Geology and Revelation / or, the Ancient History of the Earth, considered in the geological facts and revealed religion.

by Gerald Molloy

And first it is important to have, at least, a general idea of the appearance which Mechanical Rocks present to the eye. We shall take three familiar examples, Conglomerate, Sandstone, and Clay. Conglomerate, or Pudding-stone as it is sometimes called, is composed of pebbles, gravel, and sand, more or less compacted together, and generally forming a hard and solid mass. The various materials of which it is composed, though united in the one rock, nevertheless remain their own external forms, and may be distinctly recognized even by the unpractised eye. Sandstone, as the name implies, is made up of grains of sand closely compressed and cemented together. The quality and appearance of this rock vary very much according to the size and character of its constituent particles. Often the grains of sand are as large as peas, or even larger; sometimes they are so minute that they cannot be distinguished without the aid of a lens. For the most part they consist of quartz, with grains of limestone intermixed; and they are usually rounded, as if by the action of running water. Clay is a rather vague and general term, now commonly employed to denote any finely-divided mineral matter which contains from ten to thirty per cent. of Alumina, and is thereby rendered plastic, and capable, when softened with water, of being moulded like paste with the hand. It occurs in many different forms among the strata of the Earth, according to the different minerals that enter into its composition and the different influences to which it has been subjected. Marl and Loam may be taken as well-known illustrations: the former is a clay in which there is a large proportion of calcareous matter; the latter is a mixture of clay and sand. Sometimes by pressure clay is condensed into a kind of slaty rock called Shale, which has the property of being easily split up into an immense number of thin plates or laminæ.

It should be remembered that there is not always a perfect uniformity in the structure of these rocks. In Conglomerate, for example, the pebbles may be as large as cannon balls, or they may be only the size of walnuts. So, too, we have every variety of fineness and coarseness in the quality of Sandstone. Again, both Conglomerate and Sandstone are often largely adulterated with clay, and on the other hand, clay will sometimes contain more than its usual proportion of sand or lime. Lastly, these materials are in one place compacted into hard and solid rock, in another they are found in a loose and incoherent condition.

But amidst all these varieties of form and texture, the rocks we have been describing generally preserve their peculiar characteristics, and with a little experience can be easily recognized. They are found to constitute a very large part, perhaps we might say the larger part, of the stratified rocks in every country that has hitherto been explored by Geologists. Wherever we go we are met by the same familiar appearances;—beds of Conglomerate, Sandstone, Clay, Marl, Shale, recurring again and again through a series of many hundred strata, sometimes in one order, and sometimes in another; sometimes without any formation of a different kind intervening, and sometimes alternating with limestone or other rocks of which we shall speak hereafter.

Such is the general character and appearance of those strata which are known among Geologists as Aqueous Rocks of Mechanical origin. Now, it must at once strike the reader, that these rocks are made up of just those very materials—the same both in kind and in form—that we have already shown to be daily prepared and fashioned by a vast and complex machinery in the great workshop of Nature. He will remember how enormous blocks are detached from the mountain side, or from the cliffs on the seashore, and broken up into fragments; how the fragments in time become pebbles, sand, and mud; and how these are caught up by rivers, tides, and currents, and carried far away to sea. Here we have certainly all the materials that are necessary for the building up of Conglomerate, Sandstone, and Shale. We have seen how they are prepared by the hand of Nature, how they are moulded into shape, how they are transported from place to place. Let us now pursue the sequel of their history, and follow them on to the end.

It is plain they cannot remain forever suspended in water; sooner or later they must fall to the bottom. Yet they will not all fall together. For though all are carried downward by the one force of gravity, those materials that are smaller and lighter will be more impeded by the resistance of the water. The pebbles and coarse gravel will be the first to reach the bottom, then the sand, and last of all the fine, impalpable mud. Thus, as the current sweeps along in its course, the sediment which it bears away from the land will be in a manner sorted, and three distinct layers of different materials will be deposited in the bed of the ocean;—first, nearest to the shore, a layer of pebbles and coarse gravel, then a layer of sand, and last of all a layer of fine mud or clay. This is the first step in the construction of stratified rock. To complete the work nothing more is necessary than the consolidation of these loose and incoherent materials. If this could be accomplished, then we should have a solid stratum of Conglomerate, a solid stratum of Sandstone, and a solid stratum of Shale formed in the bed of the ocean.

With regard to this operation, however, we cannot hope for the advantage we have hitherto enjoyed, of actual observation. The process of consolidation, if it take place at all, is going on in the depths of the Sea. But though it is thus removed beyond the reach of our senses, it is not beyond the reach of our intelligence. We may borrow the torch of Science, and search even into the hidden recesses of Nature’s secret laboratory.

In the first place, a partial consolidation of clay and sand, and even of gravel, may take place under the influence of pressure alone. Many of us are familiar with this truth, but few, perhaps, are aware how extensively it is illustrated in the practical arts of life. Here are some curious and interesting examples. The minute fragments of coal which are produced by the friction of larger blocks against one another, and which may be obtained abundantly in the neighborhood of every coal mine, are now manufactured into a solid patent fuel by the simple process of forcible compression. Again, the dust and rubble of black lead, formerly cast aside as useless, are now carefully collected, and by no other force than pressure are converted into a solid mass, fit to be employed in the manufacture of lead-pencils. “The graphite or black lead of commerce,” says Sir Charles Lyell, “having become very scarce, Mr. Brockedon contrived a method by which the dust of the purer portions of the mineral found in Borrowdale might be recomposed into a mass as dense and as compact as native graphite. The powder of graphite is first carefully prepared and freed from air, and placed under a powerful press on a strong steel die, with air-tight fittings. It is then struck several blows, each of a power of a thousand tons; after which operation the powder is so perfectly solidified that it can be cut for pencils, and exhibits, when broken, the same texture as native graphite.”[35] An instance yet more to our purpose occurs in the experiments made to try the force of gunpowder. Leathern bags filled with sand are put into the mortar that is to receive the cannon-ball at a distance of fifty feet from the mouth of the gun; and the sand is often compressed by the percussion of the ball into a solid mass of Sandstone.[36] Now the deposits of which we are speaking cannot fail to be subjected to a very powerful and a very constant compressing force. For, since the process of deposition is always going on, the matter which is deposited to-day will to-morrow be covered with a new layer, and in the course of ages it may lie beneath an immense pile of mineral matter, hundreds or even thousands of feet in thickness.

But in fact there is another and more important agent at work. When the harder and more compact blocks of Conglomerate and Sandstone are subjected to a close analysis in the laboratory of the chemist, it is found that they are strongly cemented together, sometimes by a solution of lime filling up the interstices between the grains or pebbles, sometimes by a solution of silica, sometimes by a solution of iron. Now this discovery affords a useful clue when we come to study the present operations of Nature. It is to the agency of a mineral cement we must look for the perfect consolidation of Mechanical Rocks. Let us see if such a cement can be found.

It is well known that the water of rivers, lakes, and springs, is more or less charged with carbonic acid gas; and therefore, when it comes in contact with limestone, it dissolves a portion of the lime and holds it in solution. Hence it follows that in every part of the world there exists an abundant store of calcareous cement. Again, our readers must have observed the brownish, rusty color sometimes produced by streams on the surface of rocks and herbage. This is the result of the iron with which the streams are impregnated: and we are informed by scientific inquirers that water containing a solution of iron prevails very generally in almost all countries. The solution of silica in water is not so common; because pure silica cannot be dissolved by water except at a very high temperature. Nevertheless, it has been clearly demonstrated by observation, that silica, where it occurs in certain combinations with other mineral substances, may be dissolved readily enough: for instance, in the decomposition of felspar, and of all rocks in which felspar is an ingredient, silica is carried off in a state of solution.[37] And since these rocks are very numerous, and distributed over every part of the earth, we may fairly conclude that a solution of silica exists very abundantly in nature.

Now when we bear in mind that we have on the one hand in the Crust of the Earth, solid strata of Conglomerate and Sandstone, exhibiting the evident operation of these mineral cements; and on the other hand, near the surface, the loose materials of Conglomerate and Sandstone as if ready to be cemented, and close at hand the cementing mineral itself in a convenient form, it is not unreasonable to assume that the process should actually take place;—that water highly charged with iron, or lime, or silica, should filter through the loose gravel and sand, depositing its mineral cement as it passes along, and converting the newly-formed strata into compact and solid rock.