It scarcely needs to be stated that there is underground water almost everywhere, and that everywhere it is producing effects similar to those produced by surface water. That water really circulates underground, and passes not merely between the rocks, but in crevices and tunnels which it has no doubt to a large extent opened for itself along natural joints and fissures, is proved by the occasional rise of leaves, twigs, and live fish in the shafts of artesian wells. These facts prove that the water travels leagues and leagues under the surface of the earth. The temperature of underground springs is an indication of the depth from which they rise. Very cold springs probably derive their water from glaciers or snow-covered summits. The hottest springs are found in volcanic districts, but there are warm springs far away from such districts. Assuming a rise of one degree of heat for each 60 ft. in depth, the source of a spring whose tem perature is 120° would be 4,200 ft. below the surface, and water at the nailing point should rise nearly 13,000 feet. The underground circulation of water has great interest 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. As in the case of rain, under ground water acts both chemically and mechanically. Leaving processes and coming directly to results we find, since every spring is busily engaged in bringing mineral sub stances from below ground to the surface, that there must evidently be a vast amount of subterranean waste, and many tunnels, channels, and caverns must, in consequence, be formed. To take one illustration: the warm springs of Bath, with a mewl tempera ture of 120° Pain.., .are impregnated with sulphates of lime and soda, and chlorides of sodium and magnesium. Prof. Ramsay has estimated their annual discharge of min eral matter to be equal to a square column 9 ft. in diameter and 140 ft. in height. It is in calcareous regions that the extent of the subterranean loss can be most strikingly seen. Sometimes a district of limestone is drilled with vertical cavities (" swallow holes" or " sinks") formed by the solution of the rock by the descent of carbonated rain-water. Surface-drainage is there intercepted, and passes at once underground, where, in course of time, an elaborate system of channels may be dissolved 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 Mammoth 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 engulfed. 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 'sometimes, as in Florida, with volume enough to be navigable almost up to their outflow. Iu such circumstances lakes maybe formed over the broken-in 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 ossiferous 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 usu ally most 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 per ceive that even more important than these visible caverns and grottoes are the silent unobtrusive changes so constantly in progress in the solid heart of the 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 putting others in their place. In its passage along fissures arid channels of the rocks, the underground water not merely dissolves mate rials chemically and removes them in solution, it likewise loosens sonic of the finer particles from tire sides of these subterranean conduits and carries them along in mechanical suspension. We 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 those which 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 conies out to the surface near the base of aline of abrupt cliff. The water which finds its way down into this layer will use it as its channel of escape, and travel ing 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 piecemeal the sandy layer on which that overlying mass rests, but, as it were, lubricates the rock, beneath. 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 landslips. Many
illustrative examples might be cited. Thus, in the year 1839 a mass of chalk on the Devonshire coast slipped over a bed of clay into the sea, leaving a rent three-quarters of a mile long, 150 ft. deep, and 240 ft. wide. The shifted mass, bearing with it houses, roads, fields, was cracked, broken, and tilted in various directions, and was thus pre pared for further attack and removal by the waves. On ninny parts of the coasts of Britain there are landslips on a large scale which doubtless took place many centuries, ago, or even, in some cases, beyond the times of human history. The undercliff of the isle of Wight,. the cliffs w. of Brandon Head, county Kerry, the basalt escarpments of Antrim, and the edges of the great volcanic plateaus of Mull, Skye, and Easily, furnish illustrations of such prehistoric landslips. Of continental examples, the well known fall of the Rossberg, behind the Righi in Switzerland, is one of tire most memorable. After a rainy summer in 1806, a large part of one side of the mountain, consisting of sloping beds of hard red sandstone and conglomerate, resting upon soft sandy layers, gave way. Thousands of tons of solid rock suddenly swept across the valley of Goldau, four villages, with about 500 of their inhabitants. In 1855, a mass of debris, 8,500 ft. long, 1000 ft. wide, and 600 ft. high, slid into the valley of tire Tiber, which, dammed back by the obstruction, overflowed the village of San Stefano to a depth of 50 ft., until drained off by a tunnel.
The surface drainage of the globe is through brooks and rivers, which carry to lakes and seas, not only the surplus surface-water, but immense quantities of material torn from the land. Like all other moving water. streams have both a chemical and a mechanical action. The substances held in solution in river-water include carbonates of lime, magnesia, and soda; silicates. peroxides of iron and manganese; sulphates of lime, magnesia, potash, and soda; chlorides of sodium, potassium, calcium, and mag nesium; silicate of potash; nitrates, and organic matter. As an average, there are 21 parts of mineral matter in 100,000. of water, and carbonate_ of lime. makes up one-half of all the solid matter. It has been calculated that the Rhine carries annually to the sea enough carbonate of lime to make 332,000,000,000 of oyster-shells of ordinary size. Sulphate of lime is the next most abundant mineral. Au English scientist estimates that there may be every year dissolved by rain one hundred tons of rocky matter to each square mile of the earth's surface. The mechanical action of ronuing streams needs no explanation. The enormous deposits made near the mouths of great rivers, and the constant effort to preserve channels and harbors, are always before us. The deltas of the Nile and the Mississippi are instances of the enormous transporting power of rivers. Three thousand miles from the gulf of Mexico the Missouri river starts a yellow stream of mud, gathering more ,and more as it goes on, and with the added vol ume of the Mississippi, the Ohio, and other streams, bears its mud to the gulf.
In Africa, Livingstone found rivers whose composition seemed to be more of sand than of water. The power of running water for abrasion or wearing away is well illustrated in the case of the falls of Niagara, where the stream may have fallen over the Queens town cliff when the river first sought its way to the sea. But much more probably the escarpment and waterfall began to arise simultaneously and from the same geological structure. As the escarpment grew in height, it receded from its starting-point. The river ravine likewise crept backward, but at a more rapid rate, and the result has been that at present the cliff, worn down by atmospheric causes, stands at Queenstown, while the ravine extends 7 m. further inland, with a width of from 00 to 400 yards, and a depth of from 200 to 300 feet. in this, as in other cases, the waterfall has cut its way backward up the course of its stream, and will continue to do so as long as the struc ture of the gorge continues as it is now—a thick bed or beds of limestone resting hori zontally upon soft shales. The softer strata at the base are undermined, and slice after slice is cut off from the cliff over which the cataract pours. It has been estimated that, at their present rate Of recession, the Niagara falls must have taken about 35,000 years to cut their way backward, and excavate the gorge between their present position and Queens town. In other cases, waterfalls have been produced by the existence of a harder and more resisting band or barrier of rock crossing the course of the stream, as, for instance, where the rocks have been cut by an intrusive dike or mass of basalt. In these and all other cases, the removal of the harder mass destroys the waterfall, which, after passing into a series of rapids, is finally lost in the general abrasion of the river-channel. The most marvelous river gorges in the world are those of the Colorado region in North America. The rivers there flow in ravines thousands of feet deep and hundreds of miles long, through vast table-lands of nearly horizontal strata. The Grand Canyon (ravine) of the Colorado river is 300 miles long, and in some places more than 6,000 feet in perpendicular depth. The country is hardly to be crossed, except by birds, so profoundly has it been trenched by these numerous gorges. Yet the whole of this excavation has been effected by the erosive action of the streams themselves.