Modern stratigraphical geologists, both those who study the Palæozoic and those who are occupied with the Neozoic rocks, are largely concerned with the establishment of zones;inother words, the zonal method of subdivision has been recognized as the best means of correlating the stratigraphical succession in one area with that of another. But when it comes to giving an exact definition of a zone there does not appear to be the same unanimity of opinion, and consequently it seems desirable that some wellconsidered definition of a zone should be put forward and discussed.