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knowledge out to its ultimate results； to have a truer； and
therefore a better； conception of this wonderful universe； and of
man's relation to it； than the most learned student who is deep…
read in the records of humanity and ignorant of those of Nature。
The language of the chalk is not hard to learn； not nearly so hard
as Latin； if you only want to get at the broad features of the
story it has to tell； and I propose that we now set to work to
spell that story out together。
We all know that if we 〃burn〃 chalk the result is quicklime。
Chalk； in fact； is a compound of carbonic acid gas； and lime； and
when you make it very hot the carbonic acid flies away and the lime
is left。
By this method of procedure we see the lime； but we do not see the
carbonic acid。 If； on the other hand； you were to powder a little
chalk and drop it into a good deal of strong vinegar； there would
be a great bubbling and fizzing； and； finally； a clear liquid； in
which no sign of chalk would appear。 Here you see the carbonic
acid in the bubbles； the lime； dissolved in the vinegar； vanishes
from sight。 There are a great many other ways of showing that
chalk is essentially nothing but carbonic acid and quicklime。
Chemists enunciate the result of all the experiments which prove
this； by stating that chalk is almost wholly composed of 〃carbonate
of lime。〃
It is desirable for us to start from the knowledge of this fact；
though it may not seem to help us very far towards what we seek。
For carbonate of lime is a widely spread substance； and is met with
under very various conditions。 All sorts of limestones are
composed of more or less pure carbonate of lime。 The crust which
is often deposited by waters which have drained through limestone
rocks； in the form of what are called stalagmites and stalactites；
is carbonate of lime。 Or； to take a more familiar example； the fur
on the inside of a tea…kettle is carbonate of lime； and； for
anything chemistry tells us to the contrary； the chalk might be a
kind of gigantic fur upon the bottom of the earth…kettle； which is
kept pretty hot below。
Let us try another method of making the chalk tell us its own
history。 To the unassisted eye chalk looks simply like a very
loose and open kind of stone。 But it is possible to grind a slice
of chalk down so thin that you can see through ituntil it is thin
enough； in fact； to be examined with any magnifying power that may
be thought desirable。 A thin slice of the fur of a kettle might be
made in the same way。 If it were examined microscopically； it
would show itself to be a more or less distinctly laminated mineral
substance and nothing more。
But the slice of chalk presents a totally different appearance when
placed under the microscope。 The general mass of it is made up of
very minute granules； but； imbedded in this matrix； are innumerable
bodies； some smaller and some larger； but； on 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 Globigerina 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 foliageproving that the mere mineral water
may； under certain conditions； assume the outward form of organic
bodiesso 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 Globigerinae than that of analogy is forthcoming。 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 depths 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 nautical purposes； scientific accuracy
could not be expected from the armed lead； and to remedy its
defects （especially when applied to sounding in great depths）
Lieut。 Brooke；＇61＇ of the American Navy； some years ago invented
a most ingenious machine； by which a considerable portion of the
superficial layer of the sea…bottom can be scooped out and brought
up from any depth to which the lead descends。
In 1853； Lieut。 Brooke obtained mud from the bottom of the North
Atlantic； between Newfoundland and the Azores； at a depth of more
than ten thousand feet； or two miles； by the help of this sounding
apparatus。 The specimens were sent for examination to Ehrenberg ＇62＇
of Berlin； and to Bailey of West Point；＇63＇ and those able
microscopists found that this deep…sea mud was almost entirely
composed of the skeletons of living organismsthe greater proportion
of these being just like the Globigerinae already known to occur
in the chalk。
Thus far； the work had been carried on simply in the interests of
science； but Lieut。 Brooke's method of sounding acquired a high
commercial value； when the enterprise of laying down the telegraph…
cable ＇64＇ between this country and the United States was under