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
"Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer
TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
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