Common Minerals and Rocks

by William O. Crosby

Descriptions of Rocks.

1.—Sedimentary or Stratified Rocks.

1. Mechanically formed or Fragmental Rocks.—These consist of materials deposited from suspension in water, and the process of their formation is throughout chiefly mechanical. The materials deposited are mere fragments of older rocks; and, if the fragments are large, we call the newly deposited sediment gravel; if finer, sand; and, if impalpably fine, clay. These fragmental rocks cannot be classified chemically, since the same handful of gravel, for instance, may contain pebbles of many different kinds of rocks, and thus be of almost any and very variable composition. Such chemical distinctions as can be established are only partial, and the classification, like the origin, must be mechanical. Accordingly, as just shown, we recognize three principal groups based upon the size of the fragments; viz.:—

(1) Conglomerate group.

(2) Arenaceous group.

(3) Argillaceous group.

This mode of division is possible and natural, simply because, as we observed in an early experiment, materials arranged by the mechanical action of water are always assorted according to size. When first deposited, the gravel, sand, and clay are, of course, perfectly loose and unconsolidated; but in the course of time they may, under the influence of pressure, heat, and chemical action, attain almost any degree of consolidation, becoming conglomerate, sandstone, and slate, respectively. The pressure may be vertical where it is due to the weight of newer deposits, or horizontal where it results from the cooling and shrinking of the earth’s interior. The heat may result from mechanical movements, or contact with eruptive rocks; or it may be due simply to the burial of the sediments, which, it will be seen, must virtually bring them nearer the great source of heat in the earth’s interior, on the same principle that the temperature of a man’s coat, on a cold day, is raised by putting on an overcoat. The effect of the heat, ordinarily, is simply drying, coöperating with the pressure to expel the water from the sediments; but, if the temperature is high, it may bake or vitrify them, just as in brick-making. Sediments are consolidated by chemical action when mineral substances, especially calcium carbonate, the iron oxides, and silica are deposited between the particles by infiltrating waters, cementing the particles together. This principle is easily demonstrated experimentally by taking some loose sand and wetting it repeatedly with a saturated solution of some soluble mineral, like salt or alum, allowing the water to evaporate each time before making a fresh application. The interstices between the grains are gradually filled up, and the sand soon becomes a firm rock. But the student should clearly understand that, in geology, gravel, sand, and clay are just as truly rocks before their consolidation as after. It is plain then that in each of the principal groups of fragmental rocks we must recognize an unconsolidated division and a consolidated division.

(1) Conglomerate group.—The rocks belonging in this group we know before consolidation as gravel, and after consolidation as conglomerate.

Gravel.—The pebbles, as we have already seen, are usually, though not always, well rounded or water-worn; and they may be of any size from coarse grains of sand to boulders. As a rule, however, the larger pebbles, especially, are of approximately uniform size in the same bed or layer of gravel, with, of course, sufficient fine material to fill the interstices. Although the same limited mass of gravel may show the widest possible range in chemical and mineralogical composition, yet hard rocks are evidently more likely than soft rocks to form pebbles; and hence quartz and quartz-bearing rocks usually predominate in gravels. Specimen 28.

Conglomerate.—Consolidated gravel. Children should be led to an appreciation of this point by a careful comparison of the forms of the pebbles in the gravel and conglomerate. The conglomerate seems to contain a larger proportion of fine material than ordinary gravel. But this is because the gravel is usually, as with our specimen, taken from the surface of the beach, where, of course, the pebbles are clean and separate; but if it had remained there to be covered by a subsequently deposited layer, enough fine stuff would have been sifted into the holes to fill them. And in the finished gravel, just as in the conglomerate, the pebbles are usually closely packed, with just sufficient sand and clay, or paste, as the material in which the pebbles are imbedded is called, to fill the interstices. The paste is usually similar in composition to the pebbles, with this difference: hard materials predominate in the pebbles and soft in the paste.