_Mark Twain's Letters, Volume 4, 1870-1871_, p. 373 states Nast and
Clemens met in the "fall of 1867."  If you examine Nast's career,
he was in the midst of producing what was known as the "Grand
Caricaturama" on Broadway in New York.  The show opened on Broadway
on December 4, 1867 and ran for two weeks.  See the website at:

http://staging.thomasnast.com/Activities/NastCaricaturama/default.asp

Nast's huge panel portraits and caricatures were related to slavery
and the Civil War and were accompanied by an actor reading a script --
a 19th century huge slide show of sorts.  The show received less
than stellar reviews. Nast then considered taking the Caricaturama
to Washington (which did not pan out) and then he did take it to
Boston.  The time frame Nast was most likely considering Washington
possibilites is the same time frame Mark Twain reported he has been
to New York and has received an offer from Nast to "do the lecturing
for his show."  Clemens was in New York in both December 1867 and January
1868.

A reading of SLC's letter to Thomas Nast 10 years later In Nov. 1877
indicates that Twain has "two or three pictures" already in mind that
he would like Nast to produce for his own lecture.  Given the time
it would have taken Nast to have produced panoramic art work for a
Twain lecture, it's not surprising the idea went nowhere.

However, Twain was still considering the idea of illustrated lectures
(via magic lantern) decades later and wrote the script for at least one.

Barb