[Illustration: Fig. 9. Sketch of ripples.] The ripple mark is usually an indication of a sea-beach, or of water from 6 to 10 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.[21-A] FOOTNOTES: [11-A] The kaolin of China consists of 71.15 parts of silex, 15.86 of alumine, 1.92 of lime, and 6.73 of water (W. Phillips, Mineralogy, p. 33.); but other porcelain clays differ materially, that of Cornwall being composed, according to Boase of nearly equal parts of silica and alumine, with 1 per cent. of magnesia. (Phil. Mag. vol. x. 1837.) [11-B] See W. Phillips's Mineralogy, "Alumine." [14-A] Consult Index to Principles of Geology, "Stratification," "Currents," "Deltas," "Water," &c. [21-A] Siau. Edin. New Phil. Journ. vol. xxxi.; and Darwin, Volc. Islands, p. 134. CHAPTER III. ARRANGEMENT OF FOSSILS IN STRATA--FRESHWATER AND MARINE. 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 and semi-opal formed of infusoria--Chalk derived principally from organic bodies--Distinction of freshwater from marine formations--Genera of freshwater and land shells--Rules for recognizing marine testacea--Gyrogonite and chara--Freshwater fishes--Alternation of marine and freshwater 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.