The term coprolite was found to be defined in many different ways, and this led to a review of the pertinent literature on the subject, with special reference to composition, origin, and geological significance of these petrified animal excrements. This review includes 63 references. Coprolites from southern Washington, collected from the bed of Salmon Creek, range from 0.5 to 10 cm in diameter. Their mineralogical and chemical composition was established by microscopic, chemical, X-ray and spectrographic analyses, and found to be almost exclusively siderite with traces of limonite. We thus have a perfect example of complete replacement, since the shapes do not leave any doubt as to their true animal excrement origin.