Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/282

Rh 268 partly due to atmospheric moisture, but it is chieﬂy carried on by rain. And as rain is so widely and almost uni- versally distributed over the globe, this chemical action must be of very general occurrence. Nature of the Changes e_[fcclecl.—~Confiuing our attention GEOLOGY to its three chieﬂy active ingredients, we find that rain water reacts chemically upon rocks by—1. 0.ridation.—The ‘ prominence of oxygen in rain-water, and its readiness to unite with any substance which can contain more of it, render this a marked feature of the passage of rain over rocks. A thin oxidized pellicle is formed on the surface, and this, if not at once washed off by the rain, sinks deeper until a crust is formed over the stone. As already remarked, this process is simply a rusting of those minerals which, like metallic iron, have no oxygen, or have not their full complement of it. 2. Deo.m'datz'on.—Orga11ic matter having an afﬁnity for more oxygen decomposes peroxides by depriving them of some part of their share of that ele- ment, and reducing them to protoxides. These changes are especially noticeable among the iron oxides so abundantly diffused among rocks. Hence rain-water, in sinking through soil and obtaining such organic matter, becomes thereby a reducing agent. 3. Solution.—This may take place either by the simple action of the water, as in the solution of rock- salt, or by the inﬂuence of the carbonic acid present in the rain. Of the latter (Carbo72at2'o72) a familiar example is the corrosion of marble slabs down which rain has trickled for a time. The carbonic acid dissolves some of the lime, which as a bicarbonate is held in solution in the carbonated water, but is deposited again when the water loses its carbonic acid or evaporates. It is not merely carbonates, however, which are liable to this kind of destruction. Even silicates of lime, potash, and soda, combinations existing abundantly as constituents of rocks, are attacked; their silica is liberated, and their alkalies or alkaline earths, becoming carbonates, are removed in solution. 4. H3/dratz'on.——Some minerals, containing little or no water, and therefore called anhydrous, when exposed to the action of the atmosphere, absorb water, or become hydrous, and are then usually more prone to further change. Hence the rocks of which they form part become disintegrated. Weathering.—The weathering of rocks is dependent upon two sets of conditions——(I) meteorological, as the range of temperature, abundance of moisture, height above the sea, and exposure, and (2) lithological,——the composition and texture of the rocks themselves. As regards the composi- tion of rocks, those which consist of particles liable to little chemical change from the inﬂuence of moisture are best ﬁtted to resist weathering, provided their particles have suﬂicient cohesion to withstand the mechanical processes of disintegration. Siliceous sandstones are excellent examples of this permanence. Consisting wholly or mainly of the durable mineral quartz, they are sometimes able so to with- stand decay that buildings made of them still retain, after the lapse of centuries, the chisel-marks of the builders. Some rocks which yield with comparative rapidity to the chemical attacks of moisture show no marks of disintegration on their surface, which remains clean and fresh. This is particularly the case with limestones. The reason lies obviously in the fact that limestone when pure is wholly soluble in acidulous water. Rain falling on this rock removes some of it in solution, and will continue to do so until the rock is dissolved away. It is only where the limestone contains impurities that a weathered crust of more or less insoluble particles remains behind. Hence the relative purity of limestones may be roughly determined by comparing their weathered surfaces, where, if they contain much sand, the grains will be seen pro- jecting from the calcareous matrix, and where should the ' rock. [111. DYNAMICAL. rock be very ferruginous, the yellow hydrous peroxide or ochre will be found as a powdery crust. In limestones containing abundant encrinites, shells, or other organic re- mains, the weathered surface commonly presents the fossils standing out in relief. This seems to arise from the crys- talline arrangement of the lime in the organic structures, whereby they are enabled to resist disintegration better than the general mechanically aggregated matrix of the An experienced fossil collector will always search well those weathered surfaces, for he often ﬁnds there, delicately picked out by the weather, minute and frail fossils which are wholly invisible on a freshly broken sur- face of the stone. Many rocks weather with a thick crust or even decay inwards for many feet or yards. Basalt, for example, often shows a yellowish-brown ferru- ginous layer on its surface, formed by the conversion of its felspar into kaolin and the removal of its silicate of lime as carbonate, by the hydration of its olivine and augite and their conversion into serpentine, saponite, or some other hydrous magnesian silicate, and by the conversion of its magnetite into limonite. Granite sometimes slmws in a most remarkable way the distance to which weather- ing can reach. It may often be dug into for a depth of 20 or 30 feet, the quartz crystals and veins retaining their original positions, while the felspar is completely kaolinized. It is to the effects of weathering that the abundant fan- tastic shapes assumed by crags and other rocky masses are due. Most varieties of rock have their own characteristic modes of weathering, whereby they may be recognized even from a distance. To some of these features reference will be made in a subsequent section. II. MECHANICAL Ac'rio.T.—When a rock has been so corroded by weathering that the cohesion of the particles on its exposed surface is destroyed, these particles are washed off by rain. This detritus is either held in suspension in the little runnels into which the rain—d1-ops gather as they begin to ﬂow over the land, or is pushed by them along the surface. In this way the rain carries off by mechanical movement what it has already loosened by chemical action. III. RESULTS or RAIN-ACTION.—-It is evident that the general result of the fall of rain upon a land-surface must be a disintegration and consequent lowering of that surface. At ﬁrst we may be inclined to imagine that this waste must be so slow and slight as to be hardly appreciable. But a little observation will suffice to furnish many proofs of its existence and compara- tively rapid progress in some places. We are familiar, for example, with the pitted channelled surface of the ground lying immediately under the drip of the eaves of a house. We lmow that the fragments of stone and gravel are left sticking up prominently because the earth around and above them has been washed away, and because, being hard, they resist the actior_ of the falling drops and screen the earth below them. On a far larger scale we may notice the same kind of operation in districts of conglomerate, where the larger blocks, serving as a protection to the rock underneath, come to form as it were the capitals of slowly-deepening columns of rock. In the same way in certain valleys of the Alps a stony clay is cut by the rain into pillars, each of which is protected by, and indeed owes its existence to, a large block of stone which lay originally in the heart of the mass. These columns are of all heights, according to the positions in which the stones may have originally lain. There are instances, however, where the disintegration has been so complete ‘that only a few scattered fragments remain of a once extensive stratum, and where it may not be easy to realize that these fragments are not transported boulders. In Dorsetshire and I-Viltshire, for