Wiskonsin includes about sixty-two town. ships. The entire lead region, then, comprehends about eighty town. ships, or two thousand eight hundred and eighty square miles; being about one-third larger than the State of Delaware. The extreme length of this lead region, from east to west, is eighty-seven miles; and its greatest width, from north to south, is fifty-four miles.

The boundary of this region commences on the Mississippi river, where the south line of township eighty-seven north, range four east of: the fifth principal meridian, crosses that stream, immediately below the mouth of the Little Tête des Morts; and runs thence six miles due west, thence six miles north, thence six miles west, thence northwest diagonally through township eighty-eight, range two east, and township eighty. nine, range one east, both of the fourth principal meridian, until the line strikes the fifth principal meridian, where the line dividing townships eighty-nine and ninety crosses said meridian line; thence six miles north, thence six miles west, thence three miles north, thence three miles west, thence three miles north, thence three miles west, thence three miles north, thence three miles west, thence three miles north, thence three miles cast, thence north to the Mississippi, which it strikes about seven miles below the mouth of Wiskonsin river ; thence, crossing the Mississippi, it runs diagonally through township five, range six west, of the fourth principal meridian, to the northeast corner of said township; thence six miles east, thence three miles north, thence eighteen miles east, thence three miles north, thence three miles east, thence three miles north, thence nine miles east, thence six miles south, thence twelve miles east, thence eighteen miles east, passing along the northern base of the Blue Mounds; thence twelve miles south, thence twelve miles east, thence twelve miles south, thence six miles west, thence six miles south, thence twelve miles west, thence six miles south, striking the northern boundaryline of the State of Illinois at the point where the line between ranges five and six cast of the fourth principal meridian crosses said boundary-line; thence, with said boundary-line, six miles west, thence twelve miles or thereby south, to the southeast corner of section thirteen, township twentyseven north, range four east of the fifth principal meridian; thence six miles west, thence three miles south, thence sixteen miles or thereby east, to the east bank of the Mississippi river, about five miles below the mouth of Fever river, and about a mile and a half below the place of beginning already designated, on the western bank of the Mississippi.

*A few fractional townships, originally included in my special reports, within the lead region, have been, on re-examination, thrown out, as not strictly belonging to the district which is likely to afford productive veins of lead ore.

This lead region is, in general, well watered; namely, by the Peccatonnica river, Apple river, Fever river, Platte river, Grand river, the headwaters of Blue river, and Sugar creek; and on the Iowa side, by the Little Mekoqueta, and the lower portion of Turkey river: all of these streams, being tributaries of the Mississippi.

The highest points within this region are the summits of the Blue Mounds, two hills of a conical shape, composed of chert and other varieties of flint rock, in the northeast portion of the tract, and rising to the height of one thousand feet above the Wiskonsin river. The Platte Mounds, also of conical form, and about six hundred feet high, occupy nearly the centre of the lead region.

These isolated and towering mounds, so conspicuous a feature in the landscape of Wiskonsin, are evidence of the denuding action to which, under the crumbling hand of time, the surface of our globe is continually subjected, and which the more durable siliceous masses of these hills of flint have been enabled partially to resist.

It will be perceived by consulting chart No. 1, that the northern boundary of the Wiskonsin lead region is nearly coincident with the southern boundary-line of the blue limestone where it fairly emerges to the surface. No discoveries of any importance have been made after reaching that formation; and when a mine is sunk through the cliff limestone to the blue limestone beneath, the lodes of lead shrink to insignificance, and no longer return to the miner a profitable reward for his labor. Indeed, the small quantities of lead ore which have occasionally been found in the blue limestone, occur in veins not much; thicker than writing. paper, which have insinuated themselves into the slender seams of the stratification. This coincidence between the northern boundary of the productive lead region, and that of the cliff limestone, is an example of the practical utility and application of the geological divisions of the different formations. Even if not a single shaft had ever been sunk in Wiskonsin, it might have been predicted, with probability, that this change in the formation would be strictly accompanied with a corres. ponding change in the productiveness of the lead veins.

Mr. Carne has observed, regarding the metalliferous veins of Cornwall, that it is a rare circumstance when a vein, which has been productive in one species of rock, continues rich long after it has entered into another ; and this change, he adds, is even remarked when the same rock becomes harder or softer, more slaty or more compact. Hence it was very unlikely's that the Wiskonsin lead ore, so rich in the cliff limestone, should retain the same rich character in the blue limestone, even had the structure of this last been equally adapted to the bearing of lead. But, in truth, rocks of a schistose character, composed of extensible layers, and devoid of vertical fissures, like this blue limestone, seldom contain lead ore in quantity. Phillips, in his recent geological treatise, from which we have already made several quotations, justly remarks : “ It is not because of any peculiar chemical quality that limestone yields most lead ore on Aldstone Moor, but because of its being a rock which has retained openness of fissure. Gritstones, in many mining fields near Aldstone Moor, are equally productive; but shales, as being soft extensible layers, have closed up the fissures, and their crumbling faces appear to have rejected the crystallizations which attached to the harder limestone, gritstone, and chert.”

These remarks apply, with force, to the fissured cliff rock of Wiskon-, ,

sin, compared to the softer and more slaty-structured blue limestone beneath it.

It will also be remarked, that the designated lead region is almost exclusively confined to the northern half of the cliff limestone formation of Iowa and Wiskonsin; which northern half is occupied by its middle and lower beds. The upper beds (lying in the southern portion of the district) do not, as already intimated, furnish productive veins of lead ore. The crevices in these upper beds seem to be less numerous, and either empty or filled with iron ore, (hydrated brown oxide,) or calcareous spar, (crystallized carbonate of lime,) to the almost entire exclusion of veins of lead.

It follows, from the above observations, that the mines in the northern portion of the district are less likely to be productive to a great depth, than those along its southern and western boundaries.

It follows, also, that, in the southern portion of the district not included by me in the productive lead region, mines of value may yet be discovered, by sinking shafts through the upper beds of the cliff limestone to the lead-bearing beds beneath, unless, indeed, these lower beds should prove to be beyond the sphere of action where the lead has been produced. This latter contingency is possible ; yet the richness of the mines in the southern and western portion of the lead district, (at Apple river and Dubuque, for example, as compared with some of the northern mines, seems to indicate that the ore may still continue rich in the descending beds. Since, however, this is, as yet, an unsolved problem; and even if it were solved, as it would require much capital to sink shafts to the necessary depth, and since mines of this depth would doubtless be inundated with water, and require steam-engines to drain them, I have not considered it my duty to include this southern portion of the district within the bounds of the productive lead region; although, hereafter, should the easily acces. sible lodes be exhausted, and the demand for lead rapidly increase, it may become so.

With regard to the magnesian limestone which underlies the blue limestone and sandstone strata, and comes to the surface in the extreme northeastern portion of the district, its similarity in structure and composition to the cliff limestone, including its disposition to form vertical fissures, and its probable identity with the rock formation in the Missouri lead region, might induce the expectation that it, also, would be rich in lead ore. It may be so; but the frequent occurrence of iron ore (brown oxide) in those townships where this formation prevails,t (as in the upper beds of the southern portion of the district,) together with the little success

* The analyses of this rock showed that one of the specimens is a purer magnesian limestone than the cliff rock itself. The results were as follows:

First analysis. Second analysis, Carbonate of lime

- 43.00

52.40 Carbonate of magnesia

• 39.40

42.00 Silex -

- 14.50

1.80 Alumina, with a trace of iron ..

. 1.80

1.34 Loss by ignition, (chiefly water)

. 1.90 1 - 1.70 Loss .

. .30



100.00 + It is not improbable, from its similarily to the lead-bearing rock in Missouri, that this lower magnesian limestone, if it be extensive north of the Wiskonsin river beyond the limits of our survey, may there yield productive veios of lead ore.

« AnteriorContinuar »