The Ramsay book cited by Barb is helpful for "crossing." As a navigation
term it means exactly that, and the examples cited by Ramsay indicate some
crossings were easily seen by the ripples in the current while other
required complicated mapping on the navigation charts. "Crossing-marks" were
used to show the location of those not so easily seen from the surface
behavior of the water. But because the channel location changed (along with
points) they are closely related as navigation terms because they tend to
occur (and change) in conjunction with each other. But the geographical and
navigational meaning of "point" is very nearly the same; the meanings for
"crossing" are not.
Thanks go to Barb for citing Robert Ramsay's work. I have a dozen
steamboating reference books, most with glossaries, but they do not all
agree, and they certainly do not all agree with Ramsay, who provides
numerous citations providing the context in which Twain used the terms.
Steamboat jargon was the subject of hot debate in Twain's day. I have his
own copy of a steamboating novel in which he wrote withering critical
comments on the misuse of the jargon on nearly every page in the first 30
pages of the book, and I've read a contemporary steamboating history that
regarded Twain as ill-informed and incompetent.
Kevin Mac Donnell