Woonsocket. The Virginia deposits occur in the eastern part of Page County and other localities along the Blue Ridge; the North Carolina deposits are found in the Greensboro district and in Ashe County.

The titaniferous magnetite deposits almost without exception are associated with basic igneous rocks of the gabbro-anorthosite group. In the Adirondacks the anorthosite was the earliest of the intrusive igneous rocks, followed immediately by the gabbro and later by the syenite and granite. The ores are associated largely with gabbro dikes and masses which occur at intervals in the anorthosite area, but several large deposits in the Lake Sanford district are inclosed directly in the anorthosite. The small deposits associated with gabbro dikes have a tabular form with the longer diameter parallel to the wall of the dike; the large bodies in the anorthosite are irregular in form and have ill-defined borders. The deposits are of approximately the same age as the inclosing rocks, and both are simply different phases of the same rock mass. The ore consists of magnetite and ilmenite, both of which occur in the gabbro and anorthosite as accessory minerals. The deposits are simply local segregations of these minerals formed during the cooling of the igneous mass; they grade from pure iron ore (magnetite and ilmenite) through a gabbro or anorthosite rich in magnetite and ilmenite to an ordinary gabbro or anorthosite carrying these minerals in minor quantities.

The richest titaniferous iron ores contain about 60 per cent iron, this being somewhat lower than the percentage of iron in the nontitaniferous ores. The titanic oxide (TiO,) is rarely below 8 or 9 per cent, and for the majority of deposits averages 15 per cent. The variation in titanic oxide content is due to a variation in the relative amounts of ilmenite and magnetite in the ores.

The titaniferous iron ores of Page County, Va., occur in small irregular deposits in a basic syenite (unakite). They are impure, being mixed with considerable amounts of minerals composing the inclosing rock.

Contact magnetite.-Magnetite deposits of the Cornwall type are found only in southeastern Pennsylvania. The principal deposits occur near Boyertown and Reading, Berks County; Cornwall, Lebanon County; and Dillsburg, York County. Small deposits are found near Warwick, Chester County, and Hummelstown, Dauphin County. The ores occur at the contact of intrusive masses of Triassic diabase where they have intruded replaceable sediments. The principal deposits occur along the northern edge of the Mesozoic Newark belt, where the diabase is intruded into limestones and limy shales of Cambro-Ordovician age, but locally small deposits are found at the contact of the intrusive masses with Triassic sandstones and shales. The York County and Warwick deposits lie in Mesozoic strata; the Cornwall deposit is situated at the top of "No. II" limestone of the Pennsylvania section just under "No. III" ("Hudson") shale; the other deposits occur near the base of "No. II" limestone.

The following description of the deposits is given by Spencer.a The Cornwall type of iron ore is so called from the important Cornwall mine in Lebanon County, Pa. The ores are essentially magnetite, but they contain pyrite in

a Spencer, A. C., Magnetite deposits of the Cornwall type in Pennsylvania: Bull. U. S. Geol. Survey No. 359, 1908, p. 10.

amounts which make it necessary to roast them before they can be used in the blast furnace. Some specular hematite occurs in certain of the mines, but the amount of this mineral is relatively unimportant.

The ore occurs in large and small masses of varying form, either entirely inclosed by stratified sedimentary rocks or lying in such rocks where they come in contact with masses of intrusive diabase. The ore minerals appear to have been formed by more or less complete chemical substitution in the body of the rock, the portion of the rock not replaced constituting the principal gangue of the ore. Aside from the deposition of the iron minerals, the limy strata associated with the ore bodies show remarkably little metamorphism, and though a few characteristic minerals of contact metamorphism occur, they are so uncommon as to almost escape observation. The Cornwall mines have yielded more than 20,000,000 tons of ore from what is essentially a single great ore body, though it contains extensive partings of barren rock. The other deposits are all much smaller, though several of them are still of important size.

The iron content of these ores is extremely variable, but as the ore is mined probably averages not far from 45 per cent. Rather constant chemical characteristics are low phosphorus, high sulphur, silica, and lime and magnesia, and the presence of copper.

Limestone magnetite.-Limestone magnetite deposits are found near Big Stony Junction, Giles County, near Abingdon, Washington County, and elsewhere in southwestern Virginia.

The ore occurs in the upper formations of the Shenandoah group of limestones and in clays derived from them. The deposits are apparently small, though indications of ore are found over a wide area. The ore usually occurs in beds in the limestone and in lumps of a few pounds each in the clay. . Iron carbonate superficially altered to limonite occurs in the limestone associated with the magnetite.

The ore consists of a mixture of magnetite and hematite and is of high grade, being very low in phosphorus. The iron content ranges up to 62 per cent or more; the phosphorus content is low.

IRON CARBONATE.

Iron carbonate of the "Coal Measures."-Bedded iron carbonate occurs in the coal horizons of the Carboniferous rocks in western Pennsylvania, northern West Virginia, eastern Ohio, and northeastern Kentucky. The mining of these ores was at one time an important industry, but has practically ceased except for small operations in southeastern Ohio.

The ores are carbonate of iron, spathic iron ore or siderite, except along the outcrop and for variable distances in, depending on the character of the overlying material, where they have altered to hydrous ferric oxide, limonite.

Carbonate ores occur in all the formations from upper Mississippian to Permian, but the most important commercial horizons are in the Pottsville and Alleghany formations of the lower part of the Pennsylvanian. The most important horizon is at the top of the Vanport or "Ferriferous" limestone in the lower part of the Alleghany formation. This ore is known as "buhrstone ore," and occurs throughout the region mentioned. Many other important horizons of varying horizontal extent exist.

The ores have been divided into four classes,a viz: (1) Limestone ores, (2) block ores, (3) kidney ores, and (4) black band ores.

(1) The limestone ores are those which occur upon or very near the top of a limestone stratum. There are several important hori

a Phalen, W. C., Economic geology of the Kenova quadrangle, Kentucky, Ohio, and West Virginia: Bull. U. S. Geol. Survey No. 349, 1908, p. 122.

zons at which this ore occurs, but the most important is that just above the Vanport limestone. At this horizon ore occurs in many places in the Carboniferous area of Pennsylvania, Ohio, West Virginia, and Kentucky. The Vanport limestone ranges up to 8 or 10 feet in thickness, and the ore above it varies from a few inches to several feet in thickness. In some places extreme variations in thickness may occur in the ore bed in short distances, thus giving it a pockety character. Locally, either or both ore and limestone are absent over considerable areas. The ore is altered to limonite along the outcrop, and it is this portion chiefly which has been mined. The limonite is usually brown or red and dense and close grained. It is fine and uniformly rich in iron. Most of this ore has been removed. The carbonate ore is dense, close grained, and bluish or grayish in color. The metallic iron in the limonite ranges from 40 to 50 per cent; that in the carbonate from 25 to 40 per cent. Other limestone ores occur at several horizons, but they are much more limited in their extent, occurring only locally over small areas.

(2) The block ores are so called because they cleave into square or rectangular prisms when mined. They consist of carbonate, except at the outcrop, where they are altered to limonite. Most of the block ores are in the Pottsville formation and the lower part of the Alleghany, and they are best developed in the western part of the district. They are more persistent than the limestone ores, and individual beds retain uniform thickness over wider areas. The beds, however, differ greatly in regard to thickness and quality of ore, though as a rule they are leaner than the limestone ores. The chief impurity is sand. In general the thick beds contain a lower grade of ore than the thin beds.

(3) The kidney ore occurs in peculiar rounded masses in clay or shale. The deposits, though occurring at distinct stratigraphic horizions, are not in definite beds like the preceding, but are scattered through zones 3 to 6 feet in thickness. They are best developed in the Alleghany formation above the Vanport limestone. Like the preceding ores, they are altered to limonite along the outcrop. As regards grade these ores compare very favorably with the limestone and block ores.

(4) The black band ores include beds of carbonate of iron, which contain less or more bituminous and earthy matter. They generally occur interbedded with bituminous shale. The ore is carbonaceous siderite, occurring in layers a few inches in thickness interbedded with shale layers through a zone varying in thickness up to 10 or 15 feet. The ore is generally of a higher grade than the other carbonate

ores.

Iron carbonate in sediments of the Coastal Plain.-Lenses and layers of iron carbonate in the form of kidneys occur interbedded with Cretaceous sediments of the Coastal Plain in Maryland, near Muirkirk, and other places between Baltimore and Washington. Considerable quantities of ore have been mined, but at present operations have been suspended.

Iron carbonate in limestone in southwestern Virginia.-A few small deposits of iron carbonate occur in the Chickamauga limestone in southwestern Virginia in association with limestone magnetite ores. They are altered near the surface to limonite.

IRON SULPHIDE.

Iron sulphides. Both pyrite and pyrrhotite occur in many localities in the crystalline rocks of the Piedmont region, and pyrite alone is found locally in small deposits in Paleozoic limestone and quartzite. The deposits in the crystalline rocks are usually in veins; those in the sediments are usually disseminated. They give rise to limonite gossan deposits.

Iron sulphides are used in the manufacture of sulphuric acid and the iron oxide residue (blue billy) is frequently used in blast furnaces for the production of pig iron.

CENTRAL DISTRICT.

The iron ores of the central district may be classified as follows: Hematite:

Soft, hard, and specular hematite associated with the pre-Cambrian iron formations of the Marquette, Menominee, Penokee-Gogebic, Mesabi, Vermilion, Cuyuna, and Baraboo ranges, Lake Superior district.

Specular hematite associated with porphyry in the Ozark region, southeastern
Missouri.

Specular hematite associated with Cambrian sandstone in the Ozark region.
Clinton hematite in east-central Wisconsin and in Missouri.

Red hematite in the Carboniferous rocks of central Missouri.

Brown ore:

Brown iron ores of the Claiborne formation (Tertiary) in northeastern Texas and northwestern Louisiana.

Brown iron ores of the Ozark region.

Bog ore in central and northwestern Wisconsin.

Limonite gossan associated with the lead and zinc deposits in Wisconsin, Illinois, and Iowa.

Magnetite:

Magnetite formed by regional and contact metamorphism of the pre-Cambrian hematite deposits in the Marquette, Mesabi, and Gunflint ranges, Lake Superior district.

Titaniferous magnetite in gabbro in northern Minnesota.

Iron carbonate:

Iron carbonate and silicate composing the unaltered iron formation in the Lake Superior district.

Iron sulphide:

Pyrite and marcasite associated with the lead and zinc deposits in Wisconsin, Illinois, and Iowa.

HEMATITE.

Lake Superior hematite.-The Lake Superior hematite deposits constitute by far the most important type of iron ore in the United States and yield about four-fifths of the total annual product. They are grouped into seven minor districts or ranges, viz: The (a) Vermilion, (b) Mesabi, and (c) Cuyuna ranges of northern Minnesota; the (d) Penokee-Gogebic, (e) Marquette (including the Republic and Swanzy areas), and (f) Menominee (including the Crystal Falls, Iron River, Metropolitan, and Florence areas) ranges of northern Michigan and Wisconsin, and the (g) Baraboo range of southern Wisconsin. Other districts with similar ore occur in Ontario and north and northeast of Lake Superior.

The rocks of the Lake Superior district range in age from Archean to Cambrian with the following succession:

Succession of rocks in Lake Superior iron-ore district.

Cambrian:

Potsdam sandstone.

Algonkian:

Keweenawan series (sediments, trap, gabbro, etc.).
Huronian series:

Archean:

Upper Huronian quartzite, iron formation, and slate.
Middle Huronian quartzite, iron formation, and slate.

Lower Huronian quartzite, conglomerate, dolomite, slate, iron formation, and
intrusives.

Laurentian series (granite, gneiss, and porphyry).

Keewatin series (greenstone, basic schists, and iron formation).

Of these rocks only the upper and middle Huronian and the Keewatin contain productive iron-ore deposits. Iron ores occur in the upper Huronian, in the Mesabi, Cuyuna, Penokee-Gogebic, and Menominee districts; in the middle Huronian, in the Baraboo district; in both the upper and the middle Huronian, in the Marquette district; and in the Keewatin, in the Vermilion district.

The iron ores are confined to the iron formations, in which they occur as local concentration deposits, resulting largely from the leaching out of silica from the iron formation, though partly from additional precipitation of iron oxide. The iron formations are bedded deposits consisting chiefly of a mixture of chert or quartz and ferric oxide segregated in bands or mingled irregularly. Banded iron formation which has become highly crystalline through metamorphism and in which the bands are bright red is known as jasper. The ordinary reddish-gray iron formation consisting of banded or irregularly intermingled chert and iron oxide is known as ferruginous chert; on the Mesabi range the local name "taconite" is applied to it. There are many other subordinate phases of the iron formations resulting from metamorphism or from an admixture with other sediments. Locally, masses of cherty iron carbonate and hydrous ferrous silicate (greenalite) occur, which are supposed to be remnants of the original form in which the iron formations were deposited. Greenalite is characteristic of the Mesabi district, and cherty iron carbonate occurs in all the other districts. The ferruginous chert was formed by the weathering of the iron carbonate and greenalite. Where the cherty iron carbonate has been altered by contact or regional metamorphism, local areas of amphibole-magnetite rocks occur. Where the ferruginous chert was metamorphosed, jasper is found. Frequently layers and lenses of slate are found interbedded with the iron formation, and these show all gradations to ferruginous chert. Paint rock, a decomposed s'ate deeply stained and impregnated with ferric oxide, is characteristic of many of the iron-ore deposits. It forms from slate lenses at the same time that the alteration of the surrounding iron formation to iron ore and ferruginous chert is taking place. The last and most important phase of the iron formation is the iron ore itself, which occurs locally along the outcrop of the iron formation where meteoric waters have had a chance to operate and where favorable conditions for concentration prevail. The rocks have suffered folding, faulting, and metamorphism to varying degrees in the different ranges, and these have influenced

a Leith, C. K., A summary of Lake Superior geology with special reference to recent studies of the ironbearing series: Bimo. Bull. Am. Inst. Min. Eng., No. 3, 1905, p. 453.