Iron Mountain, a remarkable deposit of specular iron ore on the S. E. border of Washington co., Missouri, about 40 m. S. W. of St. Genevieve, the nearest point on the Mississippi. The locality is connected with St. Louis by rail. It is described by Dr. Litton in the second annual report of the geological survey of Missouri (1855), and by Prof. Raphael Pum-pelly and Dr. Adolf Schmidt in the volume on " Iron Ores and Coal Fields " of the new geological survey (1873). The later account, being based on more extensive and minute examinations, with the advantage of exposures made by continued mining operations, is the more accurate. The Iron mountain deposit occurs in the azoic rocks, which reach their most extensive surface development in the region forming the northern part of Madison, Iron, and Reynolds, and the southern part of St. Francis and Washington counties, constituting the exposed portions of the skeleton of the eastern part of the Ozark range, and appearing as knobs 1,400 to 1,800 ft. above sea level, and 300 to 700 ft. above the valleys at their bases. These knobs form an archipelago of granitic and porphyritic islands in the lower Silurian strata of dolomite and gritstone, which surround and separate them.
These azoic crystalline rocks, having been apparently above the level of the sea since before the upper Silurian period, at least, have been exposed to the action of atmospheric agency for enormous periods, and under this long-continued influence have undergone remarkable changes. The gradual removal of the soluble constituents has left important residuary deposits in the Silurian strata of clay, flint, crystalline quartz, iron sulphuret, galena, etc, and in the pre-Silurian rocks of iron ore. The destructive action of atmospheric agencies is. developed both in the disintegration and decomposition of large bodies of rocks en masse, and in the formation and subsequent gradual disintegration of polygonal blocks on the surface. In Iron mountain the former is characteristically the case, the porphyry of which the hill (now 250 ft. high, and covering about 500 acres) largely consisted, having been entirely changed to clay. The accumulation of insoluble residuum from this decomposition and denudation has left a remarkable surface deposit of iron ore, covering the whole of the hill in a mantle of ore detritus, associated with clay. The internal structure of the hill is indicated by the mining excavations at and near the summit, and on the spur called Little Iron mountain.
It appears to consist of deposits of most irregular form, lying, as magnetic observation has seemed to show, in zones extending N. N. E., being nearly the course of the ridge of porphyry of the spur of which Iron mountain is the southwestern termination. At the summit an immense mass of solid ore is exposed; and the decomposed porphyry adjoining it is traversed in all directions by veins of various sizes and irregular shape, forming a network of ore and rock. Little Iron mountain, a western continuation of the main hill, contains similar irregular veins and masses of ore, the smaller of which frequently show crystals of apatite. At or near the surface the apatite has been removed, leaving the impressions only of the crystals, frequently lined with delicate drusy quartz: hence, in such seams, the surface ore shows least phosphorus. All the ores of this region are characterized by great purity. Those of Iron mountain are in general very rich and very uniform, nearly free from sulphur both on the surface and in depth, and carrying a variable proportion of phosphorus, which rarely exceeds 0.12 per cent. The ore is specular, containing over 90 per cent. (sometimes 97 per cent.) of peroxide of iron.
Porphyry is enclosed in it in large masses which can be easily separated, apatite occasionally in small crystals, and quartz in drusy aggregations resulting from infiltration into small cavities or fissures. All Iron mountain ore is magnetic, some of it strongly so, with distinct polarity, though the greater part of it acts but slightly on the needle. The amount of ore in this deposit is beyond calculation. The main body has a thickness of at least 50 ft., and continues indefinitely in depth. Some distance beyond the base of the mountain an artesian well, sunk to the depth of 152 ft., passed through iron ore and clay 16 ft., sandstone 34 ft., dolomite 7 1/2 in., gray sandstone 7 1/2 in., "hard blue rock" 37,ft., "pure iron ore" 5 ft., porphyry 7 ft., iron ore 50 ft. - Pilot Knob, in Iron co.,.6 m. S. of Iron mountain, is a conical hill nearly circular, about 600 ft. high and a mile in diameter at the base, composed chiefly of porphyry, porphyry conglomerates, and beds of hard specular iron ore. The top is conglomerate 140 ft. thick, consisting of more or less angular pebbles of porphyry cemented with iron ore, and containing frequent layers and bodies of ore.
The lowest layer of the conglomerate is in some places workable as iron ore, the matrix consisting mainly of finely divided specular oxide, with clay. Below this lies the ore bed proper, 46 ft. thick, and divided into two beds, about 31 ft. above the lower foot wall, by a persistent slate seam 10 in. to 13 ft. thick. The upper bed is distinctly stratified flag ore, and considerably leaner than the lower, which presents a very compact and hard stratified peroxide. Under this ore bed is a series of porphyries. The maximum superficial extent of the ore bed appears to be about 200,000 square yards. It dips S. W. about 13°. The ore differs in quality from most of the other specular ores in the state. It is steely gray with a tint of sky-blue, very faint lustre, crystalline to granular structure, lamellar and jointed, and very brittle. The streak is uniformly dark red. The fine ore from the conglomerate has polaric magnetism, and all the Pilot Knob ores are slightly attracted by a magnet, when they are ground fine. Those from the beds immediately above and below the slate seam, however, do not disturb an ordinary compass needle. - At Cedar hill, Shepherd mountain, and other localities, similar deposits of magnetic specular iron ore occur.
Dr. Schmidt holds that these specular ore deposits have been formed by infiltration from chalybeate waters. At Iron mountain he believes such waters to have deposited oxide of iron in numerous large and small fissures in the porphyries. At Pilot Knob he believes the iron ore beds to have been formed by gradual replacement of the constituents of the porphyry with ore, effected by solutions similar to those which caused Iron mountain and other deposits. This view is supported by chemical as well as geological considerations. - The Iron mountain company in 1871 shipped 262,477 tons of ore; in 1872, 371,474 tons, of which 143,431 tons were consumed in Missouri. In 1873 there were 16 furnaces employing the ores of this region, 7 using charcoal and 9 stone coal, able to produce in a year of ten months about 176,000 tons of iron. The production of 1872 was 126,652 tons, from 211,177 tons of ore, indicating a yield of about 60 per cent. of iron from the ore. These specular oxides of the azoic rocks of Missouri are proving immensely important, not only to the western states, but also to the general iron industry of the country.
The demand for the ores is large, on account of their richness and purity, which render them suitable for admixture with magnetites, etc, of other localities, and adapt them especially for the manufacture of pig iron for use in the Bessemer process, and of wrought iron by the direct process.