GLACIAL PERIOD. Each of the more important varieties of rocks and mineral species is described in separate articles; since, how ever, geologists and mineralogists have introduced an enor mous number of unnecessary names for minor varieties of rocks and minerals, as well as for the smaller subdivisions of the stratified rocks, it has not been found possible to have separate articles for all of them but in the index references are given to the places where they are described or referred to. The longer articles contain selected bibliographies of the classics of the subject and modern works in which full details will be found. For each of the major subdivisions of the stratified rocks (geo logical systems), a separate article is provided: e.g., CAMBRIAN, TRIAS,EocENE,while accounts of the geology of the principal coun tries of the world will be found in the articles on those countries.