At which point in Edgar Rice Burroughs Tarzan series does the series change from "Adventure fiction" to "Science fiction"?

Penned before there was a Science fiction genre, the first few of the 24 Tarzan books written by Edgar Rice Burroughs were tales of adventure. As the series went on, Tarzan began dealing with peoples and inventions the world still has yet to see. By today's standards, at what point in ERB's Tarzan book series does the series change from "Adventure fiction" to "Science fiction"?