In Chapter 4, "Over Hill and Under Hill", I suddenly read this:
When he peeped out in the lightning-flashes, he saw that across the valley the stone-giants were out and were hurling rocks at one another for a game, and catching them, and tossing them down into the darkness where they smashed among the trees far below, or splintered into little bits with a bang.
I backtracked several pages in order to attempt to find any mention of "stone-giants" or "giants", but this seems to be the first time they were mentioned.
What "stone-giants"?! Are they giants made out of stone? Like the trolls that turned into stone? Bigger? Moving mountain-giants?
At first, I thought this was a poetic way of describing the mountains themselves and the rocks falling down from them, but the narrator seems to be talking about literal giants made out of stone? Which are just suddenly mentioned without as much as a basic description?
While there is a mention of "various horrors/dangers" in the mountains, I didn't expect them to just have giants made out of stone existing there, without some sort of explanation or history. The sentence reads just as if they had been already established, the way it says "the" stone-giants.
I searched an online copy of the book and there is no mention of the literal word "giant" before that in the same chapter or the previous one.