The geologist dealing with stratified rocks has two quite distinct tasks before him. He is called upon to distinguish, define, and name the various rock formations. For this task he has small need of help from the paleontologist. His second task, however, is more difficult—that is, the classification and correlation of his formations after they have been defined. For this task the work of the paleontologist is in most cases indispensable. The order of sequence may be a means of classifying formations in a continuous section, but so soon as the continuity is broken, either by faulting or by concealment of the rocks under surficial cover, fossils are required for identification of the particular division of one section which may be repeated in the next. Formations differ, from place to place, in the various characters by which they are defined, and because likeness of . . .