A children's book about a witch on a glass mountain?

Anyone remember a book about a witch that lived on a glass mountain?
I read it back in the late seventies early eighties and loved it, would like to read it to my neice!
I think the witch was ‘rescued’ by one (maybe two) small girls that climbed up the mountain to see her.
I think the cover illustration showed the witch atop the mountain on a rocking chair.