Chapter VI · (continued 2)

Under the influence of percolating, heated waters, new minerals are formed in sedimentary or igneous rocks, and rocks once metamorphosed may undergo additional changes. Mineral matter previously widely disseminated through rocks is, under the action of percolating, heated water, brought together and the regeneration and crystallization of a large variety of ores and minerals result. The birthplace of a large variety of ores and minerals is in the zone of metamorphism. It is in metamorphic rocks that the geologist looks for gems, the precious metals, crystalline marble, magnetic iron, etc.

For the most part, however, the native metals and ores of the precious and many of the common metals are too widely disseminated in the metamorphic rocks to be of commercial importance, and a still further concentration, principally in fissures and other cavities, is necessary before they can be of value to man. This secondary concentration is much the same as in the case of the deposition of lead and zinc ores in cavities in sedimentary rocks, and results largely from the solution and redeposition, sometimes by replacement, of mineral matter by heated waters.

Certain ores and rocks contained in metamorphic terranes owe their concentration to previously acting processes of concentration, but have undergone chemical changes in place. Illustrations of this class of ores, etc., are furnished by the magnetite and hematite contained in the metamorphic rocks on the eastern border of the Appalachians, in New England, eastern Canada, and the Lake Superior region. These ore bodies, frequently of great size, in some instances furnish evidence of having been originally lenticular masses of bog-iron ore, or ferric carbonate, associated with sedimentary beds, and originally concentrated, as already mentioned, at the surface through the action of water charged with carbon dioxide, but principally on account of the influence of heat have been changed to a higher degree of oxidation and now appear as hematite, as, for example, in the iron districts of the northern portions of Michigan, Minnesota, Wisconsin, and the Ozark Hills, or still further altered as in the richest of all iron ores, magnetite, so abundant in the metamorphic rocks of the Appalachian region, about the Adirondack hills, widely and in extensive bodies in eastern Canada, about the south shore of Lake Superior, in Texas, etc.

In certain instances, as has been shown by C. R. Van Hise and others, hematite ore, like that of the Lake Superior region, has resulted from the alteration of ferrous carbonate which had replaced limestone by a chemical process of solution and double decomposition.

As bodies of iron ore in the form of the carbonate, or limonite, may occur in rocks of any age, and as rocks of any age may be metamorphosed, it follows that hematite and magnetite may be present in any formation which has been subjected to metamorphosing conditions.

Limestone when metamorphosed is changed to a crystalline marble, frequently white in colour owing to the dissipation of its previously contained organic matter. The white marbles so extensively utilized in Georgia, Vermont, etc., are of this nature. Other similar metamorphosed layers occur in several of the ranges of the Pacific mountains from Mexico to Alaska.

The influence of metamorphism on deposits of coal when the heat has been of moderate intensity serves to drive off a large part of the volatile matter present and converts the coal into a substance resembling coke, as has happened adjacent to dikes or intruded sheets of igneous rock in the Richmond coal-field, Virginia, in New Mexico, Washington, etc. When the heat is somewhat more intense, the coal is changed to what is termed graphitic anthracite, as in the Rhode Island coal-fields, and when still greater or long-continued, results in the production of graphite, as in the Algonkian rocks about the Adirondack hills and over a wide region in eastern Canada.

An important result of metamorphism is the production of new minerals in the rocks acted on. Many of the metamorphic terranes consist essentially of quartz, feldspar, and mica, which have been formed by the rearrangement of the mineral matter contained in the rocks during their previous state. Besides these constituent minerals there are frequently others present, such as the garnets, tourmaline, emerald, sapphire, corundum, etc., which are of economic importance. In a large number of instances the minerals of metamorphic rocks are contained in veins of one class or another, in part resulting from segregation in the rocks themselves while yet in a heated condition, and in part deposited in fissures or other openings as a result of secondary concentration through the action of heated waters. The principal difference between the minerals concentrated in the metamorphic rocks and those deposited in cavities in unaltered sedimentary beds seems to be that in the former instance the percolating water which carried the material in solution had a higher temperature than in the latter case.

Among the numerous mineral substances of value in the arts, occurring in the metamorphic terranes of North America, other than building stones and the previously concentrated deposits, such as iron ore, graphite, etc., mention can only be made at present of the following:

Mica, which is used in thin sheets for the windows of stoves and furnaces, and when ground and mingled with other substances furnishes a good insulating material for electric wires, fireproofing, and also used as a lubricant, etc., occurs in large quantities in the metamorphic rocks of New Hampshire and Ottawa, and less abundantly in North Carolina, South Dakota, Wyoming, Idaho, etc. It is widely distributed, but to find transparent colourless sheets of large size is difficult.

Talc and soapstone, consisting of the hydrated silicate of magnesia, and useful for hearths, mantels, fire-brick, linings for stoves, laundry-, bath-, and acid-tubs, etc., and when ground, employed as an adulterant of soap, paper, rubber, and as a lubricant, etc., occurs widely in the metamorphic terranes on the eastern side of the Appalachians, in Canada, and at numerous localities in the Pacific mountains. The chief centres of production at present are in Pennsylvania, New Jersey, New Hampshire, and Vermont.

Asbestos, valuable on account of its fibrous structure and non-conductivity of heat, which make it an excellent insulator, and largely used in the manufacture of fireproof paper, cloth, etc., occurs in connection with serpentine, in metamorphic terranes, and is extensively mined in the Thetford district, Quebec.

Corundum, consisting of aluminum oxide, and having essentially the same composition as the sapphire and ruby, and a less pure variety of similar composition termed emery, is largely used as an abrasive in polishing metal, sharpening tools, etc., and also as "sand-paper" in working wood, occurs in commercial quantities, largely in crystalline limestone, at Chester, Mass., in Georgia, North Carolina, and several other localities. Although corundum is next to the diamond in hardness, and therefore highly favourable, when reduced to a powder, for polishing various substances, the demand for it has in recent years been diminished owing to the manufacture of an equally if not superior material termed commercially carborundum.

Among the crystals used as gems, which occur in the metamorphic rocks of North America but thus far in minor quantities, and as a rule of inferior quality, may be enumerated sapphires, rubies, tourmalines, garnets, quartz, etc.

Apatite, a mineral rich in phosphoric acid, and largely used in the manufacture of fertilizers, occurs associated with limestone in the metamorphic rocks of Quebec and Ontario in the form of veins, beds, and irregular pockets, and a few years since was extensively mined, but now, owing to foreign competition, is held in reserve.

By far the most valuable of the minerals and native metals that occur in the metamorphosed terranes is gold. Although this metal has been found in paying quantities in association with nearly every kind of country rocks and in terranes of all ages, the place of its original concentration from a previously widely disseminated condition is to a great extent in the zone of metamorphism. It occurs principally as native gold, although usually alloyed with silver, but is frequently contained in iron pyrites. In the crystalline rocks, such as gneiss, schist, slates, granite, etc., it occurs in flakes and grains, but so far as its occurrence in commercial quantities is concerned its deposition has for the most part been secondary, and the metal, usually in association with quartz, is found in veins, lodes, contact deposits, etc., and owes its concentration to chemical agencies not well understood, acting in connection with percolating water. That this general statement is correct is clearly shown by the fact that gold occurs in crystals, flakes, grains, etc., most frequently in quartz and iron pyrites, which, as can be shown in a number of ways, have crystallized from solution. The gold and its commonly associated mineral in countless instances occupy fissures and must have been carried to such localities after the surrounding rock had been fractured. So intimate is the association of gold with metamorphic rocks that this is one of the main guides in searching for it, although, as already stated, it is frequently present in other rocks as well. With the disintegration of the metamorphic terranes the gold is set free, and may be still further concentrated by streams so as to form the well-known placers.

A very large proportion of both the quartz and placer mining of North America is in regions occupied by metamorphic rock. This is true of all gold-mines, previously quite largely exploited, of the Atlantic mountain region from Georgia to eastern Canada. The mines of California are also largely in schistose rocks, as are also those to the northward, throughout the Pacific mountains, to British Columbia and Alaska, including the recently established mining district at Cape Nome.

With placer gold, and probably derived largely, if not entirely, from metamorphic rocks, there are frequently found grains of platinum. The annual production of this metal in the United States and Canada has a value of about $5,000.

The study of the distribution of native metals and ores in the metamorphic rocks of North America indicates that in general the older rocks, as the Archean, for example, are less rich than the younger terranes, such as the schist, etc., of the Sierra Nevada and Cascade Mountains. This seems to indicate that the older rocks were once deeply buried and their more soluble substances removed by ascending waters, and in part redeposited in higher terranes. Erosion has since carried off the rocks which were mineral-charged and laid bare the depleted terranes beneath. This hypothetical explanation of the general poverty of the Archean rocks is coupled with another consideration, namely, that the younger metamorphic terranes, where they have been elevated, as in the Pacific mountains, are more broken than the Archean rocks, and afford more cavities in which minerals may be deposited. Whether this is a complete explanation or not remains to be demonstrated, but observation shows that the Archean terranes—all of which as yet discovered are composed of either metamorphosed or igneous rocks—are, in comparison with younger metamorphosed rocks, relatively poor in minerals and ores of commercial importance.

Among the economic products of the rocks are included mineral waters. The direct commercial value of such waters, not including their use for baths, etc., in the United States, is about $7,000,000 annually. The demand for these waters depends largely on the mineral substances they hold in solution, and which in many instances is in process of transference from one locality to another. Much might be written in this connection in illustration of the fact that the processes by which minerals, ores, etc., have been concentrated are still in progress.

LITERATURE

An extensive literature is available concerning the geology, minerals, ores, etc., of North America, but only a few of the more important publications can here be referred to. The numerous publications of the United States Geological Survey, the Geological Survey of Canada, and the Geological Survey of Mexico contain vast amounts of valuable information. Several of the States of the United States have independent surveys and have published numerous reports. Of journals containing articles of American geology, the more important are: The Journal of Geology, published at the University of Chicago; The American Geologist, published at Minneapolis, Minn.; The American Journal of Science, published at New Haven, Conn. The publications of a large number of learned societies in Canada and the United States should also be consulted.

The most useful bibliographies of North American geology are:

Darton, N. H. Catalogue and Index of Contributions to North American Geology, 1732-1891. Published as Bulletin No. 127 of the United States Geological Survey, Washington, D. C., 1896.

Dowling, D. B. General Index to the Reports of Progress [of the Geological Survey of Canada], 1863-1884. Published by the Canadian Geological Survey, Ottawa, Canada, 1900.

Warman, P. C. Catalogue and Index of the Publications of the United States Geological Survey, 1880 to 1901. Published as Bulletin No. 177 of the United States Geological Survey, Washington, D. C., 1901.

Weeks, F. B. Bibliography of North American Geology, Paleontology, Petrology, and Mineralogy for the Years 1892-1900, Inclusive. United States Geological Survey, Bulletins No. 188 and 189, Washington, D. C., 1902.

Of the numerous general treatises on the geology, the following will be found especially helpful to the student:

Dana, J. D. Manual of Geology. Fourth edition. American Book Company, New York, 1895.

Kemp, J. F. The Ore Deposits of the United States and Canada. Scientific Publishing Company, New York, 1900.

Le Conte, J. Elements of Geology. Fifth edition, revised and partly rewritten by Prof. H. L. Fairchild. D. Appleton and Company, New York, 1903.

Merrill, G. P. A Treatise on Rocks, Rock-Weathering, and Soils. The Macmillan Company, New York, 1897.

Shaler, N. S. Outlines of the Earth's History. D. Appleton and Company, New York, 1898.