I've searched all the Mark Twain texts on my computer and can't find
anything that closely resembles the quote that Alan is asking about.
However, that passage does have an atmospheric resemblance to this exchange
between Horace Bixby and the cub in chapter 8 of _Life on the Mississippi_:

"How do you follow a hall at home in the dark? Because you know the shape of
it. You can't see it."

"Do you mean to say that I've got to know all the million trifling
variations of shape in the banks of this interminable river as well as I
know the shape of the front hall at home?"

"On my honor, you've got to know them better than any man ever did know the
shapes of the halls in his own house."

"I wish I was dead!"