THE SILURIAN BEACH. 
31 
spoke to liim of the presence of men on his desert 
island. We walk on the old geological shores, 
like Crusoe along his beach, and the footprints 
we find there tell us, too, more than we actually 
see in them. The crust of our earth is a great 
cemetery, where the rocks are tombstones oil 
which the buried dead have written their own 
epitaphs. They tell us not only who they were 
and when and where they lived, but much also of 
the circumstances under which they lived. We 
ascertain the prevalence of certain physical condi- 
tions at special epochs by the presence of animals 
and plants whose existence and maintenance re- 
quired such a state of things, more than by any 
positive knowledge respecting it. Where we find 
the remains of quadrupeds corresponding to our 
ruminating animals, we infer not only land, but 
grassy meadows also, and an extensive vegeta- 
tion ; where we find none but marine animals, 
we know the ocean must have covered the earth ; 
the remains of large reptiles, representing, though 
in gigantic size, the half aquatic, half terrestrial 
reptiles of our own period, indicate to us the ex- 
istence of spreading marshes still soaked by the 
retreating waters ; while the traces of such ani- 
mals as live now in sand and shoal waters, or in 
mud, speak to us of shelving sandy beaches and 
of mud-flats. The eye of the Trilobite tells us 
that the sun shone on the old beach where he 
