subaqueous deposits; but the comparative number of individuals 
whose remains became embedded even in such deposits as those 
which arc formed at the bottom of fresh water lakes, or estuarine 
deltas, must always have been infinitesimally small. Were our 
knowledge of the present mammalia of Europe confined to 
information we could gain by dredging in existing lakes or 
estuaries, or by investigating the deltas of our present rivers, it 
would be very imperfect indeed ; and it would bo still more hopeless 
to search for modern mammalian remains in the North Sea or the 
English Channel. But when we remember that no sedimentary 
deposit lias ever been formed except of materials which have been 
denuded from pre-existing formations, that the formation of ono 
deposit must always be the exact equivalent of the destruction 
of another, and that consequently the present constituents of 
existing strata (excepting those directly derived from igneous 
rocks) have been arranged and re arranged times without number, 
during the long ages which geology reveals to us, it will be obvious 
that the strata which now compose the earth’s crust must bo 
the merest fragment of the total thickness of sedimentary 
rocks which have formerly existed. And it is those deposits 
which are most likely to contain the remains of terrestrial 
quadrupeds which have been most exposed to the different 
denuding agencies. 
Further, what a large proportion of the strata which have escaped 
destruction is inaccessible to us. The geological records are 
not carefully arranged like books in a well-ordered library, 
where we can put our hand at once on any volume we need. 
They may rather be compared to the mounds of Nineveh, 
which contain what the ravages of time have left to us of 
the historical recoids of Assyria; but these records are buried 
beneath the accumulated rubbish of centuries, and we are 
thankful if the chance discovery of a tablet happens to refer to 
the special subject in which wc arc interested. 
Take the chalk, for example, with which wc are more familiar 
than wc are with deposits further from our county. It is often said 
that the Norfolk subsoil is chalk : most of our geological maps show 
