394. ELEMENTARY GEOLOGY 
Physical Geography. Fossils also teach us facts 
about the past changes in land and water. The 
presence of land is shown by the fossils of certain 
creatures which habitually live near the coast, or by 
the presence of land animals and plants that must 
have drifted into the sea, and thus been buried in 
marine sediments. With marine fossils we prove the 
presence of ocean water; the existence of lakes, where 
now there is dry land, may often be told by the fossils 
of fresh-water creatures. 
Some animals live in clear, others in muddy waters. 
Certain creatures, like corals, exist where warm ocean 
currents flow; and so, by their distribution we may 
sometimes trace the extension of these great equatorial 
streams. Again, in two places not far apart, the fossils 
of the same period are quite unlike, and we feel cer- 
tain that they were separated by some barrier, as the 
Bermudas are now separated by deep sea from Cape 
Hatteras. Then finally, the species of the two zones 
begin to intermingle, and from this we know that the 
barrier has been removed. 
Still another way in which fossils help us to an 
understanding of past changes, is by determining the 
value of an unconformity (p. 521). Two sets of strata 
rest together, one on the other; but between the sets 
there is a break or unconformity, which represents a 
time of dry-land condition between the deposit of the 
