This is the sequel to "The People of the Abyss." A book about the human race reborn after a world-destroying cataclysm. A consulting engineer and his secretary wake up from a long sleep in his office in skyscraper in New York City. The building is decaying and the city is a vast forest. They started out at about 1911 and it appears that they have slept for centuries and all the cities have been destroyed. They encounter some savages who attack them forcing them to flee, eventually to Boston. During further exploration they end up in a great abyss where the sun never shines and are captured by some strange people of which one is an old partiarch speaks English. They eventually escape the abyss bringing the old man with them and promising to return to bring the others up to the surface to rebuild civilization.
Used availability for George Allan England's Out of the Abyss