with it no refreshing coolness. A little after midday, a dark cloud, emitting from its edges a pale phosphorescent light, rose from the lake, and advanced against the wind. Its approach was indicated by a loud roaring, and, when it reached our encampment, the trees swayed to and fro, and many were prostrated around us. The air was filled with flying leaves and branches. Voyageurs and men instinctively rushed into the river, and remained until the fury of the storm had abated.

Thunder-storms of great violence are not unusual; and the large tracts of prostrate timber frequently met with in the forests, and known as "windfalls," indicate the path of the tornado.

Sudden gusts of wind spring up on the lake, and hence the oldest voyageurs are most inclined to hug the shore.

Instead of seeking for a solution of these phenomena by a resort to natural causes, they ascribe them, like the Scandinavians of old, to the freaks of a crazy old woman, who is endowed with ubiquity:

"Now here, now there, and everywhere."

Before the middle of September, a change in the elements becomes observable. The light and sportive breezes are succeeded by heavy gales, which sweep over the lake, and render coasting exceedingly hazardous.

Auroras, even in, midsummer, are of frequent occurrence, and exhibit a brilliancy and extent rarely observed in lower latitudes. The commonest phenomena are these: A dark cloud, tinged on the upper edge with a pale luminous haze,' skirts the northern horizon. From this, streaks of orange and blue-colored light flash up, and often reach a point south of the zenith. They rapidly increase and decrease, giving to the whole hemisphere the appearance of luminous waves, and occa sionally forming perfect coronæ. They commence shortly after sunset, and continue through the night. The voyageurs regard them as the precursors of storms and gales, and our own observations have confirmed the result. Occasionally broad belts of light are seen spanning the whole arc of the heavens, of sufficient brilliancy to enable one to read.

In the winter these phenomena are much more frequent, and the ground appears tinged with a crimson hue. The aurora indicates a disturbance of the equilibrium in the distribution of terrestrial magnetism, and, according to Dové, may be regarded, not as an externally manifested cause of this disturbance, but rather as a result of telluric activitymanifested on one side by the appearance of light, and on the other by the vibrations of the magnetic_needle.*

On one or two occasions we have witnessed the rare and beautiful phenomenon of parhelia, or mock suns.

For a full exposition of these phenomena, consult Humboldt's Kosmos, vol. I.

CHAPTER III.

GEOLOGY OF THE COPPER REGION.

Maps.-Classification of the rocks. Their composition. - Keweenaw 'Point.-Range and extent of the trap.-Local details.-District between Portage lake and the Montreal river.-Range and extent.Metallic contents, and the association of copper.-Porcupine mountains. -Isle Royale.-Its similarity in geological structure to Keweenaw Point.-Range and extent of the trap.-Metallic contents.

That portion of the Lake Superior land district whose geology we purpose to delineate in the following report is represented on the accompanying maps, entitled

1. A geological map of Keweenaw Point.

2. A geological map of the region between Portage lake and the Montreal river.

3. A geological map of Isle Royale.

These maps comprise the territory known as the copper region.

The iron region, though of less extent, but of equal economical value, will form the subject of a subsequent communication.

The rocks which constitute the solid framework, so to speak, of this district, are divisible into two classes, widely different in their origin and composition-the igneous and aqueous.

Under the first division may be included the several varieties of trap using this term as a generic one-such as greenstone, granular and amygdaloidal trap, basalt, &c. These rocks appear to have been generated within the bowels of the earth by the action of fire, and in some cases to have been protruded in vast irregular masses, forming conical or domeshaped mountains; at other times, in continuous lines of elevation; while in others they appear to have flowed like lava-currents in sheets over the sands then in the progress of accumulation. The mineral substances which compose these ancient lavas are very various in their nature, but in general it may be said that the predominating rock is one composed of an intimate mixture of labrador, hornblende, and chlorite, though the latter is not an invariable accompaniment.

To the second class, or aqueous formation, may be referred the sandstones, shales, and limestones of this district. They occur in stratified beds, divided into layers, strata, laminæ, &c. The materials appear to have been transported by currents and deposited on the floor of the ocean, where they subsequently became consolidated.

In addition to these, there is another class of rocks which have undoubtedly resulted from the joint operation of igneous and aqueous causes. The materials appear originally to have been ejected through rents and fissures in the crust of the earth to the surface, where they were subsequently transported and ground up by currents and deposited in stratified beds. This class of rocks is termed by M. Prevost pluto-neptunean; and

* Article "Formation," Dictionnaire Universel d'Histoire Naturelle.

·

to this division may be referred the conglomerates and chlorite beds associated with the trap.

The metamorphic rocks, or those which were supposed originally to have been deposited by water and subsequently modified by heat, causing them to resemble igneous products, are developed only to a limited extent in the copper region; but in the iron region they are displayed on a scale of vastness, and form the most interesting feature in the physical history of the district.

The mineralogical character of the trappean rocks, being a complex and difficult subject, will be separately described in the chemical part of the report. At present it may, however, be briefly stated that they are in general made up of an intimate mixture of labrador and hornblende, forming a dark-colored homogeneous mass, in which the separate minerals cannot be distinguished by the eye. Chlorite, though not an invariable accompaniment, is often present in a considerable quantity. Magnetic oxide of iron is also a very common ingredient, and sometimes in visible particles, though generally its presence is only betrayed by the action of the rock on the magnetic needle. The variable proportion and nature of the mineral ingredients give rise to a great diversity in the external characters. of the mass, which diversity is still further increased by the different circumstances under which different portions of a rock identical in mineral character may have passed from the fluid to the solid state.

The same rock may be found in every shape of transition, from the most compact and homogeneous structure to a light porous mass, filled with cavities, or amygdules, which have often, posterior to the cooling of the rock, been filled with various mineral substances.

For the sake of convenience in describing the local details, and in order to adhere, as much as possible, to the terms already familiarly used by those engaged in mining explorations in this district, we shall include, under the name of trap range, or trappean rocks, all the different varie. ties of igneous rocks which form the great belt extending from the extremity of Keweenaw Point to the Montreal river, and which also form the greater portion of Isle Royale. When the rock is vesicular in its structure, it is called amygdaloid; when compact, crystalline, or homogeneous, the hornblende predominating, it is called greenstone; when columnar or jointed, as on Isle Royale, it is called basalt. If the homogeneous base contain distinct crystals of feldspar disseminated, it becomes a true porphyry; and the largely crystalline and feldspatic varieties are known as sienitic.

These and many other varieties occur abundantly throughout the district, in belts imposed one upon another. Their position and the changes they have caused in the contiguous detrital rocks will be noticed in describing the detailed geology.

Range and extent.-Commencing at the head of Keweenaw Point, we find the trappean rocks, with the associated conglomerates, emerging to the surface in bold stair-like cliffs, affording many scenes of wild and picturesque beauty. This peculiar physiognomy is characteristic of the whole trap region. Humboldt long ago remarked that each zone had its particular types of animal and vegetable life, but that the inorganic crust of the globe showed itself independent of climatic influences. Everywhere, basalt rises in twin mountains and truncated cones; everywhere porphy

ritic trap appears in grotesquely arranged masses, and granite in rounded summits.*

The outer belt of trap, occupying the extreme northern portion of Keweenaw Point, (see map,) is less than a mile in width, and preserves a great degree of uniformity throughout its entire course. It forms a segment of a circle, of which the Bohemian mountains may be regarded as the centre. The southern points of Manitou island are dotted with patches of this igneous rock, while the greater portion of the belt has crumbled beneath the action of the lake surf. From the extremity of Keweenaw Point, it extends westerly for about eighteen miles in a curvilinear direction, and passes into the lake at the eastern point of Sand bay. Throughout most of this distance it is protected from the action of the surf by a thick belt of conglomerate, but at several points the water has broken through this sea-wall and excavated spacious harbors in the igneous belt. Copper, Agate, Grand Marais, and Eagle Harbors are included in this belt, and owe their origin to a common cause.

This belt is composed of the varieties of igneous rock known as amygdaloid and brown granular trap. The amygdaloid is best developed in the upper portion of the belt, where it comes in contact with the conglomerate, presenting a dark scoriaceous mass, full of vesicles, somewhat compressed, and bearing a close resemblance to certain modern volcanic products. These vesicles are, for the most part, filled with carbonate of lime, chlorite, agates, carnelians, and amethysts, and minerals of the zeolite family. As we penetrate deeper into the belt, the vesicular structure disappears, and the rock passes into a dark brown granular trap, consisting of an intimate union of hornblende and labrador. This is its general character; and to cite the numerous places where it has been observed, would be to encumber the report with unnecessary detail.

This belt is traversed by veins containing copper and silver, several of which have been mined, but in every instance unsuccessfully. At Eagle Harbor, a company wrought a vein, which, for a time, yielded a rich per centage of copper. Between 5,000 and 6,000 pounds were taken from the vein within a comparatively limited space; but as the miners sank deeper, the copper disappeared. The range of the vein was limited on the south by the conglomerate, and on the north by the lake. At the surface it was two feet in width, and filled in with laumonite and native copper. The shaft was commenced about two hundred feet north of the junction of the rocks, and extended to the depth of ninety seven feet. At that depth the vein had contracted to three inches, and was barren of copper. The best mining-ground is undoubtedly beneath the bed of the lake; but to reach it would require a deep shaft, a long gallery, and an expensive apparatus for ventilation. The company were not disposed to embark in an undertaking, the labor of which was certain, while success was precarious. Several other veins in the northern range were explored by the company, with no better results.

At Hawes's island, near Agate Harbor, a vein was opened by the Cypress River Company, which yielded rich specimens of copper and silver, but, in the downward progress, they disappeared.

With a single exception, (northwest quarter of section 58, range 30,) we have excluded every tract within this belt from the list of mineral lands, believing that it contains no veins which will be permanently productive.

* Aspects of Nature, vol. II.

To the west of Sand bay about ten miles, and north of the first trappean range, narrow belts of trap have been observed in two places, to wit: on section 28, township 58, range 32; and on sections 1 and 6, between ranges 32 and 33, township 57. The nature of the ground is such that they can be traced but a short distance inland. Whether they are a prolongation of the belt just described, or detached, intercalated masses, it is impossible to determine. The character of the rock is highly amygdaloi dal, and chlorite enters largely into its composition. Imperfect indications of veins exist, one of which was explored by the Lake Shore Mining Company yielding little or no copper.

In the more compact varieties, a concretionary structure is sometimes observable. Parallel bands of different colors, a few inches in width, traverse the mass in waving lines, or are arranged in circular forms. This same arrangement is seen more strikingly illustrated in the trap on Hays's Point, near Copper Harbor. The direction and arrangement of these lines is illustrated in

Fig. 3,

the above wood cut. This structure is occasionally found in all igneous rocks, and undoubtedly results from chemical affinity, by which the particles assume a concretionary arrangement.

About a mile south of this trap belt, and separated from it by a deposit of conglomerate and coarse sandstone, which, in places, expands to a thickness of more than 3,000 feet, occurs the northern trap range of Keweenaw Point. It will be seen, by inspecting the map, that these two belts of igneous rocks, in their westerly prolongation, preserve a remarkable parallelism.

This range does not appear to have been the result of one, but of successive overflows; for we not only find the igneous materials arranged in parallel bands, and exhibiting great diversity in external characters, but we also find numerous intercalations of conglomerate of inconsiderable thickness, but extending for miles in a linear direction-these mixed products being associated in regular beds, having a common bearing and inclination, so that the inexperienced observer is inclined to refer the whole to a common origin. This deception is still further increased by observing lines of pseudo stratification in the trap conforming to those of the associated sedimentary rocks.*

This range starts from the head of Keweenaw Point, below Manitou island, and, sweeping round in a crescent form nearly conforming to the trend of the coast, crosses the western arm of Portage lake, where it

*This pseudo-stratification has been observed by De la Beche in the granite of Cornwall, and is supposed by him to result from a tendency in the materials of a cooling mass to arrange themselves in beds, particularly near the surface.-Geology of Cornwall.