Charles Willard Diffin 
Two Thousand Miles Below [EPUB ebook] 

Rawson learns to his cost that the life-spark of a fabled race glows in the black heart of a dead, Western volcano. "In the grey darkness the curved fangs of a sabre-toothed tiger gleamed white and ghostly. The man-figure that stood half crouched in the mouth of the cave involuntarily shivered. "Gwanga!" he said. "He goes, too!""(Goodreads)