It so happens that calcareous skeletons, exactly similar to the
_Globigerinae_ of the chalk, are being formed, at the present moment, by
minute living creatures, which flourish in multitudes, literally more
numerous than the sands of the sea-shore, over a large extent of that
part of the earth's surface which is covered by the ocean.
The history of the discovery of these living _Globigerinae_ and of the
part which they play in rock building, is singular enough. It is a
discovery which, like others of no less scientific importance, has
arisen, incidentally, out of work devoted to very different and
exceedingly practical interests. When men first took to the sea, they
speedily learned to look out for shoals and rocks; and the more the
burthen of their ships increased, the more imperatively necessary it
became for sailors to ascertain with precision the depth of the waters
they traversed. Out of this necessity grew the use of the lead and
sounding line; and, ultimately, marine-surveying, which is the recording
of the form of coasts and of the depth of the sea, as ascertained by the
sounding-lead, upon charts.