Imagens das páginas
PDF
ePub

The Medicine Bow mountains, on the west side of Laramie plains, appear to contain a heavier forest growth than the Black Hills, and should iron ore be found on that side of the plains it might prove a better region for the manufacture of iron than the eastern side. I proceeded as far west as Fort Halleck. on a branch of the Medicine Bow creek, and at the northeast base of Medicine Bow mountains, and sought to ascertain the locality of iron ore on this creek reported by Mr. Engelmann, who accompanied the expedition. Mr. Duval, who is still in government employment at Fort Halleck, was entirely ignorant of any such discovery. I learned, however, of the occurrence of coal beds, said to be of large size, in the prairie hills six miles northeast from Fort Halleck, and made several ineffectual attempts to find them. The east side of the hill, where they were discovered, I found covered deep with snow, and though I remained eleven days at Fort Halleck in hope of a favorable time for exploration, it was for nearly the whole period almost impracticable for one to cross the prairies and dangerous to leave the stage road. The wind every day blew with extraordinary fury, sweeping the snow forward and piling it in deep drifts in the cañons and gulches, and on the lee or east side of the hills. For days together a man could with difficulty stand up against it, and the driving snow often prevented his seeing one hundred yards in any direction. During this time the themometer ranged from zero to 10° below. It was the first day of December when I reluctantly left this portion of the Rocky mountains, in despair of making in it any useful geological observations at so advanced and inclement a season.

Before going to Fort Halleck, I accompanied the surveying party of Mr. B. B. Brayton through the Black Hills, on the Cheyenne Pass, leading from Lodge Pole creek to Salt lake; and it is for the sake of completing my account of the plains west of the Black Hills, before proceeding to that of the mountain district and the plains to the cast of it, that I have introduced above my

remarks upon the country about Fort Halleck.

At the Cheyenne Pass the Black Hill range, extending due north and south, presents a very uniform slope on its western side, but little interrupted by cañons, such as are of frequent occurrence in other portions of the Rocky mountains. This slope nearly to the summit is that of the limestone strata, which uplifted from their horizontal positions in the plains, here form the outermost layers of the range. In a gorge near the base of the mountain they are exposed to the thickness of full twenty feet, which is probably but a small portion of the real thickness of the formation. The rock is in broad, flat blocks, admirably suited for building-stones, and much of it, though never crystalline, appeared as if it might make a substantial marble of fair quality, but not of bright colors. I could discover no fossils in it. Beneath this rock, exposed in precipitous ledges along the gorges, and curving up from under it at the summit, is a red silicious sandstone resembling the Devonian red sandstone of the Alleghanies. It covers the surface of the hills lying north of the pass, spreading out over broad areas in nearly horizontal strata. These towards the east abut in bold cliffs, and next beyond them in this direction appear the granitic and porphyritic rocks which make up the central portion of the range. The only distinct fossils I could find in the sandstone were small encrenites. Neither this formation nor the limestone is likely to afford any useful minerals, though the latter may possibly prove a repository in occasional localities of hematite. Professor James Hall, of Albany, to whom I have submitted specimens of both the limestone and sandstone, refers the group to the carboniferous formation of the age of the true coal measures. The rocks which compose the mass of the Black Hills are red granites, red sienites, and red porphyritic sienites. They form not only the high ragged peaks and groups of rough hills that lie to the north and south of the pass, but the smoother surface and prairie-like hills of the pass itself are also underlaid by the same formation. A peculiar feature it everywhere exhibits is a decided tendency to disintegrate and crumble into

In some

coarse, angular fragments. The surface is very generally covered with these, which make a poor soil enough, but the very best of roads. The wagon road through the pass, though unimproved by any labor upon it, is for the most part unsurpassed in smoothness and durability by any macadamized road. This tendency to disintegrate is also the cause of the numerous peaks and monumentshaped masses of all sizes standing on the steep mountain slopes and summits, and also scattered over the smoother and level portions of the mountains.

Some of these appear, like the boulders of northern latitudes, perched upon ledges from which they could be easily tipped off; others resemble icy masses along a frozen coast as they melt away on the approach of spring. instances, when the disintegration has gone on most rapidly at the base of towerlike masses, huge blocks have parted from the main body to which they belonged and have fallen down, exposing a fractured and nearly smooth face, sometimes of several hundred square feet area.

In general, the outline produced by the disintegration is rounded like that of rolled boulders. I could find in these formations no metallic veins, nor any features that would lead me to look for iron ores in the central part of the range. No mica nor talcose slates accompany the granites, and the only variation in the rocks is as they become more or less sienitic or porphyritic in their composition.

Numerous quartz veins, however, are seen toward the eastern side of the pass crossing the road in a northerly and southerly direction, and projecting above the surface, which in their vicinity is covered with loose pieces of this mineral. These veins resemble the gold-bearing quartz veins of the southern States, but are unlike those of the Colorado mining district.

The granites also of that part of the Rocky Mountain range are very different from those of the Black Hills, being of light colors and gneissoid in structure.

On passing out from the central range toward the plains on the east side, one everywhere meets facing the mountains a range of high, precipitous cliffs of red sandstone, the lower layers often conglomerate. These rocks present a thickness of full 500 feet, and as the lowest -strata are not exposed, the formation may be much thicker than this.

It is evidently a repetition of the same sandstone group that caps the summit on the west side, and passes under the limestone that forms the western slope. This rock, too, lies in the same relative position to the sandstone on the east side, capping the cliffs in some instances, and also forming a parallel outer range of bills, the strata still dipping cast. All along the east side of the Black Hills, as far as I observed them, and further south where these hills are lost in the main Rocky Mountain range, this group of marginal cliffs is traced, and everywhere they present a striking feature in the topography, all the more marked by the bright red color of the sandstone.

Their forms at the Cheyenne Pass, and again at Boulder creek, Colorado, are represented in the accompanying sections and sketch, and their range is designated in the ground plan or map. The hills appear to have once formed a continuous unbroken line, the western summit presenting a bold escarpment, the base of which is covered by the debris fallen from above.

This constitutes the gentler slope seen in the section at the western foot of the hills. The eastern slope is that of the strata, and the surface on this side is frequently in chief part that of the rock itself, scantily overgrown with sage bushes, cactus, and grass that have taken root in the crevices. Behind the first range, with an intervening valley, sometimes nearly a mile wide, but much less further south where the dip of the strata is very steep, is a second range of precisely similar form, and near the Cherokee Pass, where the stage road crosses the mountains, I have observed a succession of four or five such ranges, the outermost one dying away in reduced dimensions in the prairie to the east. Their covering of snow prevented my studying the structure.

At the entrance to the mountains at Clear creek, Colorado, there is seen extending several miles north and south, outside of this range of hills, another group of basaltic formation.

The bills composing it are all remarkable for their peculiar tabular form, being perfectly flat on the summit, which is bounded on all sides by vertical walls, apparently a hundred feet high, of rudely columnar green-stone or hornblende rock. The summits are sometimes several hundred acres in area, and at others (as on one of most striking appearance just east of Golden Gate City) the extent does not seem to be more than three or four acres. In this, however, one may be deceived by the great height of the hill, which is probably full seven hundred feet above its base. These are the only hills of this formation I saw in the Rocky mountains. Their position is represented in the map, and their form among the sections accompanying it. The soil near their base is quite fertile, and is often cultivated for soine distance up their very steep slopes.

This group, as also the more extended range of sandstone hills behind them, traced nortů and south, are seen to be interrupted at intervals of half a mile to a mile and a half by gaps, all of which are worn down to about the same level, which may be 300 to 500 feet below the summits. The mountain streams find their outlet through these gaps, and all the roads into the mountains pass up by the same openings.

The rounding away of the ends of the hills in the gaps toward the east, the direction of the dip, keeps exposed the strata, which in the face of the escarp. ment further west occupy a much higher position, and to an observer facing the escarpment the impression is conveyed that in each hill the strata at its northern end dip north and at the southern end dip south. In the middle of the face they appear to be horizontal, the basset edges only being in view.

T'he peculiar form of these hills is obviously due to powerful denudation directed from the central range eastward. On the shorter western slope of the Black Hills the effects of the same action in the opposite direction are less strikingly exhibited in the abutment of the same sandstone formation, which, as already noticed, is seen on that side near the summit of the range. Other evidences of extensive movements over the surface from the main Rocky mountain ranges eastward, will be presented in describing the formations examined further south.

The lower members of the limestone formation at the east entrance of the Cheyenne Pass are remarkably intermixed with various forms of silex, as flint, jasper, carnelian, and chalcedony, which sometimes present a rude agate structure. The flints are of many different colors; the jasper is in fine blocks of clear red. The sides of some of the hills are covered in places with fragments of these minerals, the flints and limestone often attached together.

The operations of the party I accompanied being limited to the pass, I had no opportunity of extending my observations into the plains on the east side of the Black Hills. I had already become satisfied that it is in the plains, and not in the mountains, that the minerals I was in quest of are to be found, and after abandoning further explorations west of the mountains I proceeded to the region south of Laporte to investigate the character of the beds of coal and iron ore there opened and worked. The range of the formations, I had learned, would carry these beds northward near the Black Hills, and a knowledge of their properties, which could be obtained in a comparatively settled country, though still covered with snow, would be useful in directing further explorations in the wild districts about the Cheyenne Pass to one provided with the necessary facilities for conducting them in a more propitious season.

It was after leaving the pass that I learned from Mr. Duvall, at Fort Halleck, of the occurrence of iron ore in large quantity on the branches of the Chugwater, about twenty miles north from Camp Wallach, which is an old deserted campai the east entrance of the Cheyenne Pass. His description of the ore as heavy and massive, with no appearance of a vein or “ lead,” would apply very well to the localities I afterward examined on South Boulder and Rock creeks, Colorado, and I imagine the ores of the two districts are of the same character. I was afterward directed by two other old explorers of these regions to the same locality, as the only one where they had observed any iron ores, and another pointed out the same district as containing coal also. It therefore appears to be the most promising spot for subsequent explorations north of the stage route.* At several other localities in the plains there are reports of coal being found. Somewhere on Lodge Pole creek it is actually worked, to a small extent, for supplying in the winter the stage stations near the mouth of this stream. There is also a bed in the plains about seven miles north from the stage road, between Laporte and Latham. This I endeavored to find, but there was no road to it and the country was covered with snow. A number of these localities are designated upon the map on both sides the Black Hills, as also nortlı of the range on a small branch of the north fork of the Platte, known as Trading house creek. They indicate satisfactorily the great extent of the area over which beds of coal may be sought with good prospect of finding it. It has been supposed that a bed of it might be found in the black shales exposed along the road eight miles south from Laporte, but the few imperfect fossils I found here discourage this expectation, as they are referred by Professor Hall to the tertiary formation.

It is only in the vicinity of Denver, which affords a market for this fuel, that beds of it are worked to any extent. On Coal creek and on South Boulder creek, both about 22 miles north from this town, a number of beds are opened on the former creek, about 14 miles east from the base of the mountains, and on the latter within three miles of it. On Coal creek the outcrop of the coal is at the base of a high hill or ridge, rising back toward the east, and washed at its foot by the creek, which seems to have excavated its bed for some distance in the soft materials of the coal and of the fire-clay beds that underlie and overlie it. One of the openings commences at the base of this ridge in a heavy body of blue fire-clay, which forms the roof of the coal; and penetrating this, passes into the coal bed itself, which presents a thickness of five feet ten inches pure coal, with no mixture of slate. The tunnel has been carried in over one hundred feet, and for this distance the strata are seen to incline at a gentle dip not exceeding 20 or 30 toward the north. The coal is of a brilliant jet black, and is easily mined in large lumps, which appear to be firm and sound. I am informed, however, that after exposure a few weeks to the air, the lumps crumble to fine coal, and for this reason no large stock of it is kept in the coal-yards at Denver. Should the coal not be found to improve in this respect when mined to greater depths, or in other beds not yet opened, there may be difficulties in applying it to the smelting of iron ore, from the small particles clogging the furnace; it may also work to disadvantage in locomotives by sifting through the grate bars. The coal seems to contain but little bitumen, burning with little smoke, no unpleasant odor, and a yellow flame. It does not melt or coke, and, however bigh the draught, produces no clinker. The ashes of most of the beds are usually white and bulky. A blacksmith, who uses it, informed me that he can obtain a welding heat with it in a forge, but with difficulty. Sulphur is observed in it in small quantity in the form of exceedingly thin disks of iron pyrites disseminated through the seains. Particles of mineral rosin are much more abundant, scattered through the coal, the pieces being of the size of pinheads. 'Several other beds of coal bave been discovered in the same vicinity; and one of these, a few rods to the southeast from the point already described,

'On referring to the report of Captain Stansbury since the above was written, I find on the closing page the following remarks respecting the locality: “In the bed of the ChugWater, and on the sides of the adjacent hills, were found immense numbers of rounded black nodules of magnetic iron ore, which seemed of unusual richness."

H. Ex Doc. 253- -4

is worked to some extent. This coal bed appears to be an upper one, but though so near the other it has an entirely different dip, which is about 18° east. It is in two positions, the upper being seven feet thickness of coal, separated from the lower, which is 41 feet thick by a stratum of dark blue fire-clay 18 inches thick. The fire-clay appears to be of exceller.t quality for the manufacture of fire-brick. The mine is worked by following the coal-bed down the slope, and the coal is drawn up on cars by a capstan. No trouble is experienced from water, though the opening already extends about 100 feet under the hill. The other bed also is dry. I found the workmen attempting to coke the coal of the second bed described, which they thought possessed a better coking quality than that of the other bed. The coke, though very inferior, and obtained only in small pieces, was purchased by the blacksmiths around in preference to the raw coal. A strong fuel, such as good coke, is of great value in this region, as is shown by the fact of its transportation all the way from Kansas to the machine shops at Central City, in the Colorado gold region, where more thau 100 tons of it have already been consumed, at a cost of $160 per ton.

The strata accompanying the coal-beds differ in some respects, so far as I could see them, from the strata of the true coal formation. There was the same variety of fire-clay, but the beds of it under the coal contained none of the stigmarial everywhere else found in this position. I saw no stems and leaves of ferns, but in the fire-clay, over the coal, I obtained imperfect fragments of blackened deciduons leaves. Clay-iron stone in nodules and layers occurs iu the fire-clay. I was told of two layers, together 18 inches thick in depth of fire-clay. I saw no beds of black shale, nor are any of limestone found in this formation ; a few feet above the upper coal-bed is a crumbly standstone of a light gray color. On the extension of this ridge, 24 miles further north, I examined some ledges which projected through the snow, the position of which I judge is over the

coal, and not very far from it. These are thin bedded sandstones of yellowish color, and other layers more compact of bluish shade. The latter contained fossil shells, the substance of which is sometimes well preserved. They are recognized by Professor Hall as belonging to the genera cardium, cucullea, maetra, nucula, tellina, and ammonite ; thus designating the formation to be as old as the cretaceous period. The deciduous leaves in the fire-clay determine the coal, or rather lignite beds, as belonging to the same formation.

The other coal district is in the hills along South Boulder creek, only 23 miles from the base of the Rocky mountains. Several beds have been opened, and two of them are worked for the supply of the Denver market. This locality also affords an abundance of iron ores, and has been celected for the establishment of the first blast furnace erected in the Territory. This is nearly completed, and will probably go into operation in March. The principal coal-bed is opened a few rods southeast from tbe furnace, and has been worked 100 feet down a slope of about 100 from the horizontal toward the east. The bed is 12 feet thick, almost uniform in quality, with no intermixture of slate, and presents a beautiful appearance in the brilliant lustre of the coal. A little sulphur (pyrites) may here be detected in the seams. It was from this bed that the first specimen analyzed by Dr. Torrey, whose report accompanies this, was obtained.

A second bed is opened about half mile from the furnace towards Denver. Though further from the mountains, this bed is nearly vertical; it is about 7 feet thick, and has been worked to the depth of 50 feet, the conl being raised by means of a borse-whim.

A third bed, 3 to 4 feet thick, just opened on the north side of the next hill, east from the first-named coal-bed, is found to be nearly horizontal at its outcrop. This promises to afford coal of a firmer quality than is obtained from the beds heretofore worked.

Other beds are known in this vicinity; and both here and at Coal creek some

« AnteriorContinuar »