Note: One of two "Twayne Science-Fiction Triplets" being a novel-length book made up of a short description of an imaginary world by a scientist followed by three novelettes based on that world. "Editor's Note" in Cole Checklist: "Although there was no Editor's byline given in The Petrified Planet and Witches Three the publishers stated that Fletcher Pratt can be considered the editor of both collections."

Synopsis: Dr. Clark's setting is a planet named Uller the second planet of Beta Hydri, a G-type star 21 light years from earth. It has both silicone and carbon-based life, both with quartz shells. He also throws in another planet with flourine-based life.