DEPOSITION INDICATED BY FOSSILS. 
47 
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 form¬ 
ed of Infusoria.—Chalk derived principally from Organic Bodies.—Dis¬ 
tinction of Fresh-water from Marine Formations.—Genera of Fresh-water 
and Land Shells.—Rules for recognizing Marine Testacea.—Gyrogonite 
and Chara.—Fresh-water Fishes.—^Alternation of Marine and Fresh-water 
Deposits. —^Lym-Fiord. 
Haying in the last chapter considered the forms of strati¬ 
fication so far as they tire 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 par¬ 
ticular kinds of fossils did not occur here and there at cer¬ 
tain 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 forma¬ 
tions we find layers of vegetable matter, commonly derived 
from land plants, separating strata. 
It may appear inconceivable to a beginner how mountains, 
several thousand feet thick, can have become full of fossils 
from top to bottom; but the difiiculty 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, dur¬ 
ing the process of deposition, each separate layer was once 
the uppermost, and immediately in contact with the water 
in which aquatic animals lived. Each stratum, in fact, how¬ 
ever 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 en¬ 
veloped. 
Eate of Deposition indicated by Fossils.— By attending to 
the nature of these remains, we are often enabled to deter¬ 
mine whether the deposition was slow or rapid, whether it 
took place in a deep or shallow sea, near the shore or far 
