The beds or layers which form the crust of the earth are divided into three classes: (1) Sedimentary, or stratified; (2) Igneous, or unstratified; (3) Metamorphic, or transformed.
SEDIMENTARY OR
STRATIFIED ROCKS
Sedimentary rocks are such as give evidence of having been formed by successive deposits of sediment in water. They include sandstones or freestones, limestones, clays, etc. The material for these must have been derived from some original source, and in many instances this may be traced to the disintegration of older rocks. Thus gneiss appears to be formed by the disintegration of granite. The great class of sedimentary rocks may be divided into three smaller divisions. These divisions, with the chief rocks of each division, may be tabulated as follows:
(a) Mechanically formed rocks from detrital sediments: Conglomerates, sandstones, clay, and shale.
(b) Organically formed rocks from animal and plant remains: Limestones, chalk, coral, peat, and coal.
(c) Chemically formed rocks from material once in solution: Limestones, stalactites, gypsum, rock-salt and sinter.
Most of the stratified rocks contain fossils; and since each group contains certain kinds peculiar to itself, it is by means of these organic remains that their relative ages have been determined.
Although the lowest stratified rocks are more ancient than those which have been deposited above them, the layers or beds do not always retain a horizontal position. Were such the case, it could only be by deep cuttings that we should arrive at the older strata. We however find that, owing to some convulsion of nature, stratified rocks have been thrown out of their original position, and thus crop out to the surface. Not only is facility thus afforded us to become acquainted with the nature of the lower rocks, but many of the most valuable products of the earth are by this means rendered accessible to man.
HOW THE HISTORY OF THE EARTH IS EMBEDDED IN THE ROCKS