Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/284

Rh 270 not merely between the pores of the rocks but in crevices and tu_iinels which it has no doubt to a large extent opened for itself along numerous natural joints and ﬁssures, is proved by the occasional rise of leaves, twigs, and even live lish, in the shaft of an Artesian well. Such evidences are particularly striking when found in districts without surface waters, and even perhaps with little or no rain. They have been met with, for instance, in sinking wells in some of the sandy deserts on the southern borders of Algeria. In these and similar cases it is clear that the water may, and some- times does, travel for many leagues underground away from the district where it fell as rain or siiov, or where it leaked from the bed of a river or lake. The temperature of springs affords a. convenient but not always quite reliable indication of the relative depth from which they have risen. Some springs are just one degree or less above the temperature of ice. Others in volcanic districts issue with the temperature of boiling water. Between these two extremes every degree may be registered. Very cold springs may be regarded as probably deriving their supply from cold or even siiow-covered mountains. Certain exceptional cases, however, occur where ice forms in caverns (_r/ladéres) even in warm and comparatively low districts. Water issuing from these ice-caves is of course cold.1 On the other hand, springs whose temperature is much higher than the mean temperature of the places at which they emerge must have descended far enough to be warmed by the internal heat of the earth. The hottest springs are found in volcanic districts. But even at a great distance from any active volcano, thermal springs, as they are called, appear with a temperature of 120° F ahr. (which is that of the Bath springs) or even ‘more. These have pro- bably risen froin a great depth. If we could assume a pro- gressive increase of 1° Falir. of subterranean heat for every 60 feet of descent, the water at 120° issuing at a locality whose ordinary temperature is 50°, should have been down at least 4200 feet below the surface. But from what has been stated in a previous section (p. 224) regarding the irregular stratiﬁcation of temperature within the earth’s crust, such estimates of the probable depth of the sources of springs are liable to various errors. Apart from its vast importance in a social point of view, the underground circulation of water has a profound in- terest for the geologist, from the light which it affords as to the changes that rocks undergo, and the manner in which these changes are effected. For, like all the other geologi- cal agents, it does not move on its course without doing work. We have now to inquire what is the nature of that work. A convenient arrangement will be to group its study under two lieads—(l) chemical action, and mechanical action. (1.) Every spring, even the most clear and sparkling, contains mineral matter in chemical solution, obtained from the rocks through which the water travels in its journey from the surface into the interior, and back to the surface again. The nature of the mineral ingredi- ents depends partly upon the composition of the rocks traversed, partly upon the gases, acids, or other reagents which may have been present in the rain, or may have been obtained by the water in its subterranean journey, and partly upon the depth to which the water may have reached and the temperature to which it may have been raised. Ve have already (ante, p. 267) considered the substances extracted by rain from the air and used by it in the disin- tegration of rocks. The same reagents are of course carried 1 The most remarkable example of a glaciere yet observed is that of Dobschau, in Hungary, of which an account, with a series of interest- ing drawings, was published in 1874 by Dr J. A. Krenner, keeper of the national museum in Buda-Pestli. GEOLOGY [1iI. DY1'A1IICAL. underneath the ground, when the raiii-water sinks out of sight, and continue there the processes of decomposition and alteration which they are seen to effect at the surface. But other sources are open to the subterranean water for the augmentation of its chemical reagents. (1.) In de- scending through the soil the meteoric water encounters abundant organic matter, which abstrac.s its oxygen and replaces it by carbonic acid. This interchange probably in many cases far more than compensates for the expendi- ture of these gases employed in subacrial disintegration. In so far as the water carries down from the soil any oxidizable organic substance, its action iiiust be to reduce the oxides it encounters among rocks. It is remarkable that ordinary vegetable soil possesses the povcr of remov- ing from the water which permeates it potash, silica, phosphoric acid, ammonia, and organic matter, elements which had been already in great measure abstracted from it by living vegetation, and which are again taken up by the same organic agents. (2.) Carbonic acid gas is some- times largely evolved within the earth’s crust, especially in regions of extinct or dormant volcanoes. Subterranean water coming in the way of this gas greedily dissolves it, and thereby obtains an enormously increased power of attacking even the iuost obduiate rocks. (3.) Whenever the water has its temperature considerably raised, its solvent capacity, especially for silica, is largely augmented. Hot springs often contain a large proportion of that sub- stance iii solution. (4.) The production of some of the compounds which are due to decompositions effected by the water, and are carried along with it in solution, increases its ability to accomplish further decompositions. Thus the alkaline carbonates, which are among the earliest products of the action of the water, enable it to dissolve silica and decompose silicates. The study of these alterations belongs to the subject of Mctamorphisui, of which some account has already been given (ante, p. 262). Let us look at the results achieved by them, as shown in the composition of the water which issues from different springs. Considered from this point of view, springs may be treated as (1) common or ordinary springs, that is, those which contain only such average proportions of mineral matter as occur in ordinary potable water, and (2) mineral springs, or those where the proportion of foreign ingredients is large enough to give a marked character to the water. These two groups, however, merge insensibly into each other. _ Common Sp7'i7zgs.—The materials ordinarily present in common spring water are, besides atmospheric air -and its gases, carbonate and sulphate of lime, common salt, with chlorides of calcium and magnesium, and sometimes organic matter. The amount of dissolved contents in ordinary drinking water does not exceed '5 or at most 1'0 gramme per litre ; the best waters contain even less. illineral Sp)-inf/s.—These may be roughly but con- veniently classiﬁed according to the prevailing mineral siib— stance contained iii them, which may range in amount from 1 to 300 grammes per litre.2 C'alcareous—'containing so much lime that it is deposited as a white crust as the water evaporates. Spring water when saturated with carbonate of lime contains about 105 parts in 100,000. Spi in gs of this kind are common in limestone countries. As the water flows away from its point of exit, it throws down a deposit of (":ll("aI‘(‘llS tufa or travertine, which, as it cncrusts moss, twigs, and other objects, gives them the appearance of having been t_urned into stone, whence the springs are popularly tt-rim-d “ ]lL'l1'lfyll1g.” Enormous accumulations of this kind have been forrncd in some parts of Italy, where the rock so produced is extensively quar1‘ic(l as a huildiiig material. _ Fcrrughzous or C'Izalybeate—containing a large proportion of iron in the total mineral ingredioiits. Such waters have an inky taste, and often deposit along their course a yellow, brown, or red ochry 9 Paul, in Watt’s Chem. IJict., v., 1016.