the remaining localities 21%. It will thus be seen that the statement with regard to the relative amounts produced is true, and that the reputation of the St. Francois mountains as iron-ore producers depends practically upon Iron Mountain and Pilot Knob.

IRON MOUNTAIN.

The Iron Mountain deposit of specular ore is located in St. Francois county, in section 31, township 35 N., 4 E. It is 81 miles south of St. Louis on the St. Louis, Iron Mountain & Southern railway.

The highest point of the mountain at the mine is known as the Big mountain, and rises about 200 feet above the level of the surrounding valley; west of Big mountain is a lower hill known as Little mountain. The highest point of Little mountain is 130 feet lower than Big mountain. There is, however, really no topographical distinction between the two, Little mountain being but a shoulder of the larger mountain, and both are but lower points of a larger porphyry mountain which reaches to the north. These and other features of the mine are well shown on the topographic map of the mine on the preceding page. It is on these two points that the ore deposit is found which gave the locality its name. When first discovered, the mountain, for it will be referred to by this singular noun, was completely covered by boulders of specular ore of various sizes, some of them weighing many tons each. No rock was in sight, and this led to the idea, which was only dispelled by long working, that the entire elevation was of solid iron ore, and it was fully believed that this ore extended indefinitely downward. This idea was strictly in accord with the tenets of geology as they were then taught, that metalliferous veins, including iron ores, were of eruptive origin, and their source being deep within the earth, the size of a given deposit would increase rather than diminish as the workings grew in depth. It is not necessary to add that this idea has long since given way, as observations have increased, leading to a more correct interpretation of geological facts.

The working of this deposit was first carried on in the boulders entirely. Work on the Big mountain finally exhausted the boulders in places and they were found to rest, together with the clays which filled the interstices of the boulders, upon solid masses of iron ore in some places, and, in others, upon a decomposed porphyry rock which changed with depth into the solid porphyry of the surrounding hills. Gradu ally, as the work extended, the limits of the solid iron ore were found,

and the exploration also showed limits to the boulder deposits, which were themselves at first thought to be inexhaustible. The boulder deposits were found to lie in a thick mantle over the summit and slope of the mountain. This mantle was heaviest near the highest point and gradually thinned out as the lower flanks of the mountain were reached. Not only did the deposit grow thinner at the flanks, but it was found. that the boulders were interstratified with thick layers of tough reddish clay with but occasional lumps of iron imbedded, and these of diminished size.

The stripping of the mountain showed still another feature. On the eastern and western slopes especially, the point was reached where the Paleozoic rocks were found to rest upon a deposit of iron ore and porphyry boulders. This discovery showed conclusively that the age of the iron ore veins as well as of this last boulder deposit was greater than the sandstones and limestones which surrounded the mountain in which the ore was found. In the surface mantle were found clays of a different color and texture. In these clays were fragments of specular iron, but in addition there were fragments of chert, showing clearly that these clays were a residuum of a decomposed limestone; while the boulder deposit plainly pointed to the conclusion that, while the Paleozoic seas were washing the foot of the mountain, disintegration was at work on this ore-body, and that occasional fragments would roll down from the parent mass and become imbedded in the sediments. Still further is this shown to be the case by the finding of iron fragments imbedded in the undecomposed limestones, but the strongest proof of all lies in the great boulder deposits, which, on the eastern side of the mountain, reach for over 1200 feet along the base of the Paleozoic rocks.

Reviewing the above, we find the following series of deposits of iron ore at Iron Mountain: First, the superficial boulder deposit; second, the solid ore-vein deposits; and third, the early Paleozoic boulders. These will be described in the order given.

As has been stated, the mantle of ore nowhere exceeded twenty feet in thickness, and, upon the removal of this, the true nature of the deposit was revealed, although its extent was still uncertain. The solid ore, which must be regarded as the parent mass, was found to extend in a perfect net-work of reticulating veins of various size, but

with a general northeast trend. The nature of the veins is shown in the following cut, which illustrates the appearance of the breast of ore

K

worked just south of drill hole No. 23, located on the map opposite p. 55. The "bluff" which fills the spaces between the solid veins of iron is more or less decomposed porphyry. This cut shows the lateral veins, as they may Fig. 10. Veins of specular iron ore in porphyry at Iron mountain. be called. The main vein ran nearly due northeast across the mountain. A cross-section of this main vein [Fig. 11] called the backbone vein, shows its comparatively great size, and also shows what is known as a saddle-back among miners. The legs of this vein are here from twelve to eighteen feet thick. The

space between the two legs is filled with broken ore and porphyry. This vein extended through what is now an open cut and which is marked as such on the topographical map of the mountain. The 66 open cut" is about 480 feet wide north and south. The lowest point to which the ore has been excavated is about 150 feet below the highest point of this mountain. The whole pit has not been sunk to a common level on account of the subdivision of the main or backbone vein.

Fig. 11. Section of the main vein at Iron mountain.

On Little mountain, a vein about 420 feet long on the outcrop was found. The entire depth of this ore body, measured on a slope of about 40°, is 360 feet. The thickness from foot to hanging wall is about 30 feet, with a maximum of 40 feet. Workings which extend under. ground toward the south and east reach about 240 feet farther. The outlines of these underground workings are shown in the topographi cal map of the mine by the broken lines. The foot-wall of this vein is not clean and sharp, but is made up of brecciated porphyry, with the interstices filled principally with iron ore, but also with quartz and apatite crystals, with occasional crystals of hornblende. The imme. diate hanging wall is made up of boulder-like masses, embedded in more or less indurated clay. Over this lies the Paleozoic limestone.

There is a wide break between Little and Big mountains, and the vein seems to disappear east of Big mountain, near bore-hole No. 30, shown on the map. West of Little mountain bore-holes Nos. 27, 28, 2, 22, 15, 12, 13 and 14 have failed to show either a solid vein or a workable deposit of boulders. Bore-holes Nos. 32, 30, 17, 20, 21, 18, etc.,

also fail to show extensions of large veins, and it seems probable that the first line of detrital ore marked out quite accurately the direction and extent of the veins, although the veins have not proved to be continuous.

Fig.12 Section showing relations of conglomerate beds to limestone.

Limestone.

Porphyry boulders
in clay.

Iron ore boulders.

Iron ore boulders.

The next and last of the economically important deposits at Iron Mountain are the beds of boulder or detrital ore. These are found exclusively, thus far, on the eastern slope of Big mountain. The un

Porphyry boulders. derground workings of these deposits here are accurately outlined by broken lines on the contoured map. As is there Porphyry boulders. shown, they extend about 1680 feet from the mouth of the slope eastward.

Iron ore boulders.

Porphyry boulders.

iron ore veins.

Fig. 12 shows the relation of the conSolid porphyry with glomerate beds to the Paleozoic rocks and the porphyry foot wall. At the point where the section is taken the conglomerate ore is immediately overlain by

a bluish green or gray decomposed porphyry clay, with boulders of partially decomposed porphyry. There are only occasional fragments of iron ore in this. The strata between those of ore are of the same nature, as is also the interstitial filling between the iron ore boulders. Over all are beds of limestone.

1. Magnesian limestone.

2.

Sandstone

3.

Clay shale

4.

Deposit of boulder ore

5.

The extreme width of this deposit, 80 far as is known, is about 360'. The height from foot to hanging wall is about 20 feet. This thickness does not hold throughout, however, but the foot and hanging walls come together quite rapidly on either side. The adjoining ideal cross section illustrates this.

The boulders of iron are held tightly

More or less decomposed porphyry. in a partially indurated clay, in which are

Fig. 13. Diagram illustrating channel

form of boulder deposits.

also boulders of porphyry and a jaspery rock which is still very fresh. The beds are so firm that shooting is necessary in mining. The firmness of the masses is well illustrated by the relative sizes of the galleries and pillars as shown in mine map.