Read "Dorothy and the Wizard in Oz" by L. Frank Baum online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
When they came to the mountain it proved to be a rugged, towering chunk of deep green glass, and looked dismal and forbidding in the extreme. Halfway up the steep was a yawning cave, black as night beyond the point where the rainbow rays of the colored suns reached into it.
The Mangaboos drove the horse and the kitten and the piglets into this dark hole and then, having pushed the buggy in after them -- for it seemed some of them had dragged it all the way from the domed hall -- they began to pile big glass rocks within the entrance, so that the prisoners could not get out again.