Ripple-mark.—The ripple-mark, so common on the surface of sandstones of all ages (see Fig. 8), and which is so often seen on the sea-shore at low tide, seems to originate in the drifting of materials along the bottom of the water, in a manner very similar to that which may explain the inclined layers above described. This ripple is not entirely confined to the beach between high and low water mark, but is also produced on sands which are constantly covered by water. Similar undulating ridges and furrows may also be sometimes seen on the surface of drift snow and blown sand. The ripple-mark is usually an indication of a sea-beach, or of water from six to ten feet deep, for the agitation caused by waves even during storms extends to a very slight depth. To this rule, however, there are some exceptions, and recent ripple-marks have been observed at the depth of 60 or 70 feet. It has also been ascertained that currents or large bodies of water in motion may disturb mud and sand at the depth of 300 or even 450 feet.[5] Beach ripple, however, may usually be distinguished from current ripple by frequent changes in its direction. In a slab of sandstone, not more than an inch thick, the furrows or ridges of an ancient ripple may often be seen in several successive laminæ to run towards different points of the compass. [1] W. Phillips, Mineralogy, p.33. [2] Phil. Mag., vol. x, 1837. [3] See W. Phillips’s Mineralogy, “Alumine.” [4] Consult Index to Principles of Geology, “Stratification,” “Currents,” “Deltas,” “Water,” etc. [5] Darwin, Volcanic Islands, p. 134. CHAPTER III. ARRANGEMENT OF FOSSILS IN STRATA.—FRESH-WATER AND MARINE FOSSILS. Successive Deposition indicated by Fossils. — Limestones formed of Corals and Shells. — Proofs of gradual Increase of Strata derived from Fossils. — Serpula attached to Spatangus. — Wood bored by Teredina. — Tripoli formed of Infusoria. — Chalk derived principally from Organic Bodies. — Distinction of Fresh-water from Marine Formations. — Genera of Fresh-water and Land Shells. — Rules for recognising Marine Testacea. — Gyrogonite and Chara. — Fresh-water Fishes. — Alternation of Marine and Fresh-water Deposits. — Lym-Fiord. Having in the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic remains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if particular kinds of fossils did not occur here and there at certain depths in the mass. At one level, for example, univalve shells of some one or more species predominate; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating strata.