Aqua-terrarium help


     First of all, thanks to everyone who replied about my silicone search.
 The GE brand says on the label that its OK if allowed to cure for 24 hours
before water is added.
     Next: The reason I inquired at all is because I'm setting up one of
those "aqua-terrarium;paulardium;viviquarium type of things.  I looked at
Tetra/Second Nature's Viquarium and found its inorganic form to be, excuse
me, butt ugly.  The River tank setup is much better, but I think I would
enjoy doing it myself.
     I plan to employ some small ferns in the terrestrial portion, some
hornworts (or is it liverworts) at the water's edge, and some kind of
submerged plant with low light requirements in the water (java fern or some
forgiving crypt sp.).  Perhaps I will use some Anubias as an emergent if I
can find it.
     My first stumbling block in the basic construction is the substrate.  I
dont want to use a 12" deep gravel substrate ala the Viquarium.  I was
thinking about taking some medium-sized flint rock and using silicone to make
an in-tank stand for it.
    Does anyone have any experience in dealing with the substrate on such a
       Thanks in advance,
                   Tony Thorpe.