Imagens das páginas

Section No 3.–From the Blue Mounds to the Wiskonsin river.

We have here attempted to represent two sections: one seen partly beyond the other. The nearer one is represented in darker shade than the farther one. The nearer and darker one is a section along the valley of two small streams; and the farther one is a section through the mound, and along the ridge or highest part of the bank of those streams. The outline of the darker section is marked by several barometrical stations, as follows:

First, " 435.” This was the place of encampment, near the head spring of the Peccatonnica river; that number (435) is the altitude above the waters of the Wiskonsin at Arena.

Second, “590.” This is the dividing-ridge between the waters of Rock river and Wiskonsin river.

Third, “ 421." The top of the sandstone.

Fourth, “ 132.” In a deep ravine, the channel of a small tributary to the Wiskonsin river.

Fifth,“ 31." The edge of a sandy plain, extending from the bluffs two and a half miles to ihe river.

Sixth, "0." The Wiskonsin river at Arena

The town of Arena has a place on the maps, was once a post-office, and contained a single log-house, which has since been burnt down. I found only the ashes of the log-house, an empty cart, and a few suits of “squawpoles,” (a nickname for Indian tent-poles.) Such towns are not on. common in this region.

Beginning at the top of the Blue Mound, and proceeding downward, this section presents the following strata :

410 169

I. Siliceous beds of chert, hornstone, and other varieties of flint, in very large masses. Some of them, I should say from recollection, were thirty feet long, twenty feet wide, and ten feet thick. They are stratified, and lie almost or quite in contact ; but show a disposition to the nodular form, by frequent vertical joints, and being variously interrupted. The structure is often cellular, and the cells lined with small quartz crystals, which give a rough appearance to the masses. Fossils are rather rare in this siliceous portion, but such as were found belonged to the cliff formation. This member of the cliff, which is usually found as a mere included layer of a few inches, is here developed to -

II. The cliff limestone, including lead ore

III. The blue fossilliferous limestone, very thin, and in some places entirely wanting, or apparently so. I have assigned it no thickness.

IV. Sandstone. The same as that described at Prairie du Chien. There is every evidence that this is identical with that on the Mis. sissippi, for it may be traced continuously along the Wiskonsin from one point to the other. It is remarkable for having its upper surface at an exact and even plane, very nearly level. In an excavated area, where several ravines meet in the same valley, and with the eye at any point of the upper surface of this sandstone, all other points appear in the same plane like an emptied lake, leaving a line of ice to mark its original height; even where the rock is covered by earth, the vegetation changes so abruptly in sort and color, at the surface of the sandstone, that the line may still be distinctly traced.



In the section which we are describing are represented two outliers, or natural columns running up to the top of the sandstone, and capped with small pines. The tops of several such outliers, where they occur, will be found to lie in the same exact plane. We have assigned to this stratum the thickness of .

V. Alternations of magnesian limestone and sandstones · I had little opportunity of examining this stratum, and cannot give its characters; it is probably similar to the corresponding lay. ers at Prairie du Chien.

VI. Sandstone.- Variable, and not well defined.

VII. Limestone, (probably magnesiun.)—In thick, well-defined strata, very suitable for building, but of rather a dark, ferruginous color. At the edge of the sandy plain “ 31,” it was lying on the hillside in very large tabular masses, two feet thick, and twenty or thirty feet in diameter, with sharp angles and edges; sufficient evidences of integrity, strength, and durability. In external characters, it resembled the harder specimens of the cliff limestone, but was destitute of fossils; in thickness, above water



[merged small][ocr errors][ocr errors][merged small]

The sandy plain of Arena is covered, next to the bluffs, with a forest of small timber. Near the river are several groves of small pines, of a peculiar species; and so constantly does the wind blow from the southwest, that these trees lean with great uniformity about five degrees in the opposite direction. At various places, also, in the open plain, the wind has taken such hold of the sand as to uproot the grass, and, drifting the dried sand continually forward, has formed broad valleys, terminated by a semicircular sand drift, ten or twelve feet high, at the northeast end. Through the middle of this drift is a narrow slit. As the sand-drift progresses, vegetation is buried and destroyed, and thus prepared to be in time uprooted by the persevering breeze, which, urging the drift forward, by carrying the particles over from the windward and depositing them to the leeward side, finally exposes and ex. cavates the spot which was once most deeply covered. - The sandy stratum (IV) gives rise to the alluvial sands of the Wiskonsin, and to those of the Mississippi ; indeed, it seems to be nearly or quite destitute of mica or feldspar; and the clear, glassy particles of quartz, of which it is composed, sparkle in the sun like minute brilliants. That the quartz is pure and unmixed; that the angles of the fragments are sharp and unabraded ; that the surface of the stratum is an exact plane, like the undis. turbed surface of a liquid; are interesting problems for the speculative geologist.

I have thus gone briefly through with the subjects of the altitudes and the sections, and have incidentally given such imperfect sketches of distinctions and external characters of strata as were necessarily observed in a very hasty search for their several boundaries and junctions, while taking their admeasurements; but I have not offered these sketches as in any degrce answering as substitutes for the more complete discriminations which may be expected in your report. It is true that a sixty days' labor can but begin such a task as this field offers; but from the numerous specimens which you have collected, and from your known skill in analysis, (that allimportant source of valuable knowledge:) I hope you will review and en.

[ocr errors]
[ocr errors]

large upon the subjects of which I have here done little more than ex! ... their proximate proportions.



mes the heightlich is to be imulta

In determining altitudes by the barometer, it is important (and almost indispensable, indeed) that there should be simultaneous observations both at the place, the height of which is to be determined, and at sorte point not very remote, the height of which has been previously ascer. tained. This becomes the more necessary in a northern latitude, and in the autumnal season, when the changes of a stationary barometer are fre. quent and rapid. My efforts to obtain a co-laborer, either at Galens or Dubuque, were fruitless; and the observations of Dr. Engelmana, of St Louis, although accurate in themselves, were at too great a distance Under these circumstances, my mode of operating was to observe the barometer at some known point of reference, as the surface of the river: and, proceeding with as little loss of time as possible to the height, make my second observation. When this was done in a few minutes, the proximate height was presumed to be deducible from the results; but it from the time, or other circumstances, an atmospheric change of the barometer was suspected, the instrument was taken back to the first station the point determined, and an equation made according to the result When the distance from the height to the station to which it was to be referred was several miles, then intermediate stations were established. and the observations repeated both going and returning, noting accurately the time by a chronometer. This last method was taken to determine the height of Blue Mound above the Wiskonsin river. The distance is about fourteen miles. Starting from my camp at the head of the Peccatonnica, at the foot of the mound, early in the day, I observed the barometer every hour upon my journey, until in four hours I had reached the river, and suspended the instrument close to the water's edge. I then returned by the same route, and repeated the observations at the same stations. This would have indicated any progressive change, and pretty nearly its hourly rate. As it happened in that case, the change, if any, was very small; and I obtained not only the altitude of my camp, but of three intermediate points. These stations and their altitudes are marked on the " section of the strata from the Blue Mounds to the Wiskonsin river,” at the figures “ 435," (the encampment,) « 421," “ 132," and “31.” After all, there were several heights, which, for the want of a stationary observer, I was unable to determine; that of the Platte Mounds was one of them.

The accompanying table, or chart, scarcely needs an explanation. I have placed them in the order of their height, and this happens to be nearly in the order of their latitude; the elevations becoming greater in proceeding northwardly. The geological distinctions marked upon the diagram serve to afford to the eye a direct comparison of separate and local observations, as between Pike's Mountain and Blue Mounds, where it appears. that the corresponding strata, cliff limestone, blue limestone, and sandstone, each occupies a higher place in the latter than in the cormer, even supposing the points of reference to be level; which they are

not, by the fall of the Wiskonsin, from Arena to its mouth, rendering the difference still greater. Thus is furnished important evidence in reference to the dip of strata, that it is, partly at least, westward; other evidence showing that it is also southward, we are led to the conclusion that it is between south and west.

A remark on the western use of the term mound.It was first very properly applied to the numerous artificial tumuli of a conic or flattened hemispheric figure, which abound more or less in every part of the region west of the Allegany mountains. It seems that in the

far west” the term has been promoted to the office of representing isolated natural elevations of a conical form, which, emerging from the elevated prairies of Wiskonsin and lowa, rise above the general table of the country to the height of two hundred, to four hundred, or even to six hundred feet; and, as appears in this table of altitudes, one thousand feet above the contiguous streams. They are certainly seen at the distance of forty miles, and rising, as they do, from the monotonous line of the prairie, like distant islands seen on the ocean, they are perpetually calling back the wandering eyes of the traveller, who sees them sink as he recedes, rise as he approaches, change their apparent position as he insensibly winds his course, or vary their hues of dim distance as the atmosphere drops or dissolves its misti. ness.

Deceived by the term “mound,” which should be mountain, some writer has denominated them “ interesting antiquities.” To be sure they are antiquities, dating as far back as the “transition” or “secondary" epoch of the creation.


Popular elementary definitions. The elements of terrestrial magnetism consist simply of the force, power, or intensity with which the earth attracts the magnetized needle, and of the direction in which that force acts; but, from the vast importance of the horizontal or compass-needle, both in navigation and surveying, and from the facility of suspending and experimenting with the same, it is customary to estimate certain elements of the needle in that position, although it is seldom the direction (never in our latitude) in which, if allowed to move freely in all directions, it would place itself. The quantities sought to be measured are usually four :

First. The declination, “variation," or direction of the horizontal needle, as it respects the true astronomical north or south points.

Second. The force, or intensity with which the horizontal needle is attracted by the earth, and held in its direction: this is called the horizontal intensity.

Third. The dip, or true course in which a needle, perfectly free to move in all directions, would finally rest and be held by the earth's attraction.

Fourth. The force or intensity with which the needle, in the direction of the dip, is attracted by the earth: this is called the total intensity."

* To avoid a circumlocution of language, the earth's attraction is named without expressing particularly the mutual attraction between the earth and needle.

Magnetical declination or variation. Most persons are aware that the compass-needle does not everywhere point to the true north, but varies in its direction in different places on the earth's surface, in such a manner that it rather points east of it, directly to. wards it, or west of it. The force with which the earth attracts or pulls such a needle, so as to hold it in its direction, and cause it to vibrale if it be moved out of that direction and be suffered freely to return, is called the horizontal intensity, and is measured by the quickness of the vibrations. Thus, when there are a greater number of vibrations of the same needle, in the same time, the horizontal intensity is greater, being as the squares of the numbers of such vibrations. A vibrating needle used for determining the intensity, is a “magnetical pendulum," acted upon by magnetism as a clock pendulum is by gravitation.

Magnetical dip. Make a needle of tempered steel, with pivots at the sides, so that it can turn like a cannon, and point up or down; balance it so nicely that it will stay in any position in which you place it: this must be done while the steel has no magnetism. Next, magnetize that needle by “touching" it with other magnets, as directed in the books on magnetism. Lastly, place the pivots in proper supports, exactly crosswise of the line in which the compass-needle points: it will no longer remain balanced, especially in the horizontal position, but, in the latitude of the United States, the north end will turn down, nearer to a perpendicular than to a level. This tuming down, or out of the level, is called the dip; it is measured by the num. ber of degrees which the north end descends from a level line. The dip increases as we travel north ward, until at a point north of the western part of Hudson's bay, where it points directly downward. At or near the equator there is no dip, or the dipping-needle lies level; and south of that point, the south end of the needle descends, as does the north end in the northern hemisphere.

Now, whatever direction the dipping-needle takes, it is held there by a magnetical force of the earth, which, when it is moved out of that direc. tion, draws it back again, and causes it to vibrate like a pendulum, and, finally, to settle at the proper dip. If the force be greater, the vibrations will be quicker: this force is called the total intensity, and is not usually ascertained by the vibrations of the dipping-needle, but is deduced by calculations from the horizontal intensity, and the dip, at any locality. This force, on the whole, increases as we proceed northwardly; but the hori. zontal intensity, in consequence of the increase of the dip, diminishes in the same direction. At the magnetic pole, where the dip would be 90 degrees, (viz: the dipping-needle perpendicular,) the horizontal intensity would be nothing, and the common compass-needle would point in one direction as soon as in another—the magnetical force of the earth pulling it, at all points, directly downward upon the supporting pivot.

Now, to measure these four quantities, in different localities, as accurately as possible, has been a part of my labors in the late brief survey of a part of our territories; and to communicate the result is the object of the present article.

Some sorts of iron ores have an influence on the magnetical needle, and change either its direction or its intensity. The effect of such ore increases directly as the quantity or mass, and diminishes as the squares of the dis

« AnteriorContinuar »