. Elements of geology. Geology. PART I. CHx\PTER X. 139 Slaty Cleavage. between two joints has no tendency to cleave in a direction parallel to the planes of the joints, whereas a rock is capable of indefinite subdivision in the direction of its slaty cleavage. In some cases where the strata are curved, the planes of cleav- age are still perfectly parallel. This has been observed in the slate rocks of part of Wales (See Fig. 125.), which consist of a Fig. Parallel planes of cleavage intersecting curved strata. (Sedgwick.) hard greenish slate. The true bedding is there indicated by a numb
. Elements of geology. Geology. PART I. CHx\PTER X. 139 Slaty Cleavage. between two joints has no tendency to cleave in a direction parallel to the planes of the joints, whereas a rock is capable of indefinite subdivision in the direction of its slaty cleavage. In some cases where the strata are curved, the planes of cleav- age are still perfectly parallel. This has been observed in the slate rocks of part of Wales (See Fig. 125.), which consist of a Fig. Parallel planes of cleavage intersecting curved strata. (Sedgwick.) hard greenish slate. The true bedding is there indicated by a number of parallel stripes, some of a lighter and some of a darker colour than the general mass. Such stripes are found to be parallel to the true planes of stratification, wherever these are manifested by ripple-mark, or by beds containing peculiar organic remains. Some of the contorted strata are of a coarse mechanical structure, alternating with fine-grained crystalline chloritic slates, in which case the same slaty cleavage extends through the coarser and finer beds, though it is brought out in greater perfection in proportion as the materials of the rock are fine and homogeneous. It is only when these are very coarse that the cleavage planes entirely vanish. These planes are usu- ally inclined at a very considerable angle to the planes of the strata. In the Welsh chains, for example, the average angle is as much as from 30° to 40°. Sometimes the cleavage planes dip towards the same point of the compass as those of stratifi- cation, but more frequently to opposite points. It may be stated ^s a general rule, that when beds of coarser materials alternate with those composed of finer particles, the slaty cleavage is either entirely confined to the fine-grained rock, or is very im- perfectly exhibited in that of coarser texture. This rule holds, whether the cleavage is parallel to the planes of stratification or not. In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enor
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Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
