To expand just a bit on S E Nesbit's excellent reply... You asked why Twain
might have put "point" in parenthesis. Perhaps because the original or full
term was "bridging point" to indicate a place in a river that was suited for
a crossing because that "point" of the river was either narrow or shallow.
Such "points" were often seasonal, and a "crossing" could be nothing more
than a temporary bridge or a ferry at times during the year when the force
of the current and depth allowed. Some "points" were permanent, or nearly
so, and developed into towns (Gray's Point, So-and-so's Crossing, etc.)
where steamboats could unload and connect with land routes (before the RR),
at least until a big flood would destroy the "point" and change the river.
You also asked if a crossing was a point. I'm not sure. I've seen the terms
used interchangeably, but I suspect that a crossing is a point that is being
utilized at the moment, and that in the "off-season" it's no longer used as
a crossing, but is still a "point."
When I was twelve and spending all of my spare time on Sims Bayou in
Houston, I conspired with a couple of friends to build a crossing at a 30
foot shallows. We worked like demons, using rock pilings and "borrowed"
planks. Three weeks later a rain floated our planks away and topled most of
our pilings, so we repeated our labors. About a week later the center of our
crossing was demolished, probably by escapees from the prison farm upstream
who now and then used the bayou as an escape route. We made repairs, but
another rain finished us off for good a few months after. By that time the
only remaining rocks were too far away to drag to our "point" and the
custodian of our plank source was probably getting suspicious, so there was
no third crossing. The last time I saw that section of Sims Bayou it had
been cased in concrete and the water was filthier than ever, a vision that
would have grieved Mark Twain.
Kevin Mac Donnell