In Cooperstown, New York there is a museum that still displays the so-called
"Cardiff Giant," a large stone which caused quite a stir in the 19th
century.
 I've seen it, but I don't remember too many precise details.  It may be
what
Twain was talking about; I think it was about the right time (and if so, he
was of course spending summers in nearby Elmira).  Perhaps there was another
petrified giant too, but you might try calling around in Cooperstown.  I
think it was the Farmer's Museum there; the local C of C or tourist folks
could tell you, or perhaps even a net search.

Dr. Scott Dalrymple
Southwestern College (Kansas)