Twain begins his indication of frustration with those who tried to use extrapolation to predict anything about the river by citing statistics that showed the lower Mississippi River had "shortened itself two hundred and forty-two miles" over a period of one hundred seventy-six years. Twain creates an average rate of change based on this distance and time ("a trifle over a mile and a third per year"), then uses extrapolation to draw the ridiculous conclusion that
just a million years ago next November, the Lower Mississippi was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-pole.
First of all, this is an impossible distance - the river would circle the globe many times over if it were that long. Secondly, Twain is mocking those who claim to be capable of measuring a span of time so specifically as to be able to pinpoint it to the month. Finally, Twain spends a great deal of time detailing the numerous ways in which the Mississippi constantly changed. Extrapolation of any sort was pointless - the pilots needed to know the basic shape of the river intimately, but then had to be able to observe and alter their course in response to the continual changes of the river banks, bottoms, and obstacles.