| Fig. 7.—Bedding, Joints, and Cleavage (after Murchison). |
42. Cleavage.—Fine-grained rocks, more especially those which are argillaceous, occasionally shew another kind of structure, which is called cleavage. Common clay-slate is a type of the structure. This rock splits up into innumerable thin laminæ or plates, the surface of which may either be somewhat rough, or as smooth nearly as glass. The cleavage planes, however, need not be parallel with the planes of bedding; in most cases, indeed, they cut right across these, and continue parallel to each other often over a very wide region. The original bedding is sometimes entirely obliterated, and in most cases of well-defined cleavage is always more or less obscure.
In the preceding diagram, the general phenomena of bedding, jointing, and cleavage are represented. The lines of bedding are shewn at S, S; another set of division-planes (joints) is observed at J, J, intersecting the former at right angles—A, B, C being the exposed faces of joints. The lines of cleavage are seen at D, D, cutting across the planes of bedding and jointing.
43. Foliation is another kind of superinduced structure. In a foliated rock the mineral ingredients have been crystallised and arranged in layers along either the planes of original bedding or those of cleavage. Mica-schist and gneiss are typical examples.
44. Concretions.—In many rocks a concretionary structure may be observed. Some sandstones and shales appear as if made up of spheroidal masses, the mineral composition of the spheroids not differing apparently from that of the unchanged rock. So in some kinds of limestone, as in dolomite, the concretionary structure is often highly developed, the rock resembling now irregular heaps of turnips with finger-and-toe disease, again, piles of cannon-balls, or bunches of grapes, and agglomerations of musket-shot. A spheroidal structure is occasionally met with amongst some igneous rocks. This is well seen in the case of rocks having the basaltic structure, in which the pillars, being jointed transversely, decompose along their division planes, so as to form irregular globular masses. In many cases, certain mineral matter which was originally diffused through a rock has segregated so as to form nodules and irregular layers. Examples of this are chert nodules in limestone; flint nodules in chalk; clay-ironstone balls in shale, &c.
| Fig. 8.—Dip and Strike of Strata. |
45. Inclination of Strata.—Beds of aqueous strata must have been deposited in horizontal or approximately horizontal planes; but we now find them most frequently inclined at various angles to the horizon, and often even standing on end. They sometimes, however, retain a horizontal position over a large tract of country. The angle which the inclined strata make with the horizon is called the dip, the degree of inclination being the amount of the dip; and a line drawn at right angles to the dip is called the strike of the beds. Thus, a bed dipping south-west will have a north-west and south-east strike. The crop or outcrop (sometimes also, but rarely, called the basset edge) of a bed is the place where the edge of the stratum comes to view at the surface. We may look upon inclined beds as being merely parts of more or less extensive undulations of strata, the tops of the undulations having been removed so as to expose the truncated edges of the beds. In the following diagram, for example, the outcrops of limestone seen at l, l, are evidently portions of one and the same stratum, the dotted lines indicating its former extent. The trough-shaped arrangement of the beds at s is called a synclinal curve, or simply a syncline; the arched strata at a forming, on the contrary, an anticlinal curve or anticline.