a rough average, not more than a
hundredth of an inch in diameter, having a well-defined shape and
structure. A cubic inch of some specimens of chalk may contain
hundreds of thousands of these bodies, compacted together with
incalculable millions of the granules.
The examination of a transparent slice gives a good notion of the
manner in which the components of the chalk are arranged, and of their
relative proportions. But, by rubbing up some chalk with a brush in
water and then pouring off the milky fluid, so as to obtain sediments of
different degrees of fineness, the granules and the minute rounded
bodies may be pretty well separated from one another, and submitted to
microscopic examination, either as opaque or as transparent objects. By
combining the views obtained in these various methods, each of the
rounded bodies may be proved to be a beautifully-constructed
calcareous fabric, made up of a number of chambers, communicating
freely with one another. The chambered bodies are of various forms.
One of the commonest is something like a badly-grown raspberry,
being formed of a number of nearly globular chambers of different
sizes congregated together. It is called Globigerina, and some
specimens of chalk consist of little else than Globigerinoe and granules.
Let us fix our attention upon the Globigerina. It is the spoor of the
game we are tracking. If we can learn what it is and what are the
conditions of its existence, we shall see our way to the origin and past
history of the chalk.
A suggestion which may naturally enough present itself is, that these
curious bodies are the result of some process of aggregation which has
taken place in the carbonate of lime; that, just as in winter, the rime on
our windows simulates the most delicate and elegantly arborescent
foliage--proving that the mere mineral water may, under certain
conditions, assume the outward form of organic bodies--so this mineral
substance, carbonate of lime, hidden away in the bowels of the earth,
has taken the shape of these chambered bodies. I am not raising a
merely fanciful and unreal objection. Very learned men, in former days,
have even entertained the notion that all the formed things found in
rocks are of this nature; and if no such conception is at present held to
be admissible, it is because long and varied experience has now shown
that mineral matter never does assume the form and structure we find in
fossils. If any one were to try to persuade you that an oyster-shell
(which is also chiefly composed of carbonate of lime) had crystallized
out of sea-water, I suppose you would laugh at the absurdity. Your
laughter would be justified by the fact that all experience tends to show
that oyster-shells are formed by the agency of oysters, and in no other
way. And if there were no better reasons, we should be justified, on like
grounds, in believing that Globigerina is not the product of anything
but vital activity.
Happily, however, better evidence in proof of the organic nature of the
Globigerinoe than that of analogy is forthcoming. It so happens that
calcareous skeletons, exactly similar to the Globigerinoe 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 Globigerinoe, 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.
At the same time, it became desirable to ascertain and to indicate the
nature of the sea-bottom, since this circumstance greatly affects its
goodness as holding ground for anchors. Some ingenious tar, whose
name deserves a better fate than the oblivion into which it has fallen,
attained this object by "arming" the bottom of the lead with a lump of
grease, to which more or less of the sand or mud, or broken shells, as
the case might be, adhered, and was brought to the surface. But,
however well adapted such an apparatus might be for rough
Continue reading on your phone by scaning this QR Code
Tip: The current page has been bookmarked automatically. If you wish to continue reading later, just open the
Dertz Homepage, and click on the 'continue reading' link at the bottom of the page.