Geologically speaking - I suppose - the fossil has itself replaced with
sedimentary rock.

e.g. a calcium carbonate item that is turned into silicon-dioxide. The
former is soft and the latter is hard.

For example here in East Texas, the land was once a rich swampy forest.
Depending on the
time the ocean was south-east or north-west of a specific point. The
'old' ocean extended
to North Dakota....  A land of volcanoes and flash floods.

So when I spot a 'fuzzy' white log like rock - it is Palm and when
broken the center is Butterscotch.

None of it except for physical form represents the original.

Seems to me the sub-Fossil means part of or all of the shell is in the
native material.

Calcareous Limestone and shell laden Travertine are simply limestone
deposits that traps real shells
and is not a substitution process.  Normally in river areas that bring
calcium from high mountain Reefs
down into the lower valleys.  Snow melt and rain erode the reef and
builds a layer of limestone when
the river slows and allows deposition.

  A beautiful example is on the Shell Oil building in downtown Houston.
Calcareous is used extensively in the outer skin of the building and
interior walls. The shell is still a shell,
but a dead animal.

  I have seen examples of calcareous that the shell was simply a
cavity.  As I recall,
this was from Italy and might be a Marble - Heat (Volcano) treated
converting and boiling off trapped water...
This process might have powdered the shell material and any plant
material that was real or replaced leaving
the cavity.

Martin

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