It then recedes gradually until the streams begin to discharge their spring floods. Snow usually commences falling as early as the middle of October, and the ground is covered before the frost has penetrated to a great depth. The amount of snow during the season has been represented as high as thirty feet; but, in consequence of its evaporation, and its change from a crystalline to a granular form, known as nevé. it settles, and the actual depth on the ground rarely exceeds four feet. Trappers, in crossing the inland lakes in midwinter, often break through, so slight and unstable is the covering. The temperature of the water of Lake Superior during the suminer, a fathom or two below the surface, is but a few degrees above the freezing point. The following observations show the temperature of the water at different times in different parts of the lake. In the western portion, the water is colder than in the eastern-the surface flow becoming warmer as it advances towards the outlet. The water in these experiments was taken from the surface. June 30, 1849.-To the south of Caribou Island. July 28, 1849.-Between Keweenaw Point and Aug. 13, 1849.-Midway in Keweenaw Bay 490.0 During the severe winters, the surface of the lake becomes congealed. When a gale sets in, the ice is seen to undulate and break, and the water to gush through the fissures, until finally the whole mass is set in motionthe fragments clashing against one another, accompanied by loud reports, like volleys of musketry. Long parallel ridges of ice, fifteen or twenty feet in height, are piled up along the shores. We can readily conceive how masses of rock thus entangled might be carried for considerable distances when the ice beconies detached and floats off, and how a cliff might be scratched and grooved. The waters of the lake possess great transparency, and a tin cup may be seen to the depth of ten fathoms. Coasting along the shores in a calm sunlight day, and looking over the gunwale of the boat, the voyageur seems to be suspended over the floor of the lake, and every fissure in the rock, and every glittering pebble is revealed with wonderful clearness. The light streaming through the transparent medium tinges every object with a brilliant hue. The evaporation from the surfaces of the lakes must he immense. The combined area of Lakes Superior, Huron, Michigan, and Erie is about 87,000 square miles, and of their basins not less than 335,515 square miles. It has been estimated that the quantity of water passing into the Niagara river at Black Rock is 22,440,000 cubic feet per minute, or about 80 cubic miles per annum. This is equivalent to fifteen inches perpendicular depth of water spread over the area of the whole country drained. The annual amount of rain which falls within this area is about thirty inches. One-half, therefore, of the water which falls within the basin of the upper St. Lawrence is taken up by evaporation, amounting to 11,800,000,000,000 cubic feet.† At Sant Ste. Marie, the outlet of Lake Superior, the spectator beholds a river nearly a mile in width, and of sufficient depth to float the largest vessel. In its onward progress, it winds among innumerable islands, and ultimately discharges itself, by several mouths, into Lake Huron. At Fort Gratiot, he sees the same river, under another name, after having received all of the tributaries of Michigan and Huron, contracted to a width of little more than three hundred yards, but of increased depth, and he finds it difficult to realize that it is the same river which he saw three hundred miles above. So, too, the voyageur who has coasted around Lake Superior and gauged the streams which pour their annual floods into the great reservoir, when he stands on the brink of Niagara, and witnesses the fearful plunge of the cataract, is induced to inquire what has become of the superfluous water. The difference between the temperature of the air and the lake gives rise to a variety of optical illusions, known as mirage. Mountains are seen with inverted cones; headlands project from the shore where none exist; islands, clothed with verdure or girt with cliffs, rise up from the bosom of the lake, remain a while, and disappear. In approaching Keweenaw Point, Mount Houghton is the first object to greet the eye of the mariner. Its dome-shaped summit serves as a landmark to guide him in his course. Once or twice, in peculiar stages of the atmosphere, we have observed its summit inverted in the sky long before the mountain itself was visible. On the north shore, during the summer months, hardly a day passes without witnessing illusions of this kind. The Paps, two elevated moun tains near the entrance of Neepigon bay, would at one time appear like hour-glasses, and at another like craters, belching forth long columns of smoke, which gradually settled around their cones. Thunder cape assumed shapes equally grotesque: at one time resembling a huge anvil with its handle projecting over the lake, at another it appeared as though traversed from summit to base by an immense fissure. These phenomena are more common on the lakes than on the Atlantic coast, since hardly a day passes during the summer without a more or less striking exhibition of this kind. The amount of refraction, dependent on the state of the atmosphere, is, during the greater part of the summer, extraordinarily variable. The greatest difficulty is experienced in making astronomical observations, from this cause. Observations taken in the afternoon, and generally during the night, are almost invariably worthless. The varying refraction may often be noticed in meridian observations of Vide M. Z. Allen's article in Silliman's Journal, January, 1844. Dalton found that an evaporating surface of six inches yielded in calm, dry air, at 65° Fahr., 2.62 grains of vapor per minute, and 4.12 in a high wind. the sun with the artificial horizon, when the two images will be seen to lap over and then separate from each other a great number of times during the few minutes, while the apparent motion of the sun is almost imperceptible. These variations amount to several minutes of altitude; and, of course, on such occasions, no use can be made of the observations. Observations taken in the morning, when a steady brisk breeze was blowing, and the sky free from clouds, were found to be the only ones on which any dependence could be placed. The same phenomena of rapidly-varying refraction may often be wit nessed at sunset, when the sun, sinking into the lake, undergoes a most striking and rapid variety of changes. At one moment, it is drawn out into a pear-like shape; the next, it takes an elliptical form; and just as it disappears, the upper part of its disk becomes elongated into a ribbon of light, which seems to float for a moment upon the surface of the water and then disappear. Fig. 2. The annexed cut represents the outline of the appearance of the sun as it went down in the waters of Lake Michigan, June 19, 1849. The cause of these phenomena can readily be found in the ever varying movement of bodies of differently heated air charged with different amounts of moisture. Those who navigate the lake not unfrequently notice that. they pass instantaneously from a current of air blowing briskly in one direction into one blowing with equal force from an opposite direction. The lower sails of a vessel are sometimes entirely becalined, while a brisk breeze fills the upper. Frosts, of sufficient severity to turn the leaves, usually occur as early as the middle of September. Snow commences falling by the middle of October, and for more than six months the ground is covered with a fleecy mantle. The streams become locked with ice and remain so until May. The ground does not become frozen to a great depth, and, so soon as the snow disappears, vegetation shoots into life, and the air swarms with myriads of insects. During the long days the sun shines with undiminished splendor, and the influence of its direct rays compensates for the low mean temperature. Spring and summer are mingled. The forest becomes clothed with leaves, and its solitude is enlivened by the song of birds and the hum of insects, before all traces of snow have dis appeared. Notwithstanding the proximity of the lake, the thermometer has a range of 120° in the course of the year. Often in midsummer, when, for several days, the winds come from the southwest, the voyageur experiences a suffocating heat-an enervating depression. The perspiration rolls from him even when unemployed and protected from the glare of the sun by the forest's shade. But, fortunately, these suffocating heats are of short continuance.. In the valley of the Ontonagon, on the 11th of June last, the thermometer rose to 96°. The wind was blowing from the SW., but brought * Humboldt remarks that the thermometer nowhere rises higher than 104° F., unless exposed to the influence of bodies which radiate heat. The extraordinary heats of the desert, as indicated by the thermometer, are caused by particles of sand carried through the atmosphere. vith it no refreshing coolness. A little after midday, a dark cloud, emiting from its edges a pale phosphorescent light, rose from the lake, and dvanced against the wind. Its approach was indicated by a loud roaring, nd, when it reached our encampment, the trees swayed to and fro, and nany were prostrated around us. The air was filled with flying leaves und branches. Voyageurs and men instinctively rushed into the river, nd 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 1oyageurs are most inclined to hug the shore. Instead of seeking for a solution of these phenomena by a resort to atural causes, they ascribe them, like the Scandinavians of old, to the reaks 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 occasionally forming perfect corona. 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 activity-manifested 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 Humbold:'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 gener al 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 sand. stones, 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 subse quently 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. |