Twain-L subscribers;

I signed on just yesterday and hope someone can point me in the right
direction.
I have been looking into the controversy surrounding the Norse discovery of
America
and have run into a problem tracking down a quote by Twain.  It is found on
page 215 of the "Vinland Map and Tartar Relation" (Skelton, Marston and
Painter. Yale Uni. Press, 1965, re-issued in 1996).

R.A. Skelton, the author of this portion of the book, describes a few of
the many obstacles researchers run into when they study this subject.  He
goes on to say there is, "....a certain air of unreality to much of the
debate," and to drive the point home he includes this footnote;

"In Mark Twain's words, "the researches of many commentators have already
thrown much darkness on this subject, and it is probable that, if they
continue, we shall soon know nothing at all about it."

When and where did Twain say this?  Did he actually write anything on the
Norse discovery of America, or did he say this about a different subject
altogether?
Unfortunately the source for the quote is not given in the bibliography.  I
have searched through more than a few books by or about Twain but have come
up empty-handed. Any help would be greatly appreciated.

Thanks,
Neil Good