Martin,
Geode was a poor term for Bob's hunk of limestone with a cavity in it.
Some geodes (as from Oregon?) do look like brown lumps of mud with silica
inside.  Geodes form in cavities in different types of rocks (mostly
basalts and tuffs but even some limestones) and usually have quartz
crystals lining the inside, but may have calcite and/or a number of
different minerals.  Bob's limestone formed as calcareous mud on an
ancient sea floor, which happened to have a marine snail trapped in the
mud.  The shell remained mostly empty when buried.  When the mud
consolidated as a rock, the fluids from the mud first dissolved the
shell, leaving a cavity ( a mold of the outside of the shell), then
precipitated some fine calcite crystals in the cavity.  A geode it is
not.  I have a BS in geology and a Master's degree in paleontology, if
that is supposed to help.

On Sat, 5 Dec 2009 14:05:18 -0600 "Martin H. Eastburn"
<[log in to unmask]> writes:
> I have a minor in Geology so I can tell that the Geodes are mud
> balls.
> They are 'petrified' mud.  Somewhat like the Redwoods  that turned
> into
> the petrified forest in Arizona.  Snails love to be around muddy
> areas
> and get caught up in a flood of one kind or another.  Then there is
> a
> replacement
> phase.  Internally might be calcite or quartz.  I'm surprised more
> are not
> found on a regular basis along with other small creatures.
>
> It is hard to believe these 'hard' minerals dissolve in water and
> redeposit
> elsewhere.  To bad we can't reverse it - but our creatures would
> dissolve as well.
>
> Martin
>
>
>
> Allen Aigen wrote:
> > I am sure that you can get better information, but since no one
> else
> > touched it, I will try.  First, it is probably Paleozoic.  The
> > Pleurotomariaceans (slit shells) were often dominant in
> gastropod
> > faunas.  But that means there are many genera to choose from,
> although
> > the long slit is not too common.  Secondly you need to make a cast
> of the
> > natural mold that you exposed.  If the surface is well preserved,
> and not
> > too obscured by the crystals inside or by recrystalization of the
> > surrounding aragonite mud to calcite (limestone) outside, you can
> have a
> > nice representation of the shell in rubber.  (Probably the easiest
> way is
> > to convince your dentist to mix up some of the material he makes
> > impressions with and squeeze it carefully into the molds to
> minimize air
> > bubbles.)  Then you can start with the Treatise volume on
> Gastropods and
> > work from there.  You may also want to bring the rock to a local
> college
> > with a geology program as they may be familiar with the formation
> it came
> > from, which can greatly help narrow down your search.  Good luck!
> >
> >
> > On Fri, 4 Dec 2009 12:30:31 +1100 Bob Dayle <[log in to unmask]>
> writes:
> >
> >> For any interested:
> >>
> >> I picked up a rock a couple of days ago while walking on an
> >> abandoned railroad
> >> grade. It had a different color and more 'texture' than a lot of
> the
> >> grey-blue
> >> ancient sea bottom variety, of which I have cracked many.
> >>
> >> This one weighs about a kilogram. It fractured into four pieces
> on
> >> the second
> >> 'tap' with a six-pound (~2.4 kilogram) sledge hammer. I found a
> >> geode inside,
> >> just as I had hoped; however, the shape was hardly expected.
> >>
> >> I have posted a page with photos. (Pardon the lack of image
> quality.
> >> I just
> >> received this USB microscope and I am still getting the hang of
> >> using it.)
> >>
> >> The page is at:
> >>
> >> http://cowry.org/research/snailcast.htm
> >>
> >> If anyone is a fossil geek and has an idea as to the type of
> snail
> >> shell that
> >> caused it, please drop me a line.
> >>
> >> Bob Dayle
> >> webmaster - http://cowry.org
> >>
> >>
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> --
> Martin H. Eastburn
> @ home at Lions' Lair with our computer lionslair at consolidated
> dot net
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