Ch. IK] GRADUAL DEPOSITION INDICATED BY FOSSILS. 21 



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.* 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 an- 

 cient ripple may often be seen in several successive laminae to run to- 

 wards different points of the compass. 



CHAPTER DX 



ARRANGEMENT OP 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 in- 

 fusoria — Chalk derived principally from organic bodies — Distinction of fresh- 

 water 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 firom land plants, separating strata. 



It may appear inconceivable to a beginner how mountains, several 

 thousand feet thick, can have become filled with fossils from top to bot- 

 tom ; but the difficulty is removed, when he reflects on the origin of 

 stratification, as explained in the last chapter, and allows sufficient time 

 for the accumulation of sediment. He must never lose sight of the fact 

 that, during the process of deposition, each separate layer was once the 

 uppermost, and covered immediately by the water in which aquatic ani- 

 mals lived. Each stratum in fact, however far it may now lie beneath the 

 surface, was once in the state of shingle, or loose sand or soft mud at the 

 bottom of the sea, in which shells and other bodies easily became enveloped. 



By attending to the nature of these remains, we are often enabled to 

 determine whether the deposition was slow or rapid, whether it took 

 place in a deep or shallow sea, near the shore or far from land, and 

 whether the water was salt, brackish, or fresh. Some limestones consist 



* Edin. New PhiL Journ. vol. xxxi.; and Darwin, Vole. Islands, p. 134. 



