They unfortunately don't give sources for this remark, but Wilson and
Ferris in their
Encyclopedia of Southern Culture (1989) say that "Mark Twain objected
to the South's pretensions.
Remembering the grand, absurd village names of his youth, he chose St.
Petersburg as the name for his
fictional river town, trying to catch and satirize those grandiose
dreams of splendor."  Also, the Russian
St. Petersburg was most likely prominent in newspapers in 1968,
although 6 years prior to Tom Sawyer,
with the "St. Petersburg Declaration," when numerous countries signed
rules of war prohibiting excessive
injury to combatants.  The U.S. did not.  Surely that made an
impression with MT.  But wouldn't Sam the
river pilot in 1857 know about the Salt River St. Petersburg (from maps
or whatever),  even if it had "died"
out by 1840???

Ron O.