About six miles to the south of Iron Mountain is Pilot Knob, which is an isolated peak or knob about 580 feet in elevation above the plain, and about 11,000 feet above St. Louis. The rocky strata of Pilot Knob is a dark, silicious slate, distinctly bedded, and clipping uniformly to the south at an angle of 25° or 30°.
The quartz predominates nearly two-thirds of the distance from the base up; but above that to the summit, iron is found in heavy beds, alternating with silicious matter. Some of these beds are very thick, and consist of pure micaceous and specular ore, which shows a slaty structure, while that of Iron Mountain is without cleavage.
The other localities at which ore is found in this region are Little Mountain, near Iron Mountain, and Shepherd's Mountain, near the Knob; while the "Bogy Bank" and Russell Bank produces good ore.
An analysis by Dr. Litton of the ore from Pilot Knob gave:— It will be observed from the foregoing description that these celebrated iron moun tains are not all iron, as many suppose, and as might be imagined from the tenor of the Missouri State Reports; perhaps less than one-tenth of the bulk of those mountains is solid or valuable ore; but even under such a limited estimate the amount of available ore in these celebrated deposits is practically inexhaustible, and is sufficient to supply the iron industry of Missouri, and, in fact, a great portion of the West, for ages to come, without reference to the large amount of brown hematite and other ores which are scattered through this region.
"The mountain-masses of Missouri have pre-eminently the eruptive character, and are associated with rocks which have always been considered as of unmistakable eruptive origin. The iron region of Lake Superior, which is even more extensive and more abundant in ores than that of Missouri, is another instance of the vast development of these ores in the Azoic.
"In the State of New York, in the same geological position, we find the same occur rence of the specular and magnetic oxides, and almost rivalling with those of the regions just mentioned in magnitude and importance. Here, however, the evidences of direct
eruptie origin are perhaps less conspicuous, and the deposits seem, in many cases at least, to exhibit the appearance of a secondary action having taken place since their original formation. In this region these ores have in their mode of occurrence the most striking analogy with those of Scandinavia. Like them, they generally coincide in the strike of the rocks in which they are enclosed, forming lenticular or flattened cylinder shaped masses intercalated in the formation. The enclosing rocks are similar in cha racter to those of Sweden : they are gneiss, quartzose, and hypersthenic rocks.* " Although the ores of the Azoic have not always a purely igneous origin, yet even in these cases where they bear the most evident marks of having been deposited in beds parallel with the formation, with the presence of water, we must acknowledge that pre existing eruptive masses may have furnished the material from which they were derived. That the Azoic period was one of long-continued and violent action cannot be doubted; and while the deposition of the stratified beds was going on, volcanic agencies, com bined with powerful currents, may have abraded and swept away portions of the erupted ferriferous masses, rearranging their particles and depositing them again in the depressions of the strata. This seems the most probable origin of some of those lenticular beds parallel with the stratification, where it is difficult to conceive of a fissure always coinciding with the line of strike of the formation, and where the mechanical evidences are wanting of the thrusting up of such masses of matter, which we know could not have taken place without many dislocations of the surrounding rocks which would have made themselves very apparent."?