cavities. The calcite may occur as a compact granular mass or in separate, well formed crystals. The crystals are commonly sharply pointed, each point being formed by three pairs of triangular faces, making what is known technically as a scalenohedron.* Such crystallization is known popularly as "dog-tooth spar," because of the resemblance of the crystals to canine teeth. Hubbard, in his early report, refers twice to the "hog-tooth spar" and it is not now certain whether this is simply a misprint or whether he intended to characterize a coarser variety of crystal. Another common form of crystal is the rhombohedron, looking like an oblique prism. Whatever the original crystal, upon being broken the calcite breaks with remarkably perfect cleavage planes parallel to the faces of the rhombohedron. The luster is more or less glassy and the color is various from impurities. The usual color is snow-white to a creamyellow, but it is sometimes tinged with a variety of color. Clear transparent varieties are known as "Iceland spar," but in general, the mineral is simply translucent. It cannot be scratched with the finger-nail, but is readily scratched with a pin or knife, giving a white powder, the "streak." Its specific gravity is the average for rocks, 2.7. Its composition is calcium carbonate (Ca CO), and it contains by weight, 44% of carbon dioxide (CO), and 56% of lime (CaO). This gas may be readily driven off by the ap plication of cold dilute hydrochloric acid to the solid crystal, causing brisk effervescence, so called. The acid of standard strength is prepared by taking four parts of pure water to one part of the acid, giving a 20% mixture. The Monroe calcite crystals have been deposited from percolating water holding calcium bicarbonate CaH2 (CO), in solution. This form of the carbonate is easily converted into the common variety (CaCO3) which crystallizes where conditions are favorable. Such conditions are commonly found in rock fissures and cavities where there is little agitation of the water which is charged with the carbonate. The finest crystals of calcite in the county are found at the Woolmith quarry, although some good specimens can be procured in the dolomites about Monroe. § 3. Aragonite.

This mineral was not identified by the writer amongst his collections but is said to have been found about Monroe. It has the same composition as calcite, but has less perfect cleavage and is per

*See Vol. VI, Part I, appendix, for report on crystallization of calcite. Dana's system of Mineralogy, 1892, p. 1086.

ceptibly heavier and harder. It does not show the familiar forms of the calcite crystals, the scalenohedron and rhombohedron. Its crystals may be either simple or compound, or may occur in radiating groups of acicular crystals. The latter is the common crystallization of strontianite (§ 6 of this chapter), and it is quite possible that this mineral may have been taken for aragonite. With the dilute acid calcite and aragonite behave similarly, so that they are liable to be confused with one another, unless crystallization and cleavage can be well made out. Upon being heated as with the blowpipe, they each whiten and glow, but the aragonite crumbles to powder while the calcite does not. The aragonite is liable to contain some strontia, which imparts an intense red color to the flame, making its separation from strontianite more difficult. The lime carbonate of many shells is in the form of aragonite as those of mussels and snails. Some corals and polyzoa form this same substance for their hard structures.

§ 4. Tufa.

This is a cryptocrystalline form of lime carbonate deposited over moss, twigs, leaves, etc., by spring water. The underground water carries the calcium bicarbonate in solution as explained in § 2. Upon having its pressure relieved and being exposed to the air some of the carbon dioxide escapes from the unstable bicarbonate, the monocarbonate results and this is so slightly soluble that it must be precipitated. Deposited thus quite rapidly over the surface of meadows there results an imperfectly crystalline, lumpy mass, more or less porous and open, showing the imprints and moulds of the organisms covered by the water. This is known as calcareous tufa, but where the structure is compact, one layer superimposed directly upon another, it is termed travertine. Iron is generally present in the water, the oxidation of which gives the tufa a yellow to brown stain. The structure of this material, its lightness and occurrence about existing or former springs are sufficient for its identification. A drop of cold dilute acid causes at once a vigorous effervescence, as in the case of calcite. The most extensive deposits of this substance are found to the south of Monroe, near the lake shore (Chapter VIII, §10). Where abundant it is sometimes burned into lime and occasionally used for building purposes. The writer has seen a very pretty country church near Rochester, N. Y., constructed of this tufa.

Just as calcite results from the solution and crystallization of pure limestone, so the mineral dolomite represents the re-crystallized magnesian limestone. Through the agency of percolating water, charged with carbon dioxide gas, this limestone is taken into solution. Little

2

is known definitely in regard to just how it is held, but is probable that the calcium and magnesium carbonates, of which the dolomite is composed, are separate and each in the form of the bicarbonate, CаH2 (CO) and MgH, (CO3) 2. Under certain conditions these bicarbonates are converted into the monocarbonates, the normal forms, and crystallize together so as to form crystalline dolomite in seams, fissures and cavities. Such occurrences of this mineral, however, are rare although microscopic rhombohedrons make up the granules of oölite and cover the grains of the Sylvania sandstone. The spring waters from beds of dolomite deposit calcium carbonate. in the form of tufa but no corresponding deposits of magnesium carbonate (magnesite) were observed. The crystals of dolomite very commonly show curved faces, instead of planes, have a pearly luster and give perfect cleavage. The color is white or pink generally, but may vary from the presence of impurities. The mineral is brittle. and has a white streak. It is slightly harder and slightly heavier than calcite, but is distinguished from it most certainly by use of the dilute acid. When a drop of such cold acid is applied to the solid crystal there is almost no action, but if the acid is heated or the dôlomite is powdered the effervescence is brisk. This mineral is a double carbonate of calcium and magnesium, having the chemical formula (Ca Mg) CO. It contains carbon dioxide (CO2) 47.8%, of lime (CaO) 30.4% and of magnesia (MgO) 21.7%. Expressed differently it has of calcium carbonate 54.35% and of magnesium carbonate 45.65%. § 6.

Strontianite.

This is the last of the four minerals from which effervescence may be obtained by the use of dilute acid. A drop of cold acid upon the solid substance causes vigorous effervescence as in the case of calcite and tufa. As found in this county it is of a snow white color forming irregular masses in the cavities and fissures of the dolomitic limestone. Usually it has a loose open structure, composed of globular masses, which show a radially fibrous structure upon being broken. Over their surface and dipping down into the cavities is a layer of fine pyramidal points, producing a fuzzy, frosted appearance.

When the crystals are broken they show both perfect cleavage and uneven fracture faces. The mineral is brittle, gives a white streak, has a glassy to resinous luster and is sub-translucent. The color although usually a pure white may be altered by impurities. The hardness is about that of dolomite (3.5 to 4), so that it may be easily scratched with a knife. Aside from the structure above described, which is quite characteristic, strontianite is easily distinguished from the preceding carbonates by its relatively high specific gravity (3.7), surprising one at once with its weight. If a fragment is moistened with hydrochloric acid and held in the flame of a lamp, candle or gas, or even a burning match, an intense red color is imparted to the flame. This is the familiar "red fire" of the Fourth of July, or campaign celebration and is due to the element strontium, from which the mineral derives its name. The mineral occurs at Stony Point, Point Aux Peaux and Brest, and in the Plum Creek quarries near Monroe, also in the Ida and Little Sink quarries. At the last locality the finest specimens may be obtained in lumps of snowy whiteness the size of the fist and upwards, having been formed in the cavities of the dolomite. At the Ida quarry small masses occur in cavities but some of the upper dolomitic layers are partially converted into strontianite, over the surface of which is spread a layer of slender orthorhombic crystals. This occurrence gives a clue to the origin of the mineral, suggesting that it has resulted from the action of strontium salts in solution acting upon the carbonates of calcium and magnesium. The composition is represented by the formula Sr CO2, of which 29.9% by weight is carbon dioxide (CO2) and 70.1% strontia (SrO). The "tremolite" reported by Houghton in his first report as occuring at Brest was probably this strontianite. Larger quantities than are now in sight would give this mineral a high commercial value since it may be used in recovering beet-sugar from the waste "molasses" (Chapter VIII, § 4). After the process is once started the strontium is recovered and the only loss is that occasioned by the actual sugar loss. According to Prof. Wiley this loss of the mineral strontianite would amount to about 61% of the weight of the molasses treated. In the market this strontian carbonate is worth five to six cents a pound.

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This is another compound of strontium, of much interest and beauty which is more abundant than the above. It is found in the fissures and cavities of the dolomites at Point aux Peaux and Stony Point, about Monroe and in the Raisinville and Woolmith quarries. The finest crystallizations are found at the last mentioned locality, where the mineral is intimately associated with calcite and native sulphur. These crystals are of the flat, tabular variety, with beveled ends and edges and attain a size of 5 to 6 inches in length, by 3 to 4 in breadth and have a thickness of one inch. Typically the color is a beautiful celestial blue from which the mineral derives its name. Some of the masses, however, are perfectly transparent and free from color while others are a pure white, or a deep chocolate brown. The crystals are brittle and break with distinct to perfect cleavage faces, the luster is glassy and the streak is white. The mineral is easily scratched with a knife-point (Hardness = 3 to 3.5), being, generally, a little harder than calcite. A very characteristic property is the high specific gravity (4), it being even heavier than strontianite. From the preceding five carbonates described celestite may be readily distinguished by the action of hydrochloric acid, with which it cannot be made to give an effervescence. Chemically the mineral is strontium sulphate with the formula SrSO,, containing of strontia 56.4% (SrO) and of sulphur trioxide (SO) 43.6%. Under the blow-pipe is readily fuses giving the intense red color to the flame. The occurrence of celestite in closed cavities in the dolomite, and its practical insolubility in water and acids, shows that its ingredients must have been introduced through the agency of percolating water and the mineral formed in place. A saturated solution of calcium sulphate (gypsum) would react upon the soluble salts of strontium so as to produce the celestite. The carbonate of strontium was not observed at the Woolmith quarry in association with the sulphate, there appearing to have been sufficient gypsum present to convert all of the original strontium salt into the more stable sulphate. At the Ida and Little Sink quarries only the carbonate was formed, while on the Lake Erie shore both the carbonate and sulphate are associated, indicating that the supply of dissolved gypsum was insufficient.