For Twain's SF, check into some of the posthumous works.  Things like _The
Great Dark_, _3000 Years Among the Microbes_, and _Which Was It?_ all come
to
mind as does _The Edypus Papers_, a work similar in many ways to _A Canticle
for Libowitz_.  I'm not sure where your definition of SF ends and fantasy
begins, but some of these works seem to me to be easily classified as SF.

Richard Tuerk
East Texas State University