As to those who say that shells existed for a long time and were
born at a distance from the sea, from the nature of the place and of
the cycles, which can influence a place to produce such
creatures--to them it may be answered: such an influence could not
place the animals all on one line, except those of the same sort and
age; and not the old with the young, nor some with an operculum and
others without their operculum, nor some broken and others whole,
nor some filled with sea-sand and large and small fragments of other
shells inside the whole shells which remained open; nor the claws of
crabs without the rest of their bodies; nor the shells of other
species stuck on to them like animals which have moved about on
them; since the traces of their track still remain, on the outside,
after the manner of worms in the wood which they ate into. Nor would
there be found among them the bones and teeth of fish which some
call arrows and others serpents' tongues, nor would so many
[Footnote: I. Scilla argued against this hypothesis, which was still
accepted in his days; see: _La vana Speculazione, Napoli_ 1670.]
portions of various animals be found all together if they had not
been thrown on the sea shore. And the deluge cannot have carried
them there, because things that are heavier than water do not float
on the water. But these things could not be at so great a height if
they had not been carried there by the water, such a thing being
impossible from their weight. In places where the valleys have not
been filled with salt sea water shells are never to be seen; as is
plainly visible in the great valley of the Arno above Gonfolina; a
rock formerly united to Monte Albano, in the form of a very high
bank which kept the river pent up, in such a way that before it
could flow into the sea, which was afterwards at its foot, it formed
two great lakes; of which the first was where we now see the city of
Florence together with Prato and Pistoia, and Monte Albano. It
followed the rest of its bank as far as where Serravalle now stands.
>From the Val d'Arno upwards, as far as Arezzo, another lake was
formed, which discharged its waters into the former lake. It was
closed at about the spot where now we see Girone, and occupied the
whole of that valley above for a distance of 40 miles in length.
This valley received on its bottom all the soil brought down by the
turbid waters. And this is still to be seen at the foot of Prato
Magno; it there lies very high where the rivers have not worn it
away. Across this land are to be seen the deep cuts of the rivers
that have passed there, falling from the great mountain of Prato
Magno; in these cuts there are no vestiges of any shells or of
marine soil. This lake was joined with that of Perugia [Footnote:
See PI. CXIII.]

A great quantity of shells are to be seen where the rivers flow into
the sea, because on such shores the waters are not so salt owing to
the admixture of the fresh water, which is poured into it. Evidence
of this is to be seen where, of old, the Appenines poured their
rivers into the Adriatic sea; for there in most places great
quantities of shells are to be found, among the mountains, together
with bluish marine clay; and all the rocks which are torn off in
such places are full of shells. The same may be observed to have
been done by the Arno when it fell from the rock of Gonfolina into
the sea, which was not so very far below; for at that time it was
higher than the top of San Miniato al Tedesco, since at the highest
summit of this the shores may be seen full of shells and oysters
within its flanks. The shells did not extend towards Val di Nievole,
because the fresh waters of the Arno did not extend so far.

That the shells were not carried away from the sea by the deluge,
because the waters which came from the earth although they drew the
sea towards the earth, were those which struck its depths; because
the water which goes down from the earth, has a stronger current
than that of the sea, and in consequence is more powerful, and it
enters beneath the sea water and stirs the depths and carries with
it all sorts of movable objects which are to be found in the earth,
such as the above-mentioned shells and other similar things. And in
proportion as the water which comes from the land is muddier than
sea water it is stronger and heavier than this; therefore I see no
way of getting the said shells so far in land, unless they had been
born there. If you were to tell me that the river Loire [Footnote:
Leonardo has written Era instead of Loera or Loira--perhaps under
the mistaken idea that _Lo_ was an article.],which traverses France
covers when the sea rises more than eighty miles of country, because
it is a district of vast plains, and the sea rises about 20 braccia,
and shells are found in this plain at the distance of 80 miles from
the sea; here I answer that the flow and ebb in our Mediterranean
Sea does not vary so much; for at Genoa it does not rise at all, and
at Venice but little, and very little in Africa; and where it varies
little it covers but little of the country.

The course of the water of a river always rises higher in a place
where the current is impeded; it behaves as it does where it is
reduced in width to pass under the arches of a bridge.

Further researches (989-991).