[quote="ian mckenzie"many of their speleothems are formed of opal.[/quote]
Most of them are formed of opal and they are colonies of living microorganisms metabolise quartz a produce opal. Really another world.
There are another 2 books about tepuis and caves there:
and only printed: Entraňas del Mundo Perdido, authors: Charles Brewer Carias & Marek Audy (Spanish)