. Elements of geology : a text-book for colleges and for the general reader. Geology. STRUCTURE AND POSITION. 171. -Sections of horizontal and inclined strata: ss, sandstone; sh, shale: Ls, limestone. again into still smaller lamina?. These terms are purely relative, and are therefore somewhat loosely used. Usually, however, the term stratum refers to the mineralogical character; the term layer to sub- divisions of a stratum distinguishable by difference of color or fine- ness ; and the term lamina to those smallest subdivisions, evi- dently produced by the sorting power of water. For instance
. Elements of geology : a text-book for colleges and for the general reader. Geology. STRUCTURE AND POSITION. 171. -Sections of horizontal and inclined strata: ss, sandstone; sh, shale: Ls, limestone. again into still smaller lamina?. These terms are purely relative, and are therefore somewhat loosely used. Usually, however, the term stratum refers to the mineralogical character; the term layer to sub- divisions of a stratum distinguishable by difference of color or fine- ness ; and the term lamina to those smallest subdivisions, evi- dently produced by the sorting power of water. For instance, in the annexed figure we have three strata of sandstone, clay, and limestone, each divisible into two layers diif ering in fineness or com- pactness of the material, and all finely laminated by the sorting power of water. The lamination, however, is not represented, ex- Fig. ias cept in the clay stratum, sh. There is another structure represented in the figure—viz., the cross fractures or joints. These, however, are not peculiar to stratified rocks, and will be discussed at another time. Extent and Thickness.—Probably nine-tenths of the surface of the land, and, of course, the whole of the sea-bottom, are covered with stratified rocks. Even where these are wanting it is because they have been removed by erosion or else covered up and concealed by fused matter outpoured on the surface. This proves that every portion of the surface of the earth has been at some time covered with water. The extreme thickness of stratified rocks is certainly not less than twenty miles; the average thickness is probably several miles. Kinds of Stratified Rocks.—Stratified rocks are of three kinds, and their mixtures, viz., arenaceous or sand rocks, argillaceous or clay rocks, and calcareous or lime rocks. Arenaceous rocks, in their inco- herent state, are sand, gravel, shingle, rubble, etc., and in their com- pacted state are sandstones, gritstones, conglomerates, and breccias. Conglomerates
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