The Sedimentary or Fossiliferous Rocks form the greater portion of that part of the earth’s crust which is open to our examination, and are distinguished by the fact that they are regularly “stratified” or arranged in distinct and definite layers or “strata.”  These layers may consist of a single material, as in a block of sandstone, or they may consist of different materials.  When examined on a large scale, they are always found to consist of alternations of layers of different mineral composition.  We may examine any given area, and find in it nothing but one kind of rock—­sandstone, perhaps, or limestone.  In all cases, however, if we extend our examination sufficiently far, we shall ultimately come upon different rocks; and, as a general rule, the thickness of any particular set of beds is comparatively small, so that different kinds of rock alternate with one another in comparatively small spaces.

[Illustration:  Fig. 4.—­Sketch of Carboniferous strata at Kinghorn, in Fife, showing stratified beds (limestone and shales) surmounted by an unstratified mass of trap. (Original.)]

As regards the origin of the Sedimentary Rocks, they are for the most part “derivative” rocks, being derived from the wear and tear of pre-existent rocks.  Sometimes, however, they owe their origin to chemical or vital action, when they would more properly be spoken of simply as Aqueous Rocks.  As to their mode of deposition, we are enabled to infer that the materials which compose them have formerly been spread out by the action of water, from what we see going on every day at the mouths of our great rivers, and on a smaller scale wherever there is running water.  Every stream, where it runs into a lake or into the sea, carries with it a burden of mud, sand, and rounded pebbles, derived from the waste of the rocks which form its bed and banks.  When these materials cease to be impelled by the force of the moving water, they sink to the bottom, the heaviest pebbles, of course, sinking first, the smaller pebbles and sand next, and the finest mud last.  Ultimately, therefore, as might have been inferred upon theoretical grounds, and as is proved by practical experience, every lake becomes a receptacle for a series of stratified rocks produced by the streams flowing into it.  These deposits may vary in different parts of the lake, according as one stream brought down one kind of material and another stream contributed another material; but in all cases the materials will bear ample evidence that they were produced, sorted, and deposited by running water.  The finer beds of clay or sand will all be arranged in thicker or thinner layers or laminae; and if there are any beds of pebbles these will all be rounded or smooth, just like the water-worn pebbles of any brook-course.  In all probability, also, we should find in some of the beds the remains of fresh-water shells or plants or other organisms which inhabited the lake at the time these beds were being deposited.