If he had not had distance in mind he would not have separated the falls in this way on the map." "Then if we can find the first fall we can figure pretty nearly how far the last fall is from the head of the chasm," said Rod.
"Yes.

I believe the distance from here to the first fall will give us a key to the whole thing." Rod had produced a pencil from one of his pockets and was figuring on the smooth side of a chip.
"The gold is a long way from here at the best, Wabi.

I explored the chasm for ten miles.

Say that we find the first fall within fifteen miles.

Then, according to the map, the second fall would be about twenty miles from the first, and the third forty miles from the second.