Common Minerals and Rocks

by William O. Crosby

Fig. 1.—Section through sediment deposited by rain in a roadside pool: a. surface of roadway; b. layer of small pebbles and coarse sand; c. fine sand passing into d; d. the finest sand and mud.

Original Structures of Stratified Rocks.

Stratification.—All rocks formed by strewing materials in water, and their deposition in successive, parallel, horizontal layers, are stratified; and this structure is their stratification. It is the most important of all rock structures; and there is no kind of structure the origin of which is more fully or certainly known. The deposition of sediment in carefully assorted horizontal layers is readily brought within the comprehension of children by simple experiments with sand and clay in water; and still better by the examination of the deposits formed in roadside pools during heavy rains ([Fig. 1]), and by digging into beaches and sandbars, which every child will recognize as formed of materials arranged by water. Great stress should be laid upon the fact that a lake like Erie or Champlain is simply a large pool with several more or less turbid streams flowing into it, while the single stream flowing out is clear, the sediment having evidently been deposited in the lake; and that every lake is, like the roadside pool, being gradually filled up with sedimentary or stratified rocks. But the ocean is a still larger pool, receiving mud and sand from many streams; and since we know that nothing escapes from the ocean but invisible vapor, it is plain that the mud and sand and all other kinds of sediment carried into the ocean must be deposited on its floor, and chiefly, as we have seen, on that part nearest the land. The consolidation of beaches, bars, and mud-flats is all that is necessary to convert them into stratified formations of conglomerate sandstone and slate.

Let us notice now, more particularly, the causes of visible stratification. As we can easily prove by an experiment with clay in a bottle of water, if the same kind of material is deposited continuously there will be no visible stratification in the deposit. It will be as truly stratified as any formation, but not visibly so; because there is nothing in the nature of the material or the way in which it is laid down to bring out distinct lines of stratification. Continuous and uniform deposition obtains very frequently in nature, but rarely continues long enough to permit the formation of thick beds or strata. Hence, while the stratification is almost always visible on the large surfaces of sandstone, slate, etc., exposed in quarries and railway cuttings, and may usually be seen in the quarried blocks, it is often not apparent in hand specimens, which may represent a single homogeneous layer. There is one important exception, and that is where the particles, although of the same kind, are flat or elongated. Pebbles of these forms are common on many beaches; and since they are necessarily arranged horizontally by the action of the water, they will, by their parallelism, make the stratification of the pudding-stone visible. The same result is accomplished still more distinctly by the mica scales, etc., in sandstone and slates, the leaves and flattened stems of vegetation in bituminous coal, and the flat shells in limestone.

In all other cases, visible stratification implies some change in the conditions; either the deposition was interrupted, or different kinds of material were deposited at different times. The first cause produces planes of easy splitting, or fissility, especially in fine-grained rocks, like shale. This shaly structure or lamination-cleavage may be due, in some cases, to pressure, but it is commonly understood to mean that each thin layer of clay became partially consolidated before the next one was deposited upon it, so that the two could not perfectly cohere. Parallel planes of easy splitting are, however, by themselves, of little value as indications of stratification, since the lamination-cleavage is not easily distinguished from slaty-cleavage (roofing slate) and parallel jointing, structures developed subsequently to the deposition of sediments and quite independent of the stratification. The second cause, or variations in the kind of sediment, gives alternating layers differing in color, texture, or composition, as is seen frequently in sandstone, slate, gneiss, etc.; and of all the indications of stratification these are the most important and reliable.

Fig. 2.—Section showing strata and laminæ: a. conglomerate; b. sandstone; c. shale; d. limestone.

A layer composed throughout of essentially the same kind of rock, as conglomerate or sandstone, and showing no marked planes of division, is usually regarded as one bed or stratum, although it may vary considerably in texture or color; while the thinner portions composing the stratum and differing slightly in color, texture, and composition, and the thin sheets into which shaly rocks split, are the laminæ or leaves. In [Fig. 2] the strata are designated by letters, and the fine lines and rows of dots show the constituent laminæ, while the whole section may be regarded as a small part of a great geological formation. The geological record is written chiefly in the sedimentary rocks; and the formations, strata, and laminæ may be regarded as the volumes, chapters, and pages in the history of the earth. Now every feature of a rock, lithological or petrological, finds its highest interest in the light which it throws upon the history of the rock, i.e., upon the conditions of its formation. Observe what the section in [Fig. 2] teaches concerning the geological history of that locality; premising that any chapter of geological history written in the stratified rocks should be read from the bottom upwards, since the lowest strata must have been formed first and the highest last. The lowest stratum exposed is conglomerate, indicating a shingle beach swept by strong currents which carried away the finer material. Upwards, the conglomerate becomes finer and shades off into sandstone, and finally into shale, showing that the water has become gradually deeper and more tranquil, the shore having, in consequence of the subsidence, advanced toward the land. The next two strata show that this movement is probably reversed; at any rate, the currents become stronger again, and the shale passes gradually into sandstone and conglomerate. The beach condition prevails now for a long time, and thick beds of sand and gravel are formed. The sea then deepens again, and we observe a third passage from coarse to fine sediment. This locality is now remote from the shore, the gentle currents bringing only the finest mud, which slowly builds up the thick bed of shale, in the upper part of which shells are abundant, indicating that the deposition of mechanical sediment has almost ceased, and that the shale is changing to limestone. The purity of the limestone, and the crinoids and other marine organisms which it contains, prove that this has now become the deep, clear sea; and this condition is maintained for a long period, for the limestone is very thick, and this rock is formed with extreme slowness.

The most important point to be gained here is that every line of stratification and every change in the character of the sediments is due to some change of corresponding magnitude in the conditions under which the rock was formed. The slight and local changes in the conditions occur frequently and mark off the individual laminæ and strata, while the more important and wide-spread changes determine the boundaries of the groups of strata and the formations.

Strata are subject to constant lateral changes in texture and composition, i.e., a bed or formation rarely holds the same lithological characteristics over an extended area. There are some striking exceptions, especially among the finer-grained rocks, like slate, limestone, and coal, which have been deposited under uniform conditions over wide areas. It is the general rule, however, particularly with the coarse-grained rocks, which have been deposited in shallow water near the land, that the same continuous stratum undergoes great changes in thickness and lithological character when followed horizontally. A stratum of conglomerate becomes finer grained and gradually changes into sandstone, which shades off imperceptibly into slate, and slate into limestone, etc. Where the stratum is conglomerate, its thickness will usually be much greater and more variable than where it is composed of the finer sediments. The rapidity of these changes in certain cases is well shown by the parallel sections in [Fig. 3]. These represent precisely the same beds, as the connecting lines indicate, at points only twenty feet apart.