FORMATION OF THE BROWN HEMATITES.
The hematites, or brown oxides, do not appear to exist except in the vicinity of limestone : yet there is no evidence of their originating from the limestones, since these rocks contain but a small proportion of iron. The ferriferous rocks of this region are generally those which underlie the limestones, though the overlying sandstones also contain the ores of iron in various forms. Throughout the great valley range the deposits of ore are generally found on the slates and sandstones, or between the limestones and overlying or under lying rocks. But when the limestones are stratified horizontally, covering or conceal ing these rocks, the ore is always found in fissures, as represented in figure 173.
It would therefore appear that the substance forming these ores is obtained from other formations than the limestones, but that the chemical action which separates, segre gates, or precipitates them, is supplied by the latter.
Where the limestones and the ferriferous slates and sandstones meet, these ores are generally found in "nests," deposited in beds of clay, as shown in figure 171. The great mineral range which exist on the Primal slates, or between them and the overlying limestones, and which we have traced from Alabama to Pennsylvania, is always in such beds of clay, and almost invariably accompanied by manganese. These beds of clay are peculiar to this locality and range, They are singular in character and form, and are made up of almost all colors, from the most delicate pink and red to the purest white, and from the most tenacious, adhesive, and plastic nature to the most loose, friable, soft, and treacherous quicksands. In the harder clay's, the nests of ore are usually found, but they are frequently cut through by quicksand courses. We also find in connection with these nests of ore large bodies of fine ochreous powder, resembling "Tripoli polish," and almost equal to crocus in its effects on metals, when the finer quality is selected.
The ores of these clay deposits are always in basin-shape, though often cut down and through by erosions. They do not lie deep, and are very irregular in size, constantly increasing and decreasing in extent, and only occasionally existing in great masses.
The brown hematites of the western side and centre of the valley, when existing en the slopes of the sandstone formations, or between the limestones and the underlying rocks, are not generally found in nests, or enclosed in masses of clay, and the ores are more massive in structure and harder in character, though generally less in quantity.
They are seldom, however, stratified, and are generally accompanied with limited bodies of clay, in which they are imbedded.
The ores found in limestone fissures are much the same in character, and always accompanied by clay to a greater or less extent. This form of deposit is extensively developed at the Ironton mines, and the hematite formations on the Lehigh generally, Most of the hematites furnished to the numerous furnaces on the Lehigh River are from the limestone fissures. Some of these mines, we learn, are 200 feet deep, and we have not heard of any bottom being found. We should expect such fissures to reach the underlying slates or sandstones; but we do not think any of them are very deep, since they generally exist where these lower formations are in close proximity. At great depth we have generally found such fissure deposits to terminate in iron pyrites, or the sulphurets of other minerals to predominate,—such as lead, zinc, copper, &c.
These fissures could not have been filled from the surface by precipitation, since in that case, instead of the narrow fissure containing all the ore, we might expect to find it abundant on the surface, or around the fissure. But that is not the case: the fissure alone is filled,—not with the formations which surround it, but with iron and clay, which were produced from or by the lower rocks; and we have no doubt but that the fissures themselves were created by the accumulated gases which sought vent either between the formations, as at the junction of the limestones and the underlying rocks, or through them when nearly horizontal, and offering no other means of escape. Sub limation, therefore, must have produced most of our brown hematites. But they may also have been formed by the decomposition of iron pyrites and the disintegration of the ferriferous rocks. But sublimation, internal heat, volcanic steam, and water, with the action of subsequent frosts, fires, and atmospheric changes, have, no doubt, produced much of our highly oxidized ores. We find the outcrops of calcareous, carbonaceous, argillaceous, and other ores greatly changed from their normal condition, and assuming the character of brown hematites: in fact, this seems to be the result of all oxidization when perfect; though there are other forms in which such ores exist, as red oxides, &c. &c.