PART I. CHAPTER III. 35 



Arrangement of Fossils in Strata. 



terns of ripples interfering with each other ; one more ancient 

 and half effaced, and a newer one, in which the grooves and 

 ridges are more distinct, and in a different direction. This cross- 

 ing of two sets of ripples arises from a change of wind, and the 

 new direction in which the waves are thrown on the shore. 



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 tospatangus Wood bored byteredina 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 stratifica- 

 tion so far as they are determined by the arrangement of inor- 

 ganic 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, bivalve shells of some one or more species pre- 

 dominate ; at another, some univalve shell, and at a third, corals ; 

 while in some formations we find layers of vegetable matter 

 separating strata. 



It may appear inconceivable to a beginner how mountains, 

 several thousand feet thick, can have become filled with fossils 

 from top to bottom ; 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 depo- 

 sition, each separate layer was once the uppermost, and covered 

 immediately by the water in which aquatic animals lived. Each 

 stratum, in fact, however far it may now lie beneath the surface, 

 was once in the state of 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. 



