I will pass to the general fitness of this county for iron industries. When we have iron-ore, coking-coal and limestone, all within easy transportation distance of the same point, and all to be carried "down grade" to it, there would seem to be a fair prospect for an iron center. The coal-fields of Gunnison county are already in a high state of development, and two or three branch lines run to them from Gunnison City. At this point iron should be made for $12 a ton, or even less than that. We now have one iron center at Pueblo, and it is probable that our next will be somewhere on the western slope of the mountains. We all remember the story of what the Texan said his State lacked. Gunnison City lacks only capital and a market. But the West of to-day is the East of to-morrow. Some of us have seen iron made where the prophets of only twenty years ago said it could never be made. We have seen it competing from points considered outside of the limits of competition ten years before the erection of the furnaces. We should beware then of laughing at the future of any new field. It may be that before the American Institute of Mining Engineers again meets in Denver, Colorado will be exporting her pig-iron to the Pacific slope.

NOTES ON THE GEOLOGY OF THE ASPEN MINING DISTRICT.

BY W. E. NEWBERRY, ASPEN, COLO.

(Colorado Meeting, June, 1889.)

THE Aspen Mining District is situated at the eastern edge of the system of stratified rocks of western Colorado, where this edge meets the ridge of metamorphic granite which divides the State into two unequal parts. In following the valley of the Roaring Fork westward from the continental divide towards Aspen, one passes through a country of granite and gneiss, with no sedimentary rocks until the town is nearly reached. At this point the Cambrian quartzites first make their appearance, lying on the granites, with a dip of from 30° to 45° to the northwest. Beyond these quartzites and conformably upon them lie dolomites, limestones, shales, porphyries and sandstones, gradually flattening in general dip as one proceeds westward, but much disturbed by local foldings and faultings.

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More in detail, the succession of these strata upwards from the granite is as follows, the measurements being an average of a number made by Mr. D. W. Brunton :

8. Jura-triassic sandstone, with a thickness of many hundred
feet.

The characteristics of these strata are as follows:

The Cambrian quartzite is compact and generally fine grained, and from white to pink in color. The Silurian strata consist of numerous beds of impure limestone, with bands of quartzite. The limestones are generally light colored, while the quartzites are coarse grained, and from red to white. In this series is a band of lithographic stone about two feet thick.

The Lower Carboniferous dolomite, or brown limestone, is dark colored throughout. Near the surface, and in the neighborhood of the contact with the overlying blue limestone, and near fault-planes, in short, wherever exposed to the action of atmospheric water, it is of a reddish-brown color, due to the oxidation of the iron contained in it, and it breaks up into small cubical fragments, from which property it derives its local name of "short lime." Where it has been less subjected to decomposition it is more massive, and almost black in color. It contains from 20 to 38 per cent. of MgCO,, and sometimes as much as 5 per cent. of oxide of iron. It contains nodules and lenticular bodies of chert, and, in some portions of the beds, masses of blue limestone or calcite of considerable size.

The Lower Carboniferous blue limestone is massive, without distinct cleavage, except where it is crushed by movement, when it shows something of the short structure of the above mentioned dolomite. The color is a bluish gray, and it is in some cases coarsely crystallized, and in nearly all cases shows some crystallization. In the neighborhood of the dolomite, and particularly near an ore-body, the blue limestone becomes stained with iron, and, with the cleavage it has under these conditions, it is often difficult to distinguish it from the dolomite without analysis. The crystallization is the only

safe guide, a freshly broken surface of the blue limestone showing flat, scale-like crystals, while the crystals of dolomite are like fine needle-points, without perceptible area. In the blue limestone stratum are a number of sheets of dolomite, similar in appearance to the main body of dolomite below. From the fact that these sheets are often lenticular in shape, and cross the bedding of the blue limestone to connect with each other, and from other facts in the same connection, there is reason to believe that the beds of blue limestone and dolomite were originally deposited as a homogeneous body of limestone, and that the dolomitization of the lower portion and of the sheets in the upper portion was subsequent to the deposition of the rocks. The blue limestone contains from 92 to 99 per cent. of CaCO,.

The shales and limestones lying above the blue lime are highly carbonaceous, in places streaks of coal being found of sufficient purity to be burned in a blacksmith's forge. These beds are also highly charged with iron pyrites.

Intruded into these beds, separated from the blue limestone by from 5 to 100 feet of shale, is a great mass of diorite. This rock, locally called porphyry, is light in color, with bird's eyes of light blue, and small crystals of pyrite. The porphyry forms the swell in Aspen Mountain, directly opposite the town. It is much decomposed near the surface, and has not been identified on Smuggler Mountain, across the valley from Aspen Mountain. It extends a long distance to the southward, being shown at least as far as Ashcroft, 12 miles from Aspen.

The above described strata are cut by the valley of the Roaring Fork nearly at right angles to their strike, and therefore cross under the town, and appear in regular order on either side of the valley. The geological structure of Aspen Mountain is, however, much more complex than that of Smuggler Mountain, to the north. A vertical section of Aspen Mountain, taken on a northwest and southeast line, through what appears from the town to be the summit of the mountain, would show three occurrences of the strata above mentioned, resembling the three slopes of an S-fold, with the syncline at the eastern and the anticline at the western end of Aspen Mountain, and this folding, complicated by faults at the base of the syncline and at the crest of the anticline, is the cause of this reappearance of the

strata.

The bottom of the synclinal fold has a strong dip to the north, forming a basin, concave towards the city.

The prime cause of this folding has been a great granite upheaval, which has lifted the strata at the west end of the mountain, and compressed into the form of a basin those lying to the east. The strata to the west of the granite upheaval were unable to stand the strain, and broke, forming a great fault, the line of which is now marked by the course of Castle Creek, which flows into the Roaring Fork to the west of the town.

The series of strata on the north side of the valley seems to be much less disturbed than that on the south, and the rocks follow their regular order, from east to west, without important break as far as observed.

The principal ore-bodies of the district occur in the Lower Carboniferous strata, including the dolomite and blue limestone above mentioned. The ore occurs generally in the neighborhood of, or on the contact between, these two strata. The fact that the strata are cut by numerous faults, making generally an acute angle with the direction of the dip, and more or less nearly parallel to the strike, complicates matters, and makes two classes of contacts between these rocks, one being the stratified contact, with the blue limestone above and the dolomite below, and the other the faulted contact, with the rocks in the same relative position when the fault is of the ordinary kind, and in the contrary position, i.e., with the dolomite above and the blue lime below, in the case of a reverse fault. The throw of these faults is in some cases so great that the shale is brought opposite the dolomite, and a faulted contact formed between these rocks.

The stratified contact, having the general dip of the formation, inclines at an angle of from 10° to 45° to the northwest, the general strike of the formation being about north 45° east. There is no evidence of any great motion upon one another of the rocks forming this contact, large areas of the contact being of a tight or welded. nature. In many places, however, there is evidence that the rocks have been parted, at or near the contact, probably by folding, and that the blue limestone has moved on the dolomite sufficiently to cause an increase in the size of the openings, by the correspondence of the ridges and hollows in one with the hollows and ridges in the other.

Thus a portion of the ore found in the neighborhood of the stratified contact consists of foreign material, mixed with fragments of the surrounding country-rock. But a great part of the ore, and I think I am safe in saying, the greater part of the ore shipped from the

camp, consists of an impregnation or mineralization of the countryrock in place, either the blue limestone or the dolomite, in the neighborhood of the stratified or faulted contact.

The method by which the mineralization has been produced differs in the cases of the two rocks. The dolomite does not seem to have been more readily decomposed by the mineral-bearing solutions, but owing to its short structure, and many cleavage planes, it has been more readily permeable by them, so that much of the brown limestone ore consists of small cubes of nearly unaltered dolomite, with the valuable mineral deposited mainly in its cleavage-planes. The dolomite has, however, in many cases, thoroughly lost its structure, and is often reduced to the condition of a dolomite sand, impregnated with valuable mineral. The mineralization of the blue limestone seems to have been accomplished by the replacement of a portion of the calcite, without materially altering its structure. many cases the rock retains its appearance, and the mineralization can only be discovered by assay. Even in such cases the rock usually appears more crystalline and porous than when barren.

In

The faulted contact is characterized by a steeper pitch than the stratified contact, and by the great quantity of wall-rock contained by it, and the strong evidence of motion. The welding of the walls noted above is, of course, absent.

There seems to be a general system of ore-chutes in these limestones, having a southerly direction, often interrupted by faults, but probably in many cases continuous along the fault-planes. From the developments so far made, the trend of these ore-chutes seems to be about south 65° west, swinging slightly to the south with increased depth.

The gangue of the Aspen ores is generally either limestone or sulphate of baryta, or both, passing from ores like some of those from the Aspen mine, containing as high as 90 per cent. of carbonate of lime, to others from various mines containing as a maximum 70 per cent. of sulphate of baryta. The amount of silica in the ores is generally small, rarely ruuning over 20 per cent. The bulk of the ore is very low in lead, that containing 25 per cent. being a rarity. An average of the ores at present shipped from the district would contain not far from 55 ounces of silver per ton, with less than 5 per cent. of lead, and from 12 to 15 per cent. of sulphate of baryta. average smelting charge of about $9.00 per ton is paid on these ores, delivered in Denver, which with the railroad freight of $8.00 makes a total average expense of $17.00 per ton after delivering on the

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