The numerous problems involved in the scientific classification and nomenclature of rock units have long engaged the attention of geologists. The appended partial list of papers dealing with this subject sufficiently indicates its scope and importance. Gradually, certain guiding principles have come to be recognized generally, and practices deemed good in the classification and nomenclature of rock units have become fairly well established. There is much room, however, for development of the principles and especially for the more widespread application of what seem to be the best practices in using the principles. The larger geological surveys have, necessarily, given consideration to the formulation of rules to provide for their own publications a comparative uniformity in the method of classifying and naming the rock units treated by them. But such rules have not been propounded and widely disseminated with intent to promote their use by geologists in general. . . .