The World Below — S. Fowler Wright

When George is transported to a far-future Earth, he finds himself caught between two very different species of humanity--the Amphibians, dwellers of the deeps, and the giant Dwellers, rulers of the large island that constitutes the major land mass. With the help of one of the Amphibians, George goes looking for any sign of two earlier time-travelers, neither of which has returned. His adventures among the strange flora and fauna of this brave new world constitute one of Wright's most original and compelling imaginative fictions.