river valley is even more noticeable, since it lies along the trend of the underlying strata, and was in large measure plowed out of a soft stratum by glacial action. Where it crosses the watershed, near Horicon marsh, it presents the same general features that are seen at other points, and in an almost equally conspicuous degree. Except in the southern part of the state, this valley is confined on the east by an abrupt ascent, and, at many points, by a precipitous, rocky acclivity, known as "The Ledge "-which is the projecting edge of the strata of the Niagara limestone. On the watershed referred to-between the St. Lawrence and Mississippi basinsthis ledge is as conspicuous and continuous as at other points, so that we have here again the phenomenon of a valley formed by excavation, running up over an elevation of 300 feet, and connecting two great systems of drainage.

On the east side of this valley, as already indicated, there is a sharp ascent of 200 feet, on an average, from the crest of which the surface slopes gently down to Lake Michigan. The uniformity of this slope is broken by an extended line of drift hills, lying obliquely along it and extending from Kewaunee county southward to the Illinois line and known as the Kettle range. A less conspicuous range of similar character branches off from this in the northwest corner of Walworth county and passes across the Rock river valley, where it curves northward, passing west of Madison, crossing the great bend in the Wisconsin river, and bearing northeastward into Oconto county, where it swings round to the westward and crosses the northern part of the As a general topographical feature it is not conspicuous and is rather to be conceived as a peculiar chain of drift hills winding over the surface of the state, merely interrupting in some degree the regularity of its slopes There will be occasion to return to this feature in our discussion of the drift. It will be observed that the southeastward slope is interrupted by valleys running across it, rudely parallel to Lake Michigan, and directing its drainage northward and southward, instead of directing it down the slope into the lake.

state.

The Mississippi slope presents several conspicuous ridges and valleys, but their trend is toward the great river, and they are all due, essentially, to the erosion of the streams that channel the slope. One of these ridges constitutes the divide south of the Wisconsin river, already referred to. Another of these, conspicuous by reason of its narrowness and sharpness, lies between the Kickapoo and the Mississippi, and extends through Crawford, Vernon and Monroe counties. Still another is formed by the quartzite ranges of Sauk county and others of less prominence give a highly diversified character to the slope.

Scattered over the surface of the state are prominent hills, some swelling upward into rounded domes, some rising symmetrically into conical peaks, some ascending precipitously into castellated towers, and some reaching prominence without regard to beauty of form or convenience of description. A part of these hills were formed by the removal by erosion of the surrounding strata, and a part by the heaping up of drift material by the glacial forces. In the former case, they are composed of rock; in the latter, of clay, sand, gravel and bowlders. The two forms. are often combined. The highest peak in the southwestern part of the state is the West Blue mound, which is 1,151 feet above Lake Michigan; in the eastern part, Lapham's peak, 824 feet, and in the central part, Rib hill, 1263 feet. The crest of Penokee range in the northern part of the state rises 1,000 feet, and upwards, above Lake Michigan.

The drainage systems correspond in general to these topograpical features, though several minor eccentricities are to be observed. The streams of the Lake Superior system plunge rapidly down their steep slopes, forming numerous falls, some of them possessing great beauty, prominent among which are those of the Montreal river. On the southern slope, the rivers, in the upper portion of their courses, likewise descend rapidly, though less so, producing a succession of rapids and cascades, and an occasional cataract. In the lower part of their courses, the

descent becomes much more gentle and many of them are navigable to a greater or less extent. The rivers west of the Wisconsin pursue an essentially direct course to the Mississippi, attended of course with minor flexures. The Wisconsin river lies, for the greater part of its course, upon the north and south arch of the state, but on encountering the diagonal valley above mentioned it turns southwestward to the "Father of Waters." The streams east of the Wisconsin flow southerly and southeasterly until they likewise encounter this valley when they turn in the opposite direction and discharge northeasterly into Lake Michigan, through Green bay. Between the Green-bay-Rock-river valley and Lake Michigan, the drainage is again in the normal southeasterly direction. In the southern part of the state, the rivers flow in a general southerly direction, but, beyond the state, turn westward toward the Mississippi.

If the courses of the streams be studied in detail, many exceedingly interesting and instructive features will be observed, due chiefly to peculiarities of geological structure, some of which will be apparent by inspecting the accompanying geological map. Our space, however, forbids our entering upon the subject here.

The position of the watershed between the great basins of the Mississippi and the St. Lawrence is somewhat peculiar. On the Illinois line, it lies only three and one half miles from Lake Michigan and about 160 feet above its surface. As traced northward from this point, it retires from the lake and ascends in elevation till it approaches the vicinity of Lake Winnebago, when it recurves upon itself and descends to the portage between the Fox and the Wisconsin rivers, whence it pursues a northerly course to the heights of Michigan, when it turns westward and passes in an undulating course across the northern part of the state. It will be observed that much the greater area of the state is drained by the Mississippi system.

The relationship which the drainage channels have been observed to sustain to the topographical features is partly that of cause and partly that of effect. The general arching of the surface, giving rise to the main slopes, is due to deep-seated geological causes that produce an upward swelling of the center of the state. This determined the general drainage systems. On the other hand, the streams, acting upon strata of varying hardness, and presenting different attitudes, wore away the surface unequally and cut for themselves anomalous channels, leaving corresponding diyides between, which gave origin to the minor irregularities that diversify the

In addition to this, the glacier-that great ice stream, the father of the drift-planed and plowed the surface and heaped up its debris upon it, modifying both the surface and drainage features Looked at from a causal standpoint, we see the results of internal forces elevating, and external agencies cutting down, or, in a word, the face of the state is the growth of geologic ages furrowed by the teardrops of the skies.

GEOLOGICAL HISTORY OF WISCONSIN.

In harmony with the historical character of this atlas, it may be most acceptable to weave our brief sketch of the geological structure of the state into the form of a narrative of its growth.

THE ARCHÆAN AGE.

LAURENTIAN PERIOD.

The physical history of Wisconsin can be traced back with certainty to a state of complete submergence beneath the waters of the ancient ocean, by which the material of our oldest and deepest strata were deposited. Let an extensive but shallow sea, covering the whole of the present territory of the state, be pictured to the mind, and let it be imagined to be depositing

mud and sand, as at the present day, and we have before us the first authentic stage of the history under consideration. Back of that, the history is lost in the mists of geologic antiquity. The thickness of the sediments that accumulated in that early period was immense, being measured by thousands of feet. These sediments occupied of course an essentially horizontal position, and were, doubtless, in a large degree hardened into beds of impure sandstone, shale, and other sedimentary rock. But in the progress of time an enormous pressure, attended by heat, was brought to bear upon them laterally, or edgewise, by which they were folded and crumpled, and forced up out of the water, giving rise to an island, the nucleus of Wisconsin. The force which produced this upheaval is believed to have arisen from the cooling and consequent contraction of the globe. The foldings may be imaged as the wrinkles of a shrinking earth. But the contortion of the beds was a scarcely more wonderful result than the change in the character of the rock which seems to have taken place simultaneously with the folding, indeed, as the result of the heat and pressure attending it. The sediments, that seem to have previously taken the form of impure sandstone and shale for the most part, underwent a change, in which re-arrangement and crystalization of the ingredients played a conspicuous part. By this metamorphism, granite, gneiss, mica schist, syenite, hornblende rocks, chloritic schists and other crystalline rocks were formed. These constitute the Laurentian formation and belong to the most ancient period yet distinctly recognized in geology, although there were undoubtedly more ancient rocks. They are therefore very fittingly termed Archæan-ancient-rocks (formerly Azoic.) No remains of life have been found in this formation in Wisconsin, but from the nature of rocks elsewhere, believed to be of the same age, it is probable that the lowest forms of life existed at this time. It is not strange that the great changes through which the rocks have passed should have so nearly obliterated all traces of them. The original extent of this Laurentian island can not now be accurately ascertained, but it will be sufficiently near the truth for our present purposes to consider the formation as it is now exposed, and as it is represented on the maps of the geological survey, as showing approximately the original extent. This will make it include a large area in the north-central portion of the state and a portion of the Upper Peninsula of Michigan. All the rest of the state was beneath the ocean, and the same may be said of the greater portion of the United States The height of this island was doubtless considerable, as it has since been very much cut down by denuding agencies. The strata, as now exposed, mostly stand in highly inclined attitudes and present their worn edges to view. The tops of the folds, of which they are the remnants, seem to have been cut away, and we have the nearly vertical sides remaining.

HURONIAN PERIOD.

As soon as the Laurentian island had been elevated, the waves of the almost shoreless ccean began to beat against it, the elements to disintegrate it, and the rains of the then tropical climate to wash it; and the sand, clay and other debris, thus formed, were deposited beneath the waters around its base, giving rise to a new sedimentary formation. There is no evidence that there was any vegetation on the island: the air and water were, doubtless, heavily charged with carbonic acid, an efficient agent of disintegration: the climate was warm and doubtless very moist circumstances which combined to hasten the erosion of the island and increase the deposition in the surrounding sea. In addition to these agencies, we judge from the large amount of carbonaceous matter contained in some of the beds, that there must have been an abundance of marine vegetation, and, from the limestone beds that accumulated, it is probable that there was marine animal life also, since in later ages that was the chief source of limestone strata. The joint accumulations from these several sources gave rise to a series of shales, sandstones and limestones, whose combined thickness was several thousand feet.

At length the process of upheaval and metamorphism that closed the Laurentian period was repeated, and these sandstones became quartzites; the limestones were crystalized, the shales were changed to slates or schists, and intermediate grades of sediments became diorites, quartz-porphyries and other forms of crystalline rocks. The carbonaceous matter was changed in part to graphite. There were also associated with these deposits extensive beds of iron ore, which we now find chiefly in the form of magnetite, hematite and specular ore. These constitute the Huronian rocks. From the amount of iron ore they contain, they are also fittingly termed the iron-bearing series. As in the preceding case, the strata were contorted, flexed and folded, and the whole island was further elevated, carrying with it these circumjacent strata, by which its extent was much enlarged. The area of the island after receiving this increment was considerably greater than the surface represented as Laurentian and Huronian on the accompanying map, since it was subsequently covered to a considerable extent by later formations. Penokee range, in Ashland county, is the most conspicuous development of the Huronian rocks in the The upturned edge of the formation forms a bold rampart, extending across the country for sixty miles, making the nearest approach to a mountain range to be found within the state. A belt of magnetic schist may be traced nearly its entire length. In the northern part of Oconto county,there is also an important development of this formation, being an extension of the Menomonee iron-bearing series. A third area is found in Barron county, which includes deposits of pipestone. In the south central part of the state there are a considerable number of small areas and isolated outliers of quartzite and quartz-porphyry, that, without much doubt, belong to this series. The most conspicuous of these are the Baraboo quartzite ranges, in Sauk and Columbia counties, and from thence a chain of detached outliers extends northeasterly through several counties. The most southerly exposure of the formation is near Lake Mills, in Jefferson county.

state.

THE COPPER-BEARING SERIES.

Previous to the upheaval of the Huronian strata, there occurred in the Lake Superior region events of peculiar and striking interest. If we may not speak with absolute assurance, we may at least say with reasonable probability, that the crust of the earth was fissured in that region, and that there issued from beneath an immense mass of molten rock, that spread itself over an area of more than three hundred miles in length and one hundred miles in width. The action was not confined to a single overflow, but eruption followed eruption, sometimes apparently in quick succession, sometimes evidently at long intervals. Each outpouring, when solidified, formed a stratum of trap rock, and where these followed each other without any intervening deposit, a series of trappean beds were formed. In some cases, however, an interval occurred, during which the waves, acting upon the rock previously formed, produced a bed of sand, gravel and clay, which afterward solidified into sandstone, conglomerate and shale. The history of these beds is lithographed on their surface in beautiful ripple-marks and other evidences of waveaction. After the cessation of the igneous eruptions, there accumulated a vast thickness of sandstone, shale and conglomerate, so that the whole series is literally miles in thickness.

The eruptive portions have been spoken of as traps, for convenience; but they do not now. possess the usual characteristics of igneous rocks, and appear to have undergone a chemical metamorphism by which the mineral ingredients have been changed, the leading ones now being an iron chlorite and a feldspar, with which are associated, as accessory minerals, quartz, epidote, prenite, calcite, laumontite, analcite, datolite, magnetite, native copper and silver, and, more rarely, other minerals. The rock, as a whole, is now known as a melaphyr. The upper portion of each bed is usually characterized by almond-sized cells filled with the minerals above mentioned, giving to the rock an amygdaloidal nature. The native copper was not injected in a

molten state, as has very generally been supposed, but was deposited by chemical means after the beds were formed and after a portion of the chemical change of the minerals above mentioned had been accomplished. The same is true of the silver. The copper occurs in all the different forms of rock-the melaphyrs, amygdaloids, sandstones, shales and conglomerates, but most abundantly in the amygdaloids and certain conglomerates.

This series extends across the northern portion of the state, occupying portions of Ashland, Bayfield, Douglas, Burnett and Polk counties. When the Huronian rocks were elevated, they carried these up with them, and they partook of the folding in some measure. The copper

bearing range of Keweenaw Point, Michigan, extends southwestward through Ashland, Burnett and Polk counties, and throughout this whole extent the beds dip north-northwesterly toward Lake Superior, at a high angle; but in Douglas and Bayfield counties there is a parallel range in which the beds incline in the opposite direction, and undoubtedly form the opposite side of a trough formed by a downward flexure of the strata.

PALEOZOIC TIME-SILURIAN AGE.

POTSDAM SANDSTONE.

After the great Archæan upheaval, there followed a long period, concerning wnich very little is known-a "lost interval" in geological history. It is only certain that immense erosion of the Archæan strata took place, and that in time the sea advanced upon the island, eroding its strata and redepositing the wash and wear beneath its surface. The more resisting beds withstood this advance, and formed reefs and rocky islands off the ancient shore, about whose bases the sands and sediments accumulated, as they did over the bottom of the surrounding ocean. The breakers, dashing against the rocky cliffs, threw down masses of rock, which imbedded themselves in the sands, or were rolled and rounded. on the beach, and at length were buried, in either case, to tell their own history, when they should be again disclosed by the ceaseless gnawings of the very elements that had buried them. In addition to the accumulations of wash and wear that have previously been the main agents of rock-formations, abundant life now swarms in the ocean, and the sands become the great cemetery of its dead. Though the contribution of each little being was small, the myriad millions that the waters brought forth, yielded by their remains, a large contribution to the accumulating sediments. Among plants, there were sea-weeds, and among animals, protozoans, radiates, mollusks and articulates, all the sub-kingdoms except the vertebrates. Among these, the most remarkable, both in nature and number, were the trilobites, who have left their casts in countless multitudes in certain localities. The result of the action of these several agencies was the formation of extensive beds of sandstone, with interstratified layers of limestone and shale. These surrounded the Archæan nucleus on all sides, and reposed on its flanks. On the Lake Superior margin, the sea acted mainly upon the copper and ironbearing series, which are highly ferruginous, and the result was the red Lake Superior sandstone. On the opposite side of the island, the wave-action was mainly upon quartzites, porphyries and granites, and resulted in light-colored sandstones. The former is confined to the immediate vicinity of Lake Superior; the latter occupies a broad, irregular belt bordering the Archæan area on the south, and, being widest in the central part of the state, is often likened to a rude crescent. The form and position of the area will be best apprehended by referring to the accompanying map. It will be understood from the foregoing description, that the strata of this formation lie in a nearly horizontal position, and repose unconformably upon the worn surface of the crystalline rocks. The close of this period was not marked by any great upheaval; there