
FOSSILS 101 
a wide area of fluviatile mud, dry land prevailed and eventually 
became covered with forests. Subsequently, owing probably 
to subsidence, the forest was submerged and buried under 
newer accumulations of fluviatile mud and silt. Many of the 
coal-seams of the Carboniferous period, with their underclays, 
tell a similar tale, and the same history is repeated by not 
a few of the lignites belonging to later geological periods. 
[Many coals and lignites, however, appear to represent masses 
of vegetable matter which have probably been drifted from 
the land into estuaries and shallow bays of the sea.] The 
not infrequent occurrence of arachnids, insects, lizards, and 
land-snails associated with beds of coal and lignite, is additional 
evidence of terrestrial conditions. Amber, again, is an 
abundant product of the lignite-bearing beds of Germany, 
and unquestionably represents the gum and resin which 
exuded from some of the forest trees of Tertiary times. 
(b) Lacustrine conditions—These are indicated by the 
presence of numerous freshwater molluscs and_ small 
crustaceans which are sometimes so abundant as to form 
beds of marl and limestone. Plant-remains, insects, and 
other relics of land-life, such as reptiles or mammals, often 
occur in lacustrine deposits. It is from lacustrine and 
estuarine deposits, indeed, that we obtain our fullest informa- 
tion as to the life of former land-surfaces. 
(c) Marine conditions.—Relatively deep or, at least, clear 
water is indicated by thick masses of limestone, more or less 
abundantly charged with corals, sea-lilies, and other marine 
organisms. This inference is based partly on the fact that 
these limestones are comparatively pure—that is, they 
contain relatively little insoluble matter, and this is usually 
in a very finely divided state. In short, it is evident that 
such limestones have accumulated over parts of the sea-floor 
not reached by ordinary sediment—conditions which, as a 
rule, can obtain only at a considerable distance from the 
shore, and often, therefore, in somewhat deep water. Further, 
we judge from the analogy of the present, that, as existing 
corals only flourish in clear water, their predecessors probably 
demanded similar conditions. This inference is further 
strengthened by the fact that when, towards the top of a 
bed of limestone, the rock becomes more and more impure, 
