Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/283

Rh RAILL] example, the surface of the country is in some parts so thickly strewn with fragments of sandstone and co1iglonier- ate “that a person may almost leap from one stone to another without touching the ground. The stones are fre- quently of considerable size, many being four or ﬁve yards across, and about four feet thick.” 1 They have been used for the huge blocks of which Stonehenge and other of the so-called driiidical circles have been constructed, hence they have been termed Druid Stones. Other names are Sarsen Stones (supposed to indicate that their accumulation has been popularly ascribed to the Saracens), and Grey Wethers, from their resemblance in the distance to ﬂocks of (wetlier) sheep. They are found lying abundantly on the chalk, suggestive at ﬁrst of some former agent of transport by which they were brought from a distance. It is now, how- ever, generally admitted that they are simply fragments of some of the sandy Tertiary strata which once covered the districts where they occur, and that while the softer por- tions of these strata have been carried away, the harder parts (their hardness perhaps increasing by exposure) have remained behind as Grey Wetliers, and have subsequently suffered from the inevitable splitting and crumbling action of the weather. But it is not from any single example, however strik- ing, that the real importance of rain as a geological agent can be adequately realized. To form a true conception of this momentous action, we need to watch what takes place over a wide region. The whole laiid-surface over which rain falls is exposed to waste. As Hutton long ago insisted, the superficial covering of decayed rock or soil is constantly, though slowly, travelling outward and downward to the sea. In this ceaseless transport rain acts as the great carrying agent. The particles of rock loosened by atmospheric waste, by frost or the chemical disintegration of the rain itself, are washed off to form new soil. But they as well as the particles of the soil are step by step moved downward over the face of the land till they reach the nearest brook or river, whence their seaward progress may be rapid. A heavy rain discolours the water—coiirses of a country, because it loads them with the ﬁne debris which it re- moves from the general surface of the land. In this way rain serves as the means whereby the work of the other disintegrating forces is made conducive to the general degradation of the land. The decomposed crust produced by weathering, which would otherwise accumulate over the solid rock and protect it from further decay, is removed by rain so as to expose a fresh surface to further decomposition. This decay is general and constant, but not uniform. In some places, from the nature of the rock, from the ﬂatness of the ground, or from other causes, rain works under great difliculties. There the rate of waste must consequently be extremely slow. In other places, again, the rate may be rapid enough to be readily appreciable from year to year. A survey of this department of geological activity shows how the unequal wasting by rain has helped to produce the details of the present relief of the land, those tracts where the destruction has been greatest forming hollows and valleys, others, where it has been less, rising into ridges and hills. I-lain-action is not always merely destructive. Usually it is accompanied by reproductive effects, and, as already remarked, the mouldered rock which it washes off furnishes materials for the formation of soil. In favourable situations it has gathered together accumulations of loam and earth from neighbouring higher ground—the “brick-earth,” “head,” and “ rain-wash” of the south of England—earthy 1 See Descriptive Catalogue of Rock Specimens in Jermyn Street Jluseum, 3d ed. ;_ Prestwicli, Quart. Journ. Geol. Soc., x. p. 123; Whiltaker, Geological Survey Ilemoir on parts of 1|!z'ddlese.'c, &c.,

p.l GEOLOGY 269 deposits, sometimes full of angular stones, derived from the subaerial waste of the rocks of the neiglibourheod? § 2. Under_qro2mr.l Water. When rain falls upon the land its further progress becomes twofold. The greater part of it sinks into the ground and apparently disappears; the rest ﬂows off into runnels, brooks, and rivers, and in this way moves down- ward to the sea. It is most convenient to follow ﬁrst the course of the subterranean water. Ve have seen that all rocks are more or less porous. They are moreover traversed by abundant joints and cracks. Hence, from the bed of the ocean, from the bottoms of lakes and rivers, as well as from the surface of the land on which rain falls, water is continually ﬁltering downward into the rocks beneath. To what depth this descent of the surface water may go is not known. It may reach as far_ as the intensely heated interior of the planet, for, as the researches of M. Daubrée have shown, capillary water has the capacity of penetrating rocks even against a high counter-pressure of vapour. The water at extreme depths may be under such pressure as to retain its liquid condition at a red or even at a white heat. Probably the depth to which the water descends varies indeﬁnitely according. to the varying nature of the rocky crust. Some shallow mines are practically quite dry, while others of great depth require large pumping engines to keep them from being ﬂooded by the water which pours into them from the surrounding rocks. As a rule, however, the upper layers of rock are fuller of moisture than those deeper down. The water which in this way sinks into the earth is not permanently removed from the surface, though there may perhaps be a slight loss due to absorption and chemical alteration of the rocks. It accumulates underneath, until by the pressure of the descending column it is forced to ﬁnd a passage through joints or ﬁssures upward to the surface. The points at which it issues are termed spriizgs. In most districts the rocks underneath are permeated with water below a certain limit which is termed the water-level. This line is not a strictly horizontal one like that of the surface of a lake. Moreover, it is liable to rise and fall according as the seasons are wet or dry. In some places it lies quite near, in others far below, the surface. A well is an arti- ﬁcial liole dug down below the water-level, into which the water percolates. Hence, when the water—level happens to be at a small depth the wells are shallow, when at a great depth they require to be deep. Since the rocks underneath the surface vary greatly in porosity, some contain far more water than others. It often happens that, percolating along some porous bed, the subterranean water ﬁnds its way downward until it passes under some more impervious3 rock. Hindered in its pro- gress, it accumulates in the porous bed, from which it may be able to ﬁnd its way up to the surface again only by a tedious circuitous passage. If, however, a bore—hole be sunk through the upper impervious bed down to the water- charged stratum below, the water will eagerly avail itself of this artiﬁcial channel of escape, and will rise in the hole, or even rush up and gush out as a jet (Peau above ground. Wells of this kind are now largely employed. They bear the name of Artesian, from the old province of Artois in France, where they have long been in use. That the water really circulates underground, and passes 9 See Austen, Quart. Journ. Geol. Soc., vi. 94, vii. 121 ; Foster and Topley, Quart. Journ. Geol. Soc., xxi. 446. 3 This term tmpermbus must evidently be used in a relative and not in an absolute sense. A stiff clay is practically impervious to the trickle of underground water ; hence its employment as a material for puddling (that is, making water-tight) canals and reservoirs. But it contains abundant interstitial water, on which indeed its characteristic plasticity depends.