Igneous and sedimentary rock - The differences and geological positions

When we consider the solid part of our globe we recognize immediately its division into earth,--using that word in its agricultural sense; and stones--more or less detached, or in the state of continuous rocks. The least examination shows further, that this earth itself is composed to a great extent of stones gradually decreasing in size; so that we easily arrive at the well-established conclusion that the earth and the stones have the same origin.

If then, in thought, we remove from the surface of the land the earth, whose thickness indeed is very inconsiderable, we perceive that the solid part of our globe consists exclusively of rocks.

These rocks are divided into two great classes: one formed of melted materials, like the lavas of our modern volcanoes; the other produced by seas, rivers, and lakes of ancient periods, in the same manner as we see deposits accumulated by the waters in our own time. The first are called igneous rocks; the second, sedimentary.

The Igneous Rocks, pushed from the interior of the earth in a plastic state, lift themselves above the surface of the soil in irregular shapes, in precipitous peaks, or vast cones, and sometimes in those basaltic columns whose aspect is so striking and impressive.

The accompanying cut exhibits a good example of these basaltic rocks, but we may remark that columnar basalt is comparatively rare, and that igneous rocks in general possess no regularity of structure. Very frequently they are intersected by cracks in all directions--a feature produced by the contraction resulting froom cooling.

The Sedimentary Rocks present an aspect so completely different from that of igneous formations, that they may be recognized at a great distance, even by inexperienced eyes. Being deposited in water in parallel layers, they have preserved the same disposition after being left dry. Sometimes, indeed, the horizontal layers have been singularly displaced from their original direction: sudden changes and movements of the soil have tilted them up and contorted them; but still the parallelism of the strata, and their disposition by successive layers, is nearly always clearly discernible. Fig. 3 is an example of sedimentary rock, and it shows very distinctly the regularity of structure which these rocks usually display.

Beyond this general distinction there is another, less prominent perhaps, but quite as definitely marked. The remains of myriads of animals and plants have been left age after age in the sediments of the different eras; these remains are known as Fossils. They reveal forms of life very different from those that exist in the present condition of our globe; and their dominant races, long since extinct, have furnished names for remote eras,--for all the geological ages, indeed, included between the azoic age and the age of man.

With the high temperature of the igneous formations, life was incompatible; consequently no fossil ever appears, or possibly could appear, in rocks of this kind. In sedimentary formations, on the contrary, such remains abound, and furnish another excellent means of distinguishing them from the igneous rocks.

A few illustrations will exemplify types of animal and vegetable fossils incident to different periods. Figs. 4, 5, and 6 represent animal and vegetable forms that existed in a period incalculably remote, and are found in rocks belonging to what has been called the primary or palaeozoic period.

Those represented by Figs. 7, 8, 9, and 10 appertain to the secondary formations, such as the oolite or jurassic formation, named from the Jura Mountains, in whose rocks such specimens are found; and the cretaceous period, or period of the chalk, next in order of time.

To the cretaceous period succeeded the tertiary formation, in which are found such shells as those represented by Figs. 11, 12, and 13. By this time creatures had begun to appear bearing a strong resemblance to those of the present period. Figs. 14 and 15 illustrate characteristic types.

After the tertiary formation comes the post-tertiary or quarternary formation, in which the animals are quite analogous to those of our own period.

If we inquire of chemistry what is the composition of the two grand classes of rocks above-mentioned, we obtain this simple answer:--The calcareous element predominates in all sedimentary formations (excepting the most ancient); silicious and aluminous elements in igneous formations.

Consequently, stratification, presence and often extreme abundance of fossils, great preponderance of the calcareous element, are the unmistakable characteristics of sedimentary formations. Absence of stratification, complete absence of fossils, great preponderance of the silicious and aluminous elements, are the characteristics of igneous formations.

Now if we investigate the chemical composition of precious stones, we shall find that the greater number of those which really merit this appellation, are principally formed of silica and alumina, or of one of these two substances. It follows, then, that precious stones should be found most frequently in igneous formations, or in the debris of such formations: and we naturally conclude that they will be most abundant in countries where the geological development is chiefly of this kind. Theoretically this is true; but practically the finding of precious stones depends far more upon the conditions than upon the abundance of the igneous rocks. These eagerly sought treasures are only the very rare exceptions in enormous masses of rock, and the latter must be broken up into small fragments before their riches can appear.

We know that under the influence of atmospheric agencies the most obdurate rock is gradually disintegrated; but atmospheric action has feebly contributed to the production of sands and the formation of arable lands. In different periods of its existence our globe has experienced agitations of extreme violence, the principal effects of which, after the lapse of countless centuries, are at this day perfectly discernible.

The last of these grand commotions belongs to the period which geologists have named the quarternary, an epoch relatively not far removed from our own.

Floods of water at that time spread across the continents; mountains of ice, of which the Alpine glaciers are but meagre vestiges, invaded even the most temperate zones; streams of irresistible violence--such as the great rivers of our day can scarcely give us any idea of--furrowed the earth. Under the influence of these agencies, the stupendous forces of which were all working in one direction, the grinding down and destruction of the rocks was effected over vast spaces, and to considerable depths. Now it is precisely in the debris of igneous rocks, whose reduction to sand was accomplished during this period, that the greater number of precious stones are found; and above all, the diamond.