Why would General Billy Mitchell describe Alaska in the 1930's as the "center" of the world? How could Alaska be both the "center" of the
world and relatively isolated at the same time?

The location of Alaska is in the North, meaning that it will not be as populated because of the climate there. It is also the center because on the globe you can see that Alaska is in the middle of Asia and North America.

river is shown in this type of map