I read this book ages ago, probably in the 60s:
Two young men go back in time to prehistoric times. I can't remember how they got there, but they meet up with a young man from a tribe of people and live with them. There are some politics going on, with a not-so-altruistic leader who is trying to take over. I only remember that this bad guy was killed when a volcano erupted and he was hit by a flaming, flying boulder.
I know it's not a lot to go on, but I do remember that the boy from the tribe said the leader would become rich if there was a war and the modern boys said this was still true in our times.