. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 158.—Cleavage-Planes cutting through Strata nation. Many rocks may be thus split into large coarse slabs called flag-stones, and are used for paving streets, or even sometimes as roof- ing-slates. This may be called flagstone cleavage, or lamination cleav- age. Again (p), the arrangement of the ultimate molecules of a mineral under the influence of molecular or crystalline forces gives rise to an exquisite splitting along the planes parallel to the fundamental faces of the crystal. This is called crystal
. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 158.—Cleavage-Planes cutting through Strata nation. Many rocks may be thus split into large coarse slabs called flag-stones, and are used for paving streets, or even sometimes as roof- ing-slates. This may be called flagstone cleavage, or lamination cleav- age. Again (p), the arrangement of the ultimate molecules of a mineral under the influence of molecular or crystalline forces gives rise to an exquisite splitting along the planes parallel to the fundamental faces of the crystal. This is called crystalline cleavage. Again (c), the ar- rangement of the wood-cells under the influence of vital forces gives rise to easy splitting of wood in the direction of the silver-grain. This may be called organic cleavage. Now, in certain slates and some other rocks is found a very perfect cleavage on a stupendous scale. Whole mountains of strata may be cleft from top to bottom in thin slabs, along planes parallel to each other. The planes of cleavage seem to have no relation to the strata, but cut through them, maintaining their parallelism, however the strata may vary in dip (Fig. 158). Usually the cleavage-planes are highly inclined, and often nearly perpendicular. It is from the cleaving of such slates that roofing-slates, ciphering-slates, and blackboard-slates are made. This remarkable structure has long excited the interest of geologists, and many theories have been proposed to explain it. On cursory examination of such rocks, the first impression is, that the cleavage is but a very perfect example of flag-stone or lamination cleavage—that the cleavage-planes are in fact stratification-planes, and that we have here an admirable example of finely laminated rocks which have been highly tilted and then the edges exposed by erosion. Closer examina- tion, however, will generally show the falseness of this view. Fig. 159 represents a mass of slate in which three kinds of structure are
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
