In following the play of these opposing currents of geologic thought we reach now the point where a period of brilliant progress in the knowledge of mountains and of continental structures begins in the work of J. D. Dana. In 1842 Dana returned from the Wilkes Exploring Expedition and the following year began the publication of the series of papers which for the next half century marked him as the leader in geologic theory in America. His work is of course to be judged against the background of his times. His papers mark distinct advances in many lines and are characterized throughout by breadth of conception and especially by clear and logical thinking. His work was published very largely in the Journal, of which after a few years he became chief editor. His first contribution on the subject of mountain structures, entitled “Geological results of the earth’s contraction in consequence of cooling,” was published in 1847 (=3=, 176). The evidence of horizontal pressure was first perceived in France as shown by the features of the Alps. Elie de Beaumont connected it, by means of the theory of a cooling and contracting globe, with the other large fact of the increase of temperature with descent in the crust. Dana credits the Rogers brothers with first making known the folded structures of the Appalachians, but objects to their interpretation of origin. He showed by means of diagrams that the folds are to be explained by lateral pressure, the direction of overturning indicating the direction from which the driving force proceeded. The Rogers brothers and especially James Hall, in working out the Appalachian stratigraphy, had noted that the formations, although accumulating to a maximum thickness of between 30,000 and 40,000 feet, showed evidences that the successive formations were deposited in shallow water. It suggested to them that the weight of the accumulating sediments was the cause of subsidence, each foot of sediment causing a foot of down sinking. This idea has continued to run through various text books in geology for half a century, yet Dana early saw the fallacy and in 1863 in the first edition of his Manual of Geology (p. 717) states “whether this is an actual cause or not in geological dynamics is questionable.” In 1866 in an important article on “Observations on the origins of some of the earth’s features,” Dana deals more fully and finally with this subject (=42=, 205, 252, 1866). He shows that such an effect of accumulating sediment postulates a delicate balance, a very thin crust and no resistance below. If such a weakness were granted it would be impossible for the earth to hold up mountains. Furthermore such subsidence was not regular during its progress and finally in the long course of geologic time gave place to a reverse movement of elevation.