From Wikipedia
Back to the Stone Age is a novel written by Edgar Rice Burroughs, the fifth in his series set in the interior world of Pellucidar. It first appeared as a six-part serial in Argosy Weekly from January 9 to February 13, 1937 under the title "Seven Worlds to Conquer." It was first published in book form in hardcover by Edgar Rice Burroughs, Inc. in September, 1937 under the present title, and has been reissued a number of times since by various publishers. Read more - Shopping-Enabled Wikipedia on Amazon
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Product Description
The fifth installment of Edgar Rice Burroughsβs Pellucidar series, Back to the Stone Age recounts the strange adventures of Lieutenant von Horst, a member of the original crew that sailed to Pellucidar with Jason Gridley and Tarzan who is left behind in the inner world. Von Horst wanders friendless and alone from one danger to the next among the Stone Age peoples, mighty reptiles, and huge animals that have been extinct on the outer crust for thousands of years. But woven among the tales of savage cave men in the country of the Basti, the hideous Gorbuses in the caverns beneath the Forest of Death, and the terrible Gaz is the story of the love this cultured hero feels for a barbarian slave girl who has spurned and discouraged him, working instead toward her own mysterious goal.