Lost Continents. The Atlantis Theme by L. Sprague de Camp
From Wikipedia:
Lost Continents: The Atlantis Theme in History, Science, and Literature is a study by L. Sprague de Camp that provides a detailed examination of theories and speculations on Atlantis and other lost lands, including the scientific arguments against their existence. It is one of his most popular works. It was written in 1948 and first published serially in the magazine Other Worlds Science Fiction in 1952–1953; portions also appeared as articles in Astounding Science Fiction, Galaxy Science Fiction, Natural History Magazine, and the Toronto Star. It was first published in book form by Gnome Press in 1954; an updated edition was published by Dover Publications in 1970.
The shown cover is for the 1975 paperback edition by Ballantine Nonfiction (24379), featuring two styles of Eightway in chromatic use, set on a circle. Cover artist unknown.
