Gypsum , a common mineral, frequently crystallized, oftener amorphous, and sometimes forming rock masses. Its crystallization is monoclinic; hardness, 1.5 to 2; specific gravity, 2.2 to 2.4; transparent or translucent, vitreous; on cleavage, pearly or silky; colorless and snow-white, but often red, yellow, or brown from enclosed coloring matters. Its transparent variety, called selenite, sometimes occurs in large plates, which have been used for windows. It also frequently occurs in aggregated needle-like crystals, and is then called fibrous gypsum. In its amorphous condition, when compact and translucent, it is named alabaster. More commonly it is white, opaque, and soft, and is then called snowy gypsum. Its chemical composition is expressed by the formula CaS04, 2H20; i. e, it is a hydrated sulphate of lime. Gypsum occurs in nearly All geological formations and countries. In clay and shale it is frequently found in beautifully defined detached crystals, apparently derived from the action of sulphuric acid, liberated by the decomposition of iron pyrites on carbonate of lime. It is also formed where sulphuric acid is generated or discharged from volcanic or other subterranean sources and comes in contact with calcareous matter, as about sulphur springs and craters of volcanoes.

The great repository of gypsum, however, is the water of the ocean, which always holds it in solution, and from which it has been precipitated by evaporation to form all the great masses of this substance. It is also soluble in fresh water in the proportion of 1 part to 400 or 500 of water. The most important deposits known are those of the Paris basin at Montmartre, which are of eocene age, and from which it has taken the common name of plaster of Paris; those of Nova Scotia, Virginia, and Michigan, of carboniferous age; of central New York, Ohio, and Canada West, in the upper Silurian; and in the triassic strata of the far west. It also occurs in the trias at Bex in Switzerland, Vic and Dieuze in France, Cheshire in England, and Stasfurt in Germany. In all these, as in the most important American localities, it is associated with rock salt. Gypsum is known to exist in large quantities in Mexico, South America, Africa, India, Australia, and China. - The origin of the great masses and strata of gypsum found in many countries has been a subject of much discussion. By most writers they are represented to have been produced by the action of sulphuric acid contained in the water of acid springs acting upon strata of limestone.

This theory is, however, inapplicable to all the most important deposits, which are undoubtedly derived from the precipitation of gypsum by evaporation from its solution in circumscribed basins of salt water, like the Dead sea and Great Salt lake. This is apparent in the structure of the gypsum beds, which are generally accurately stratified, and not unfrequently alternate with sheets of limestone. Gypsum is also usually associated with greater or less quantities of the salts which are found in sea water, viz., the chlorides of sodium, calcium, magnesium, the sulphate of soda, etc. Of all the solid matter contained in sea water, gypsum is the least soluble, and therefore is the first precipitated. It is thus deposited by itself, and forms continuous and regular strata many miles in extent and of great thickness. The next ingredient which would be thrown down in the evaporation of sea water is the chloride of sodium; and this we find in the strata of rock salt winch accompany beds of gypsum. The other salts mentioned have such an affinity for water that they are not found solid, but compose the bitterns of the brines of wells and springs. In New York, Canada, and Ohio, gypsum occurs chiefly in the Salina or Onondaga salt group.

This formation is made up of a series of earthy sediments interstratified with salt and gypsum, and is plainly the deposit which accumulated at the bottom of a great salt lake, which in the Silurian age reached from eastern New York to the Cincinnati axis. In the western part of this basin, at Sandusky, Ohio, the Salina group contains sheets of regularly bedded strata of gypsum, divided horizontally by thin sheets of carbonate of lime. In the carboniferous age evaporating pans where salt water precipitated its solid contents existed in Nova Scotia, Michigan, Virginia, and Arizona, and later in the triassic age, in the region now occupied by the Llano Estacado and in the Indian territory. In all those localities proof is abundant that the strata of gypsum are precipitated sediments, and that the theory which attributes the formation to the action of acid springs is a fallacy. - The uses of gypsum in the arts are varied and important. "When calcined, its combined water is driven off. If now ground to powder and again mixed with water, this water recombines with it, and the mass becomes first plastic, then solid, and takes the form of any model into which it may have been poured. This property of gypsum has many applications in the arts.

It makes the most convenient of mortars, and was extensively used by the ancient inhabitants of Mexico as well as those of Egypt in their masonry. The use of gypsum in the formation of plaster casts is too common and well understood to require special notice. When mixed with glue water, plaster of Paris is converted into stucco. If mixed with a solution of borax, alum, or sulphate of potash, and subsequently rebaked and powdered, and again mixed with a solution of alum, it forms a hard cast which takes a high polish. This composition is called Keene's cement if made with alum, Parian if with borax, and Martin's if with potash. In the preparation of plaster of Paris, the gypsum rock is ground between buhr stones until it is reduced to a fine powder. This is calcined by being heated in kettles or stills, the escaping water producing a movement like ebullition. As calcined plaster absorbs moisture from the atmosphere, it should be prepared as wanted, or carefully protected from dampness. Gypsum is sometimes used for the glazing of porcelain. But the principal consumption of it is as a fertilizer for soils. Sulphate of lime enters into the composition of grasses, potatoes, turnips, etc, and these cannot flourish in soils entirely free from it.

Its potency, however, is probably due in a far greater degree to its action in fixing volatile and escaping carbonate of ammonia. When this comes in contact with sulphate of lime, double decomposition takes place, carbonate of lime and sulphate of ammonia being formed. Its value as a fertilizer may be readily tested by distributing a quantity of it in a narrow line across a meadow. Where the plaster has fallen, the grass will frequently be so much stronger and greener, that the difference may be seen even at a considerable distance. Gypsum is not unfrequently mingled with, and sometimes shades into, anhydrite, the anhydrous sulphate of lime. Both pass under the name of plaster, the anhydrite being called hard and gypsum soft plaster. The uses to which they are applied are the same. - The commerce in gypsum in the United States amounts to about $1,000,000 per annum, almost equally divided between the miner and manufacturer, and between three districts : the Atlantic coast, where the material is derived from Nova Scotia, and the states of New York and Michigan, where it is indigenous and abundant.

The importation of gypsum into the United States from Nova Scotia and New Brunswick in 1873 was 118,280 tons, valued at $117,828. The annual production of gypsum in New York and Michigan may bo estimated at about 100,000 tons each.