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I read this in grade school so think the book was probably from the 1960s or 1970s.
The (female) protagonist would black out and go back in time. I remember druids and Stonehenge and also there was a deaf/mute girl.
any chance that it was "A Swiftly Tilting Planet" by Madeline L'Engle.
It looks like you can purchase it for slightly more than shipping on Amazon. (link)
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