The Lost Continent (Beyond Thirty) - Edgar Rice Burroughs

Originally published under the title of Beyond Thirty.

The novel, set in the year 2137, was heavily influenced by the events of World War I. In the future world depicted in the novel, Europe has descended into barbarism while an isolationist Western Hemisphere remains sheltered from the destruction. The title Beyond Thirty refers to the degree of longitude that inhabitants of the Western Hemisphere are forbidden to pass. (Summary from Wikipedia)