« PreviousContinue »
AGENTS OF DENUDATION
CHEMICAL COMPOSITION OF ROCKS—EPIGENE AGENTS—INSOLATION AND DEFLATION CHEMICAL AND MECHANICAL ACTION OF RAIN—ACTION OF FROST | OF PLANTS AND ANIMALS; OF UNDERGROUND WATER; OF BROOKS AND RIVERS—RATE OF DENUDATION—DENUDATION AND SEDIMENTATION GO HAND IN HAND.
TH E present, geologists tell us, contains the key to the past. If we wish to find out how rocks have been removed, and what has since become of them, we must observe what is taking place under the influence of existing agents of change. How, then, are rocks being affected at present? We do not proceed far in our investigation before we discover that they are everywhere becoming disintegrated. In one place they are breaking up into angular fragments; in another, crumbling down into grit, sand, or clay. Brooks and rivers and the waves upon our coasts are constantly undermining them; everywhere, in short, rocks are being assaulted and reduced. But in order to bring this fact more forcibly before the reader, it will be well to sketch, as briefly as may be, the general character of the warfare which is being waged against rocks over all the land-surface, and to note the various results that flow from this incessant energy of the epigene or superficial agents of change.
As these agents are often associated in their work, it is sometimes hard, or even impossible, to say which has played the most effective part in the demolition of rocks. Nevertheless, it will conduce to clearness if we endeavour to consider the operation of each by itself, so far, at least, as that is possible. Before doing so, however, we must glance for a moment at the general characters of rocks. We have already taken note of the fact that rocks are of various origin —igneous, derivative, and metamorphic. It is now necessary to consider their composition and structure, for, according as these differ, rocks are variously affected by epigene agents, some yielding rapidly, others being more resistant. We need not go into detail. Their composition and structure may be described in the most general terms. For our purpose it will suffice to group them roughly under these four heads: Felspathic, Argillaceous, Silicious, and Calcareous rocks. This is very far from being an exhaustive classification, but under these groups may be included all the rocks that enter most largely into the formation of the earth■s crust.
1. Felspathic Rocks. These rocks contain as their dominant constituent the mineral, or, rather, the family of minerals, known under the name of felspar. The group includes nearly all the igneous and most of the metamorphic rocks. The derivative rocks that come under the same head are of relatively small importance. The minerals entering most abundantly into the composition of the felspathic rocks are the felspars (aluminous silicates of potash, soda, and lime), various ferro - magnesian silicates, such as mica, pyroxene, hornblende, and olivine (aluminous silicates of magnesia, lime, iron-oxides, etc.), and quartz (silica, silicic acid). The crystalline igneous rocks occur either in more or less regular beds (lavas), interstratified with derivative rocks, or they penetrate these in the form of irregular veins, dykes, sheets, or large amorphous masses. The lava-form rocks are often associated with beds of volcanic de'bris (tuff, etc.). Some igneous rocks are smoothly compact in texture, such as obsidian and pitchstone, which are simply varieties of volcanic glass; others, such as basalt, consist partly of glass and partly of crystalline ingredients, and vary in texture from compact to coarsegrained; yet others are built up wholly of crystalline substances, and may be fine-grained or very coarsely granular, as granite. The crystalline schists are equally variable as regards texture. They differ, however, from the igneous rocks in structure. While the latter are confusedly crystalline, the schists show a kind of streaky structure or pseudo-lamination, their constituent minerals being arranged in rudely alternate lenticular layers.
Igneous rocks and schists are traversed by cracks and fissures which usually ramify irregularly in all directions. In many bedded igneous rocks (lavas), however, these cracks, or "joints," as they are termed, are somewhat more regular, being, as a rule, disposed at approximately right angles to the planes of bedding. In certain fine-grained rocks, such as basalt, the jointing is often very regular, giving rise to a prismatic columnar structure, as in the basalts of Staffa and the Giant■s Causeway. The main fact, however, with which we are at present concerned is simply this: that all crystalline, igneous, and schistose rocks are traversed by cracks and fissures of one sort or another. It is further to be noted that these rocks, in common with rocks of all kinds, are more or less porous, and therefore liable to be permeated, however slowly, by percolating water.
2. Argillaceous Rocks. These rocks are composed chiefly of clay, but other ingredients are usually present. Some are soft, such as ordinary brick-clay; others are of firmer consistency, and frequently show a fine fissile structure, as in common argillaceous shale; yet others are hard, tough rocks, some of which are capable of being cleaved into thin plates, as roofing-slate.
3. Silicious Rocks. These might be described in general terms as gravel-and-sand rocks. The most abundant and widely distributed rocks of this class are the sandstones—composed generally of grains of quartz (silica) cemented together by carbonate of lime, by iron-oxide, or other substance. Cementing material, however, is not always present, some sandstones having been solidified by pressure alone. The gravel-rocks, or conglomerates, usually consist of rounded fragments of quartz or some hard silicious rock. But to this there are exceptions, the stones in some conglomerates consisting of calcareous or of felspathic rocks or of a mixture of many different kinds. A silicious sandstone which has been more or less metamorphosed is termed quartz-rock.
4. Calcareous Rocks. Under this head are grouped limestones of every kind. They vary in character from soft earthy marls and chalks to hard, granular, crystalline limestones and saccharoid marbles. Some are nearly pure carbonate of lime; others contain larger or smaller percentages of quartz, clay, ironoxide, and other impurities.
The Argillaceous, Silicious, and Calcareous groups comprise the great bulk of the derivative rocks as well as a few metamorphic rocks. They are all originally of aqueous or sedimentary origin, and generally occur, therefore, in beds or strata. Like the igneous rocks, they are more or less porous, although some— especially the clay-rocks—are much less permeable than others. In addition to the planes of lamination and stratification, which characterise most derivative rocks, there are other natural division-planes or joints which cut across the strata in directions more or less perpendicular to the bedding. More irregular usually are the joints which intersect hard slates and quartzrock, these being divided generally much in the same way as schists and amorphous masses of crystalline igneous rock.