this class are common; for instance, in a belt extending from southwestern Maine to central Massachusetts. From two points in this belt, at Raymond and Phippsburg, Maine, crystallized anorthite has also been identified by analyses made in the laboratory of the United States Geological Survey. The other feldspars as well, albite, orthoclase, and the plagioclases, are known as contact minerals or inclusions in crystalline limestones, and also the micas muscovite, biotite, and phlogopite. Phlogopite is essentially a mineral of this group of rocks, its formation and that of biotite requiring the presence of magnesium compounds. To form scapolites, sodium chloride is necessary, but that may easily come from percolating waters, or from apatite. The alkalies required by the feldspars and micas may have a similar origin, or else be derived from impurities in the sediments from which the limestones were formed.

Nearly all limestones are more or less magnesian or ferruginous, facts which determine the formation of many metamorphic minerals. Magnesia, for instance, may crystallize by itself as periclase, and that species alters into brucite. Magnesia and alumina together give rise to spinel. With silica, magnesian silicates, often ferriferous, may form, such as forsterite, olivine, enstatite, and hypersthene. With lime and magnesia together, monticellite is produced, and also a wide range of pyroxenes and amphiboles. Augite, hornblende, diallage, diopside, actinolite, and tremolite are common in metamorphic limestones, and the minerals of the chondrodite-humite series are also characteristic of these rocks in many localities. The white, yellow, and brown magnesian tourmalines are other species of this class. Furthermore, the olivines, pyroxenes, amphiboles, and chondrodites alter into serpentine and talc, forming the ophicalcite marbles or verde antique.

In a Scottish dolomitic marble containing forsterite, tremolite, diopside, and brucite, J. J. H. Teall has observed a dedolomitization due to the silication of the double carbonate. That changes to diopside without change of ratios, and the partly altered rock shows the two species in juxtaposition. The metamorphosis was effected by a plutonic intrusion, and where silica was deficient, brucite appeared. Probably in the latter case magnesium carbonate was first reduced to periclase, MgO, which was later hydrated to brucite, MgO,H2. The mixture of calcite and brucite is identical with the

a Bull. U. S. Geol. Survey No. 220, p. 27, 1903. Anorthite also occurs in the marble of Monzoni, in the Tyrol. See G. vom Rath, Zeitschr. Deutsch. geol. Gesell., vol. 27, p. 379, 1875.

See, for example, G. Linck, Neues Jahrb., 1907, p. 21, on orthoclase from the dolomite of Campolongo.

e In Mon. U. S. Geol. Survey, vol 46, p. 221, 1904, W. S. Bayley described a talcose schist from the Aragon iron mine, Michigan, which was probably derived from a dolomite. An analysis of it, by G. Steiger, is given, and also its mineralogical composition. d Geol. Mag., 1903, p. 513.

predazzite of the Tyrol."

b

It may be noted here that certain of the Adirondack limestones are regarded by J. F. Kemp as having been originally siliceous dolomites, in which the silica and magnesia have segregated as pyroxene. In northern New Jersey, according to L. G. Westgate, a quartz rock and a quartz-pyroxene rock have been formed by the metamorphism of limestones.

In addition to the minerals already named, the crystalline limestones contain many other less important species. Apatite, fluorite, rutile, perofskite, titanite, dysanalyte, and zircon are among them. By the reduction of sulphates, a considerable number of sulphides may be formed. At Carrara, for instance, G. d'Achiardi found realgar, orpiment, sphalerite, pyrite, arsenopyrite, galena, chalcocite, and tetrahedrite; and also native sulphur and gypsum. Pyrrhotite and molybdenite have been identified at other localities, and in the famous Binnenthal, in Switzerland, several rare sulphosalts occur in a crystalline dolomite. In short, the list of minerals now known as existing in metamorphosed limestones must comprise at least 70 species and possibly more.

The rocks thus formed from limestones and dolomites, or from mixtures of these with siliceous material, can vary from a nearly pure, recrystallized carbonate to an indefinite aggregate of silicates alone. Even in a single bed the rocks may range from one extreme to the other. Analyses of such rocks, therefore, have little significance and are not often made. Three examples from the silicate side of the group may serve to illustrate the variety of composition:

Analyses of metamorphic silicate rocks.

A. Wollastonite gneiss, Amador County, California. Analysis by W. F. Hillebrand. Described by H. W. Turner, Seventeenth Ann. Rept. U. S. Geol. Survey, pt. 1, p. 521, 1896. Consists mainly of wollastonite, but garnet, quartz, and titanite are also present. B. Prehnite rock, Black Forest, Germany. Analysis by C. Schnarrenberger. Described by H. Rosenbusch, Mitth. Gr. badisch. geol. Landesanstalt, vol. 5, Heft 1, 1905. Estimated to contain 46.2 per cent prehnite, 37.9 albite, 13.8 actinolite, and 3.2 kaolin and nontronite. Probably formed from a marl containing 34.5 per cent of carbonates with 65.5 silicates and quartz.

C. Garnet rock, Black Forest. Analysis by Schnarrenberger. Described by Rosenbusch, loc. cit. Probably derived from an original mixture of 48 per cent carbonates and 52 of silicates, chiefly kaolin. Contains about 75 per cent garnet, 10 per cent sodapotash mica, and 15 per cent hornblende.

• See ante, p. 489.

Bull. Geol. Soc. America, vol. 6, p. 241, 1894. In the same volume, p. 263, C. H. Smyth discusses another group of Adirondack limestones which were metamorphosed along contacts with gabbro.

Am. Geologist, vol. 14, p. 308, 1894.

d Atti Soc. toscana sci. nat., Pisa, vol. 21, 1905.

See

A list is given by F. Zirkel in Lehrbuch der Petrographie, 2d ed., vol. 3, p. 448. also B. Lindemann, Neues Jahrb., Beil. Bd. 19, p. 197, 1904. For the Ceylonese localities, see A. K. Coomára-Swamy, Quart. Jour. Geol. Soc., vol 58, p. 399, 1902. J. F. Kemp and A. Hollick have described the crystalline limestones of Warwick, New York, in Ann. New York Acad. Sci., vol. 7, p. 644, 1893.

CHAPTER XV.

METALLIC ORES.

DEFINITION.

From a strictly scientific point of view, the terms metallic ore and ore deposit have no clear significance. They are purely conventional expressions, used to describe those metalliferous minerals or bodies of mineral having economic value, from which the useful metals can be advantageously extracted. In one sense, rock salt is an ore of sodium, and limestone an ore of calcium; but to term beds of these substances ore deposits would be quite outside of current usage.

In the previous chapters of this work several forms of ore deposit have been described; and therefore the present chapter is in some measure supplementary. Its purpose is to deal with the subject more fully, and especially to give details concerning certain groups of ores which have been left out of account hitherto. Little has been said so far of the sulphides, and these are among the most important of economic minerals. Their genesis, their deposition in veins or pockets, their alterations and transferences are yet to be considered.

Upon the classification of ore deposits there has been much controversy, and various systems are in vogue." To the geologist or miner this question is most important; to the chemist it is less fundamental. Regarded from the genetic side, a large part of the field has been already covered; and it is easy to see that many ore deposits, if not all, fall under the headings of earlier chapters. For example, certain metallic ores occur as volcanic sublimates; others, like the titaniferous magnetites, are magmatic segregations, or local developments of igneous rocks. The sands and gravels that yield chromite, tinstone, gold, platinum, etc., are detrital in character; many manganese and iron ores are sedimentary rocks, and from the latter metamorphic

For recent papers and works on this subject, see F. Pošepný, Trans. Am. Inst. Min. Eng., vol. 23, p. 197, 1893. J. H. L. Vogt, idem, vol. 31, p. 125, 1901. L. De Launay, Contribution à l'étude des gites métallifères, Paris, 1897. J. F. Kemp, Ore deposits of the United States and Canada, New York, 1900. W. H. Weed and J. E. Spurr, Eng. and Min. Jour., vol. 75, p. 256, 1903. R. Beck, Lehre von den Erzlagerstätten, Berlin, 1903; and its English translation by Weed, New York 1905. A. W. Stelzner and A. Bergeat, Die Erzlagerstätten, Leipzig, 1904. C. R. Van Hise, Treatise on metamorphism, Mon. U. S. Geol. Survey, vol. 47, chapter 12, 1904., W. H. Weed, Trans. Am. Inst. Min. Eng., vol. 33, p. 717, 1903. C. R. Keyes, idem, vol. 30, p. 323, 1900. G. Gürich, Zeitschr. prakt. Geol., 1899, p. 173.

beds of magnetite or hematite are derived. Some ore bodies are residues from the concentration of limestones; others represent metasomatic replacements; others again are deposited or precipitated from solutions. In short, an ore body is simply a concentration of certain compounds of certain metals effected by processes with which we are already familiar. Since, however, each metal forms its own special compounds, and exhibits reactions peculiar to itself, it is best for chemical purposes to adopt a chemical classification, with which the broad, general principles can be correlated. Each metal, therefore, will be treated by itself as a chemical individual and from a chemical point of view. Geologically it is important to know whether an ore deposit, laid down from solution, occupies the pores of a sandstone, a limestone cavern, or a fissure in the rocks; and it is also desirable to ascertain how these cavities or crevices were formed. To the chemist these considerations are for the most part irrelevant; but the conditions under which given compounds can be dissolved or precipitated are fundamental. What are the components of ore bodies? How were they produced? In what way are they redistributed? These are some of the questions which the chemist is expected to answer. The details must be studied with reference to the individual metals; but some general considerations require attention first.

SOURCE OF METALS.

a

Although the immediate derivation of metallic ores is often from sedimentary rocks, the original source of the metals is to be sought in the igneous magmas. In igneous rocks of some sort the metals were once diffused, and their presence in eruptive material is easily detected. G. Forchhammer in a series of rock samples found traces of silver, copper, lead, bismuth, cobalt, nickel, zinc, arsenic, antimony, and tin, to say nothing of the commoner metals, iron and manganese. Some of the same elements were found in the ashes of plants, which had extracted them from the soil. From these experiments Forchhammer concluded that ore bodies derived their contents from the neighboring rocks, a conclusion at which other investigators have also arrived. In an elaborate series of researches F. Sandberger' found that the dark silicates of many rocks contained lead, copper, tin, antimony, arsenic, nickel, cobalt, bismuth, and silver, and upon these facts he based his famous theory of "lateral secretion." That is, Sandberger concluded that metalliferous veins derived their metallic contents by lateral leaching from adjacent rocks. This theory, however, was subjected to much criticism by A. Stelzner, F. Pošepný,

Pogg. Annalen, vol. 95, p. 60.

Untersuchungen über Erzgänge, Wiesbaden, 1882 and 1885. See also Neues Jahrb., 1878, p. 291, on copper, lead, cobalt, and antimony in basalt.