In the Plum Creek quarries, near Monroe, celestite may be observed undergoing alteration into strontianite, a rather unexpected change, the sulphate being regarded usually as the more stable form. However, a similar change may be produced in the laboratory by boiling the powdered sulphate with solutions of the alkaline carbonates; sodium, potassium or ammonium. It seems very probable that cold water, charged with one or more of the alkaline carbonates might, in the course of time, convert the outer portions of the crystallized sulphate into the carbonate.

§ 8. Gypsum.

Just referred to as probably having been active in the production of celestite is this calcium sulphate, which, although related to it in composition, is still readily distinguished from it by simple physical properties. Either in the form of gypsum (CaSO+2H2O), or as the anhydrous variety (anhydrite, CaSO4) this calcium sulphate is widely distributed through the Monroe beds of the county. As given in Chapter III, § 5, this sulphate was deposited directly from the waters of an inland sea, along with the calcium and magnesium carbonates which constitute the beds of dolomite. It would be formed anew wherever free sulphuric acid, derived from the decomposition of pyrite or marcasite, came into contact with calcium carbonate in solution. Calcium sulphate dissolves in 400 parts of water so that surface water, percolating through the dolomites containing gypsum or anhydrite, would take more or less of it into solution. Upon standing quietly in rock cavities and fissures, crystals of gypsum known as selenite, might form. Such crystals occur sparingly at the Woolmith quarry, but were not observed elsewhere. These are perfectly transparent flat crystals, without color and having a glassy to pearly luster. They split readily into thin plates, which may be easily bent without completely separating, but the thin plates are not elastic, as in the case of mica.

The cleavage in this one direction is very perfect. The streak is white and the specific gravity below the average, being 2.3. It is distinguished from all the minerals thus far described by its inferior hardness, since it is readily scratched with the finger-nail. No effervescence can be secured from it by the use of acid. Fine granular to compact masses of gypsum, variously colored from impurities, are known as "plaster" and are valuable for fertilizing purposes. When heated to a temperature of about 392°F. the water

in chemical combination is driven off, the mineral crumbles to a white powder and is known as "plaster of Paris." By weight this water (H2O) comprises 20.9% of the mineral, combined with 32.5% of lime (CaO) and 46.6% of sulphur trioxide (SO).

§ 9. Anhydrite.

In the preparation of the plaster of Paris if the gypsum is brought to a temperature of 400° to 650°F. not only is the water of crystallization driven off, but the calcium sulphate practically loses its power to again unite with it and "set." It has closely approached the condition of the anhydrous variety (CaSO4), which is of common occurrence in nature and is known as anhydrite. As generally seen this is a compact or very fine granular translucent mass, of a pure white color and breaking with an uneven fracture. It is brittle and heavier and harder than gypsum. It cannot be scratched with the finger-nail, but is readily scratched with a knife point, giving a white powder. In well drillings it is frequently called "marble," but is easily distinguished from it by the action of the dilute acid. Anhydrite will slowly dissolve in hydrochloric acid, but gives no effervescence, while a small fragment of marble effervesces vigorously as it dissolves. This form of calcium sulphate contains 41.2% of lime and 58.8% of sulphur trioxide. It is less soluble in water than is the gypsum. The presence of anhydrite, or gypsum, in drillings, or dolomites, may be detected by boiling the powder for a short time in hydrochloric acid contained in a testtube and then allowing the acid to cool. If calcium sulphate was present a mass of very slender needle like crystals will make their appearance. It is not definitely known why the sulphate should have been deposited from the evaporating inland sea in the anhyrous form. Ochsenius has explained it as due to the fact that when the hydrous sulphate (gypsum) is in solution in concentrated sea water with certain salts of potassium it exchanges part of its water for sulphate of potassium, forming polyhalite (KCl) and other salts which remain in solution in the bittern while anhydrite is deposited.*

§ 10. Epsomite.

When the free sulphuric acid of waters percolating through a bed of dolomite acts upon the dissolved calcium carbonate there is formed calcium sulphate as described in § 8 of this chapter. As might be expected this same acid would react similarly upon the magnesium carbonate of the dolomite and give rise to mag

*Quoted from Mich. Geol. Sur., Vol. V, Pt. II, p. xvii.

nesium sulphate (MgSo, +7 H2O), or epsomite. This is so very soluble that ordinarily we should not expect it to be deposited, but it is a common ingredient of mineral waters. Sometimes it occurs as a delicate efflorescence over the surface of rocks in dry fissures, the galleries of mines and caves. In the Ida well (see Chapter III, § 8) a 100 foot layer of this substance was reported and a sample saved. The sample proved to be epsomite, but it is believed to have come from a cavity or fissure and not to represent the entire bed which was probably a dolomite rich in anhydrite. Epsomite is now on the market at 23 cts. per pound by the barrel. It contains by weight 16.3% of magnesia (MgO), 32.5% of sulphur trioxide (SO) and 51.2% of water (H2O). It occurs usually in elongated slender orthorhombic crystals, white in color, with a glassy to earthy luster, breaking with perfect to imperfect cleavage. can be scratched with the finger-nail, (Hardness 21), giving a white powder. Its specific gravity is but 1.75 so that it appears very light. On account of its easy solubility it has a pronounced taste, which is salty to bitter. Upon being heated it gives off much. acid water in which it liquefies.

C. Chlorides.

It

§ 11. Halite.

Owing to the possibility of the occurrence of rock salt (NaCl) in the northeastern part of the county (see Ch. VIII, § 12), a description of this mineral is here inserted. The common form of crystal is the cube, parallel to the faces of which it breaks readily with very perfect cleavage. When pure it is colorless and transparent but it is frequently rendered translucent and of a variety of colors from impurities. It can be scratched with the finger-nail, with some difficulty, giving a white powder. When pure its specific gravity is 2.1, so that it appears light. The most characteristic property is its familiar saline taste, being readily dissolved in water. It gives no effervescence with the acid. In the blow-pipe flame it fuses and imparts an intense yellow color, owing to its sodium, which makes up 60.6% of the mineral, while the remaining 39.4% is chlorine. It occurs in solid beds in the earth only where the subterranean waters have not had free access to it. It is believed to have been formed through the concentration of the waters of an inland sea.*

*See Chapter III, 5; also Geol. Sur. of Mich., Vol. V, Part II, pp. ix to xix.

§ 12. Sulphur.

D. Sulphur and Sulphides.

Closely associated with the celestite and calcite crystals at the Woolmith quarry are beautiful crystalline masses of brilliant yellow sulphur, contrasting gorgeously with the blue of the celestite and the creamy white calcite. Attention was first called to this occurence of native sulphur by the writer in 1895.*

The sulphur is confined mainly to the ellipsoidal cavities in a soft, open bed of dark brown dolomite from one to three feet thick. These cavities vary in size from a fraction of an inch to three feet in their greatest horizontal diameter. Some of these cavities are completely filled with the sulphur, which at times shows crystal faces, but generally the sulphur shows only the rough conchoidal fracture characteristic of such crystalline masses. More frequently the cavity has a lining of celestite and calcite and only the central portion contains the sulphur, showing that its deposition began later than that of these two minerals. It was estimated by the foreman at the time of the survey, four years ago, that between 400 and 500 barrels had been removed, most of which was carried away by visitors. The present market value of such sulphur is 23 cents per pound. This mineral is very easily distinguished from all others by its brilliant yellow, its resinous luster and conchoidal fracture. It is quite brittle, can be scratched with the finger-nail, has a specific gravity of but 2 and is translucent. It melts readily and burns with a blue flame emitting irritating odors of sulphuric dioxide (SO). It is insoluble in water and is not affected by acids. When the deposits were first examined by the writer in 1895 there entered the quarry from the "sulphur bed" a stream of water charged with hydrogen sulphide gas (HS). As the water trickled down the face of the quarry wall and flowed over the bottom there was deposited a mealy white precipitate of sulphur, resulting from the decomposition of this gas. Where this deposit was longest exposed it showed a slight yellow color. From the cavities of the dolomite specimens of whitish yellow sulphur were collected, having the same mealy appearance, but containing throughout small irregular masses in the crystalline condition. There is no room for doubt then that the sulphur has resulted from the decomposition

*American Journal of Science, Vol. L, Third Series, pp. 246-248; also in a paper presented to the Michigan Academy of Science, Lansing, December, 1895.

of hydrogen sulphide gas, introduced through the agency of percolating water, and that it has gradually passed from the mealy to the crystalline condition. The gas has been formed in the rocks through the decomposition of the next two minerals to be described.

§ 13. Pyrite and marcasite.

These are two minerals which have the same chemical composition (FeS.), being compounds of iron and sulphur. They contain by weight 53.4% of sulphur and 46.6% of iron. They are not often. seen at the surface, but are abundant enough in the rock strata to give character to the spring and well water and to be indirectly responsible for the formation of a number of the minerals described in this chapter. They each have a brassy luster and color, a specific gravity approaching 5 and are hard enough to make a scratch upon glass. They are brittle, break with an uneven fracture, are opaque and have a black streak. When heated sufficiently they are decomposed, the sulphur burns with its characteristic blue flame and leaves behind a dark mass which is now magnetic, owing to the free iron. The pyrite is generally found in beautifully perfect cubes of various sizes, having a golden color and luster, which causes it frequently to be mistaken for gold. In the dolomite of the Plum Creek quarries seams and incrustations of crystallized pyrite occur, in tarnished condition. The crystals are very small, giving the mineral a fine granular appearance, and are mainly octahedrons. This is the only locality at which pyrite was found. The marcasite is of a whiter yellow, does not occur in cubes, but usually in flattened concretions showing radially fibrous structure, and is more subject to decomposition than the pyrite. The formation of these two minerals through the agency of organic matter and oxide of iron was explained in Chapter II, § 11. When either of these minerals is in contact with water for a sufficient length of time, double decomposition takes place. The sulphur unites with hydrogen of the water to form hydrogen sulphide gas (HS), which is readily held in solution by the underground water and gives rise to "sulphur" springs and wells. The iron of the pyrite, or marcasite, is readily oxidized and if carbon dioxide be present will be converted into iron carbonate (FeCO), taken into solution and will impregnate the water with iron, forming "chalybeate springs or wells" (chapter VIII, § 10). The hydrogen sulphide itself may unite with oxygen to form sulphuric acid (H,SO), which may unite with