Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/285

Rh UNDERGROUND wivri-;P..] deposit, consisting mainly of hydrated peroxide of iron. They may be frequently obsei'vcd in those districts where beds or veins of ironstone occur, or where the rocks contain much iron in combiiia- tioii. Si'l[ccous —depositiiig silica_or ﬂint. _ Although silica_ may be dissolved and retained in solution even in cold water, it is in the hot water issuing in voleaine countries that it occurs most abiiiid- aiitly, and where true siliceous springs exist. The geysers of Iceland, New Zealand, the Yellowstone region, and other districts are illustrations. When the heated water of these thermal springs cools and c .'apor-ates, the silica is deposited as siliceous sintcr round their liasiiis, or in picturesque mounds at the point of escape. One of the siiiter beds in the geyser region of Iceland is said to be two leagues long, a quarter of a league wide, and a liundicd feet thick. _-s already stated (mite, p.263), the effect of pressure is to enable water at great depths to retain a larger amount of nnneral matter in solu- tion. Hence, when the water ascends, it deposits its mineral coi1- tents, iiot only because it cools, but because t1ie_pi'essurc is removed. 'l‘herc must in many cases be a copious deposit along the walls of the ﬁssures up which the water flows on its way to the surface. l)oiibtlcss in this way many iniiicral veins have i'ece_i'ed their suc- cessive coatings of quartz, jasper, gypsum, calcite, and other iiiinerals. L‘rz'nc—bi-iiigiiig to the surface a solution, more or less ncarly saturated, of ('lll0l'lLli‘ of sodium. Springs of this kind appcarwhere beds of solid rock-salt exist underneath. The water in its passage i-neounters the salt, dissolves it, and brings it to the surface. The brine springs of (‘hcshire in Englainl, the Salzkamnicrgut in Austria, Bex in Siritzcrland, &e., have long been well known. Some_ of the English briiics contain about one per cent. of salts, of which the chloride of sodium may range from a half to tlirec-fourths or more. Other briiies, however, yield a far larger amount; one at Clenienshall, Wiii-teiiibiii-g, gave upwards of 26 per cent. of salts, of which almost the whole was chloride of sodium. The other sub- stances contained in solution in the water of brine springs are usually such as exist also in sea-water, such as sulphate and car- bonate of lime, chlorides of magnesiuni and potassium, &e. il[c(Zz'cz'iial——a vague tcrin applied to niineral springs which have or are believed to have curative effects in differeiit diseases. Medical inen recognize various qualities, distinguished by the par- ticular substance most conspicuous in cacli—as Alkaline 1V(u.‘crs, containing lime or soda and carbonic acid, as those of Vichy or Saratoga; Bitter IVatcrs, with sulphate of magnesia and soda—- Scdlitz, Kisslngcn; Salt or Jluriatcd IV uters, with eoiuinoii salt as the leading mineral constituent—Viesbaden, Clicltenliaiii; Earthy N'vufnrs, lime, either a sulphate or carbonate, being the most marked iiigi'edicnt—l3atli, Lueca; Sulphurous IVatc7's, with snlpliur as St1l1)lll1I‘clIl.C'l. hydrogen and in sulphidcs——Aix—la-Cliapclle, IIarro- gate. 0iI.—)[inei‘al oil is carried up by some ordinary springs, and floats in dark drops on the surface of the watci'. But in some parts of the world, as in a wide region in the Northern States of the American Union and in Canada, the oil ascends with little or no water, and forms the oil-springs which in recent years have become so reinarkable anil abundant a source of illuminating oils, paraffin, and other hydrocarbon compounds. It’es2z..’{s of Chemical Jclion of Uizclergromul Wate9'.— Since every spring is busily engaged in bringing mineral substances from below ground to the surface, there must evidently be a vast amount of subterranean waste, and many tunnels, cliaimels, and caverns must in consequence be formed. To take one illustration: the warm springs of Bath, with a mean temperature of 120° Fahr., are iinpreg- nated with sulphates of lime and soda, and clilorides of sodiuni and magnesium. Professor Ramsay has estimated their annual discharge of mineral matter to be equal to a square column 9 feet in diameter and 1-10 feet in height. It is in calcareous regions that the extent of the subter- ranean loss can be most strikingly seen. Sometimes a dis- trict of limestone is drilled with vertical cavities (swallow- /iolcs or sin/rs) formed by the solution of the rock by the descent of carbonated rain-water. Surface-drainage is there intercepted, and passes at once under ground, where, in course of time, an elaborate system of cliamiels may be dis- solved out of the solid rock. Such has been the origin of the Peak caverns of Derbyshire, the intricate grottoes of Antiparos and Adelsberg, and the vast labyrinths of the M-ainnioth Cave of Kentucky. In the course of time the underground rivers open out new courses, and leave their old ones dry. By the falling in of the roofs of caverns near the surface, brooks and rivers are occasionally en- GEOLOGY 271 gulphed, which, after a long subterranean course, may issue to the surface again in a totally different surface area of drainage to that in which they took their rise, and soiiic- tiincs, as in Florida, with volume enough to be navigable almost up to their outﬂow. In such circumstances lakes may be formed over the sites of the broken-iri caverns; and valleys may thus be deepened, or perhaps even formed. Mud, sand, and gravel, with the remains of plants and animals, are swept below ground, and sometimes accumulate in deposits there. This has been the origin of ossifcrous caverns, and of the loam and breccia so often found in them. ' These wonderful results of the subterranean circulation of water appeal to the imagination, and are those usually inost dwelt upon as evincing the potency of this kind of geological agency. And yet the thoughtful observer who reflects upon this subject will perhaps be led to perceive that even more important than these visible caverns and grottoes are the silent unobtrusive changes so constantly in progress in the solid heart of rocks. As far down as percolating water reaches there is not a particle of mineral matter safe from its attacks. And as we have seen, it is hardly possible to find any rock which does not bear throughout its minute grains and pores evidence that water has ﬁltered through it, removing some substances and put- ting others in their place. (2.) II’csulls of 1|Icc/u1m'cal Action.—In its passage along ﬁssures and channels of the rocks, the underground water not merely dissolves materials chemically and removes them in solution, it likewise loosens some of the ﬁner particles from the sides of these subterranean conduits and carries them along in mechanical suspension. lVe may occasionally observe, where a spring gushes forth at the surface, that grains of sand are brought up in the clear sparkling water. This removal of material sometimes produces remarkable surface changes when it takes place along the side of a steep slope or cliff, such as occur in river valleys, or by the sea-coast. Let us suppose a thin layer of some porous material, like loose sand or ill—compacted sandstone to lie between two more impervious rocks such as masses of clay or limestone, and that this porous stratum sloping down from higher ground comes out to the surface near the base of a line of abrupt cliff. The water which ﬁnds its way down into this layer will use it as its channel of escape, and travelling along its course will issue in springs or in a more general oozing forth along its outcrop at the foot of the declivity. Under these circumstances the support of the overlying mass of rock is apt to be loosened. The water not only removes piece—meal the sandy layer on which that overlying mass rests, but as it were liibricates the rock underneath. Consequently at intervals portions of the upper rock may break off and slide down into the valley or plain below. Such dislocations are known as lcmclslfps. Many illustrative examples might be cited. Thus in the year 1839 a mass of chalk on the Dorsetshire coast slipped over a bed of clay into the sea, leaving a rent three-quarters of a mile long, 150 feet deep, and 240 feet wide. The shifted mass, bearing with it houses, roads, and ﬁelds, was cracked, broken, and tiltedin various directions, and was thus prepared for further attack and removal by the waves. On many parts of the coasts of Britain there are laiidslips on a large scale which doubtless took place many centuries ago, or even in some cases beyond the times of human history. The underclitf of the Isle of Wight, the cliffs west of Brandon Head, county Kerry, the basalt escarpments of Antrim, and the edges of the great volcanic plateaus of Mull, Skye, and Ilaasay furnish illustrations of such prehistoric laiidslips. Of Continental examples, the well-knowii fall of the Rossberg, behind the Righi in Switzerland, is one of the most memorable. After a rainy summer in 1806 a large