I was recently reminded of a YA novel I read as a child in the 1970s, and I'm curious as to what book it is. I read it in Danish, but I'm pretty sure it was translated, probably from English. The translated book was new at the time, but the original may have been older.
The novel was a historical steampunkish novel (ca. 1880-1920) about a boy stowing away on a ship (or possibly taken as a captive). There was a girl who may have been a few years older, and I seem to recall she had a green dress or green eyes (though I may be mistaken).
The fantastical element was at the ending. The villains who kept the boy and girl prisoner were involved in some dastardly plot and had their base on a deserted island in a valley that was the caldera of an inactive volcano. The path down to the crater floor was lined with ancient statues made by a lost civilization, sort of like Easter Island. At the end the boy topples the topmost statue, starting a domino effect where all the statues crash into the crater floor, cracking it, and restarting the volcano.
That's about all I remember. I hope it rings somebody's bell. My guess is that the book is British rather than American, but again, I might be mistaken