498 WATER of gypsum. The carbonate of lime in the in- flowing waters, which hold it as bicarbonate, yields by double decomposition with the sul- phate of magnesia of the sea water sulphate of lime and bicarbonate of magnesia, the former of which salts, being very sparingly soluble, readily separates as gypsum, leaving the more soluble magnesian bicarbonate to be thrown down at a later period in the process of evapo- ration, in the condition of simple carbonate. From this, with the addition of carbonates of lime and magnesia brought by the inflowing waters, are formed the dolomites or magnesian limestones which abound in the rock forma- tions of various geological periods, and appear to have been in all cases deposited in evapora- ting basins. By virtue of the last described reaction, as will be seen, both of the constit- uents of the sulphate of magnesia are com- pletely removed from the evaporating waters and fixed in insoluble forms, thus permanently modifying the composition of the water of enclosed basins, which by subsequent changes of sea and land become once more a part of the ocean, affecting the composition of its waters. In many cases the river or spring water flowing into closed basins contains neu- tral salts of lime and magnesia, so that we find in such lakes great variations in composition, from the bitter salines of the Dead sea, charged with chlorides of magnesium and calcium, to the alkaline waters containing carbonate and borate of soda which abound in central Asia, Egypt, and California. The various stratified rocks, whether of mechanical or of chemical origin, have for the most part been deposited in the waters of the open sea or of enclosed baains. All of these rocks are more or less porous, and over great areas which have never been subjected to any considerable disturbance they are still impregnated with the saline wa- ters in the midst of which they were deposited. Leaving out of question the solid soluble salts enclosed in these strata, some notion of the amount of saline water which they contain may bo had from a consideration of the degree of porosity of various rocks. Careful experiments upon the different rocks of the great American palaeozoic basin show that various sandstones are capable of holding in their pores from 2 or 8 up to 10 and 20 volumes of water for 100 of rock, while the pure limestones generally hold not more than 1 or 2, and the dolomites from 5 to 10. The porosity of many rocks is even greater than any of these, and from a compari- son of the above observations with others made in Europe, it is probably not an exaggeration to say that the stratified sedimentary rocks as a whole contain in their pores one tenth of their volume of water; so that the 40,000 or 50,000 ft. of palroozoic strata in parts of North America and Great Britain would hold enclosed the equivalent of seas of nearly a mile in depth, and the volume of water enclosed in the rocky strata of the earth's crust bears a very consider- able proportion to that of the ocean. Many of the deposits of later times, it is true, are of fresh-water origin, and the older strata in re- gions where they have been much broken and disturbed have had their saline waters replaced by fresh waters from the atmosphere. Over very large areas, however, such strata are found to contain saline waters differing from those of the present ocean, and representing the ancient sea waters. Such an area is that of the great palaeozoic basin of the United States, including the valleys' of the Mississippi, the Ohio, and the St. Lawrence, which by the evi- dence of numerous artesian wells and springs are shown to include saline waters with a pre- dominance of salts of lime and magnesia such as should belong to the earlier sea, and by their great density, in many cases much exceeding that of the present ocean, indicate the former presence of partially dried-up seas, which is further shown by the interstratified deposits of rock salt at more than one geological hori- zon. These subterranean oceans are the source of the various saline mineral waters, in which however the ancient sea waters are found very much diluted or modified by the admixture of waters from superficial sources, or of the alka- line waters already mentioned. By common usage the name of mineral waters is given to such as from the proportion or the nature of their mineral ingredients are unfitted for the ordinary uses of life. They may be divided into several classes, of which the following are the most important: 1, those approaching sea water in composition, though more or less di- luted, and holding variable proportions of salts of lime and magnesia; 2, waters holding chief- ly carbonate of soda with variable proportions of carbonates of lime and magnesia. Between these two types are a large number of inter- mediate waters, such as would result from their intermingling, and presenting various grada- tions ; the most marked being those which are at once alkaline and saline, like the springs of Saratoga, while the first type is represented by the bitter saline of St. Catharines, Ontario, and the second type by waters like Vichy and Carlsbad. In very many of these saline and alkaline waters are found small portions of the rarer elements, such as lithium, crosium, ru- bidium, strontium, and barium, with salts of iron, and occasionally arsenic, antimony, cop- per, lead, and many other metals in traces. The presence of phosphates, borates, and fluorides is also very frequent, and compounds of bromine and iodine are supposed to contribute very much to the value of certain mineral waters. Besides these principal groups of saline and alkaline waters should be mentioned those which contain sulphates of aluminum and of iron, as the so-called alum springs, and others in which free sulphuric acid is the chief ingre- dient, as the acid springs of New York and Ontario. A sulphurous impregnation may be- long to any of the classes noticed, and may be due either to the presence of free sulphide of hydrogen or to a soluble metallic sulphide.
