How sea shells become fossils enclosed in rock
Floods bury marine animals in mud that later turns to stone, leaving shells caught between two layers
A page of continuous text, without diagrams, setting out Leonardo's account of how marine shells came to be petrified in mountain rock. He explains that river floods, thick with fine mud, buried shellfish living near the sea; the animals died, and as the sea lowered and the salt water drained, the mud hardened into stone, filling the empty shells. The shells were thus caught between two masses of stone, and he notes that older, harder shells kept their natural surface while young ones were largely calcined and penetrated by the petrifying humour.
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Shelled animals of innumerable kinds
A heading naming the animals that carry their bones outside: whelks, snails, oysters, clams, pond-snails and the like, said to be of innumerable species. These are the creatures whose fossilised shells the following argument concerns.
Imprinted and petrified shells lose their outer figure
A marginal note observing that shells which have been imprinted and petrified no longer carry the surface figure that their original shell once had.
Mud, drained of salt water, hardens around the shells
When turbid floods discharged fine mud over shellfish near the shore, the animals were buried and died. As the sea lowered and the salt water drained, the mud turned to stone and filled the emptied shells; the mud inside slightly opened shells joined the surrounding mud and also became stone, so the shells remained trapped between the two masses. Older, hardened shells kept their natural surface, while young calcined ones were penetrated by the petrifying humour.