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.

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 nuraber, 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 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

crescent.

was no crumpling or metamorphism of the strata, and they have remained to the present day very much as they were originally deposited, save a slight arching upward in the central portion of the state. The beds have been somewhat compacted by the pressure of superincumbent strata and solidified by the cementing action of calcareous and ferruginous waters, and by their own coherence, but the original character of the formation, as a great sand-bed, has not been obliterated. It still bears the ripple-marks, cross-lamination, worm-burrows, and similar markings that characterize a sandy beach. Its thickness is very irregular, owing to the unevenness of its Archæan bottom, and may be said to range from 1,000 feet downward. The strata slope gently away from the Archæan core of the state and underlie all the later formations, and may be reached at any point in southern Wisconsin by penetrating to a sufficient depth, which can be calculated with an approximate correctness. As it is a water bearing formation, and the source of fine Artesian wells, this is a fact of much importance. The interbedded layers of limestone and shale, by supplying impervious strata, very much enhance its value as a source of fountains.

LOWER MAGNESIAN LIMESTONE.

During the previous period, the accumulation of sandstone gave place for a time to the formation of limestone, and afterward the deposit of sandstone was resumed. At its close, without any very marked disturbance of existing conditions, the formation of limestone was resumed, and progressed with little interruption till a thickness ranging from 50 to 250 feet was attained. This variation is due mainly to irregularities of the upper surface of the formation, which is undulating, and in some localities, may appropriately be termed billowy, the surface rising and falling 100 feet, in some cases, within a short distance. This, and the preceding similar deposit, have been spoken of as limestones simply, but they are really dolomites, or magnesian limestones, since they contain a large proportion of carbonate of magnesia. This rock also contains a notable quantity of silica, which occurs disseminated through the mass of the rock; or, variously, as nodules or masses of chert; as crystals of quartz, filling or lining drusy cavities, forming beautiful miniature grottos; as the nucleus of oölitic concretions, or as sand. Some argillaceous matter also enters into its composition, and small quantities of the ores of iron, lead and copper, are sometimes found, but they give little promise of value. The evidences of life are very scanty. Some sea-weeds, a few mollusks, and an occasional indication of other forms of life embrace the known list, except at a few favored localities where a somewhat ampler fauna is found. But it is not, therefore, safe to assume the absence of life in the depositing seas, for it is certain that most limestone has orignated from the remains of animals and plants that secrete calcareous material, and it is most consistent to believe that such was the case in the present instance, and that the distinct traces of life were mostly obliterated. This formation occupies an irregular belt skirting the Potsdam area. It was, doubtless, originally a somewhat uniform band swinging around the nucleus of the state already formed, but it has since been eroded by streams to its present jagged outline.

ST. PETER'S SANDSTONE.

At the close of this limestone-making period, there appears to have been an interval of which we have no record, and the next chapter of the history introduces us to another era of sand accumulation. The work began by the leveling up of the inequalities of the surface of the Lower Magnesian limestone, and it ceased before that was entirely accomplished in all parts of the State, for a few prominences were left projecting through the sand deposits. The material laid down consisted of a silicious sand, of uniform, well-rounded-doubtless well-rolled-grains. This was evidently deposited horizontally upon the uneven limestone surface, and so rests in a sense

unconformably upon it. Where the sandstone abuts against the sides of the limestone prominences, it is mingled with material derived by wave action from them, which tells the story of its formation. But aside from these and other exceptional impurities, the formation is a very pure sandstone, and is used for glass manufacture. At most points, the sandstone has never become firmly cemented and readily crumbles, so that it is used for mortar, the simple handling with pick and shovel being sufficient to reduce it to a sand. Owing to the unevenness of its bottom, it varies greatly in thickness, the greatest yet observed being 212 feet, but the average is less than 100 feet. Until recently, no organic remains had ever been found in it, and the traces now collected are very meager indeed, but they are sufficient to show the existence of marine life, and demonstrate that it is an oceanic deposit. The rarity of fossils is to be attributed to the porous nature of the rock, which is unfavorable to their preservation. This porosity, however, subserves a very useful purpose, as it renders this pre-eminently a water-bearing horizon, and supplies some of the finest Artesian fountains in the state, and is competent to furnish many more. It occupies but a narrow area at the surface, fringing that of the Lower Magnesian limestone on the south.

TRENTON LIMESTONE.

A slight change in the oceanic conditions caused a return to limestone formation, accompanied with the deposit of considerable clayey material, which formed shale. The origin of the limestone is made evident by a close examination of it, which shows it to be full of fragments of shells, corals, and other organic remains, or the impressions they have left. Countless numbers of the lower forms of life flourished in the seas, and left their remains to be comminuted and consolidated into limestone. A part of the time, the accumulation of clayey matter predominated, and so layers of shale alternate with the limestone beds, and shaly leaves and partings occur in the limestone layers. Unlike the calcareous strata above and below, a portion of these are true limestone, containing but a very small proportion of magnesia. A sufficient amount of carbonaceous matter is present in some layers to cause them to burn readily. This formation is quite highly metalliferous in certain portions of the lead region, containing zinc especially, and considerable lead, with less quantities of other metals. The formation abounds in fossils, many of them well preserved, and, from their great antiquity, they possess uncommon interest. All the animal sub-kingdoms, except vertebrates, are represented. The surface area of this rock borders the St. Peter's sandstone, but, to avoid too great complexity on the map, it is not distinguished from the next formation to which it is closely allied. Its thickness reaches 120 feet.

THE GALENA LIMESTone.

With scarcely a change of oceanic conditions, limestone deposit continued, so that we find reposing upon the surface of the Trenton limestone, 250 feet, or less, of a light gray or buff colored highly magnesian limestone, occurring in heavy beds, and having a sub-crystalline structure. In the southern portion of the state, it contains but little shaly matter, but in the northeastern part, it is modified by the addition of argillaceous layers and leaves, and presents a bluish or greenish-gray aspect. It receives its name from the sulphide of lead,-galena, of which it contains large quantities, in the southwestern part of the state. Zinc ore is also abundant, and these minerals give to this and the underlying formation great importance in that region. Elsewhere, although these ores are present in small quantities, they have not developed economic importance. This limestone, though changing its nature, as above stated, occupies a large area in the southwestern part of the state, and a broad north and south belt in east-central Wisconsin. It will be seen that our island is growing apace by concentric additions, and that, as the several formations sweep around the central nucleus of Archæan rocks, they swing off into adjoining states, whose formation was somewhat more tardy than that of Wisconsin.

CINCINNATI SHALES.

A change ensued upon the formation of the Galena limestone, by virtue of which there followed the deposition of large quantities of clay, accompanied by some calcareous material, the whole reaching at some points a thickness of more than 200 feet. The sediment has never become more than partially indurated, and a portion of it is now only a bed of compact clay. Other portions hardened to shale or limestone according to the material. The shales are of various gray, green, blue, purple and other hues, so that where vertical cliffs are exposed, as along Green bay, a beautiful appearance is presented. As a whole, this is a very soft formation, and hence easily eroded. Owing to this fact, along the east side of the Green-bay-Rock-river valley, it has been extensively carried away, leaving the hard overlying Niagara limestone projecting in the bold cliffs known as "The Ledge." The prominence of the mounds in the southwestern part of the state are due to a like cause. Certain portions of this formation abound in astonishing numbers of well preserved fossils, among which corals, bryozoans, and brachiopods, predominate, the first named being especially abundant. A little intelligent attention to these might have saved a considerable waste of time and means in an idle search for coal, to which a slight resemblance to some of the shales of the coal measures has led. This formation underlies the mounds of the lead region, and forms a narrow belt on the eastern margin of the Green-bay-Rockriver valley. This was the closing period of the Lower Silurian Age.

CLINTON IRON ORE.

On the surface of the shales just described, there were accumulated, here and there, beds of peculiar lenticular iron ore. It is probable that it was deposited in detached basins, but the evidence of this is not conclusive. In our own state, this is chiefly known as Iron Ridge ore, from the remarkable development it attains at that point. It is made up of little concretions, which from their size and color are fancied to resemble flax seed, and hence the name "seed ore," or the roe of fish, and hence oölitic ore. "Shot ore" is also a common term. This is a soft ore occurring in regular horizontal beds which are quarried with more ease than ordinary limestone. This deposit attains, at Iron Ridge, the unusual thickness of twenty-five feet, and affords a readily accessible supply of ore, adequate to all demands for a long time to come. Similar, but much less extensive beds, occur at Hartford, and near Depere, besides some feeble deposits elsewhere. Large quantities of ore from Iron Ridge have been shipped to various points in this and neighboring States for reduction, in addition tɔ that sme ted in the vicinity of the mines.

NIAGARA LIMESTONE.

Following the period of iron deposit, there ensued the greatest limestone-forming era in the history of Wisconsin. During its progress a series of beds, summing up, at their points of greatest thickness, scarcely less than eight hundred feet, were laid down. The process of formation was essentially that already described, the accumulation of the calcareous secretions of marine life. Toward the close of the period, reefs appeared, that closely resemble the coral reefs of the present seas, and doubtless have a similar history. Corals form a very prominent element in the life of this period, and with them were associated great numbers of mollusks, one of which (Pentamerus oblongus) sometimes occurs in beds not unlike certain bivalves of to-day, and may be said to have been the oyster of the Silurian seas. At certain points, those wonderful animals, the stone lilies (Crinoids), grew in remarkable abundance, mounted on stems like a plant, yet true animals. Those unique crustaceans, the trilobites, were conspicuous in numbers and variety, while the gigantic cephalopods held sway over the life of the seas. In the vicinity of the reefs,

there seem to have been extensive calcareous sand flats and areas over which fine calcareous mud settled, the former resulting in a pure granular dolomite, the latter in a compact close-textured stone. The rock of the reefs is of very irregular structure. Of other portions of the formation, some are coarse heavy beds, some fine, even-bedded, close-grained layers, and some, again, irregular, impure and cherty. All are highly magnesian, and some are among the purest dolomites known. The Niagara limestone occupies a broad belt lying adjacent to Lake Michigan.

LOWER HELDERBERG LIMESTONE.

On Mud creek, near Milwaukee, there is found a thin-bedded slaty limestone, that is believed to represent this period. It has neglected, however, to leave us an unequivocal record of its history, as fossils are extremely rare, and its stratigraphical relations and lithographical character are capable of more than one interpretation. Near the village of Waubeka in Ozaukee county, there is a similar formation, somewhat more fossiliferous, that seems to represent the same period. The area which these occupy is very small and they play a most insignificant part in the geology of the state. They close the record of the Silurian age in Wisconsin. During its progress the land had been gradually emerging from the ocean and increasing its amplitude by concentric belts of limestone, sandstone and shale. There had been no general disturbance, only those slight oscillations which changed the nature of the forming rock and facilitated deposition. At its close the waters retired from the borders of the state, and an interval supervened, during which no additions are known to have been made to its substructure.

DEVONIAN AGE.

HAMILTON CEMENT ROCK.

After a lapse of time, during which the uppermost Silurian and the lowest Devonian strata, as found elsewhere, were formed, the waters again advanced slightly upon the eastern margin of the state and deposited a magnesian limestone mingled with silicious and almuninous material, forming a combination of which a portion has recently been shown to possess hydraulic properties of a high degree of excellence. With this deposition there dawned a new era in the life-history of Wisconsin. While multitudes of protozoans, radiates, mollusks and articulates swarmed in the previous seas, no trace of a vertebrate has been found. The Hamilton period witnessed the introduction of the highest type of the animal kingdom into the Wisconsin series. But even then only the lowest class was represented — the fishes. The lower orders of life, as before, were present, but the species were of the less ancient Devonian type. Precisely how far the deposit originally extended is not now known, as it has undoubtedly been much reduced by the eroding agencies that have acted upon it. That portion which remains, occupies a limited area on the lake shore immediately north of Milwaukee, extending inland half a dozen miles. The cement rock proper is found on the Milwaukee river just above the city. At the close of the Hamilton period the oceanic waters retired, and, if they ever subsequently encroached upon our territory, they have left us no permanent record of their intrusion.

The history of the formation of the substructure of the state was, it will be observed, in an unusual degree, simple and progressive. Starting with a firm core of most ancient crystalline rocks, leaf upon leaf of stony strata were piled around it, adding belt after belt to the margin of The growing island until it extended itself far beyond the limits of our state, and coalesced with the forming continent. An ideal map of the state would show the Archæan nucleus surrounded by concentric bands of the later formations in the order of their deposition. But during all the