That the vast succession of mountain ranges and elevated plateaus and valleys which go to make up the Cordilleran mountain system in the United States must be the final result of a number of orographic movements occurring at different periods of the earth’s history was recognized in the earliest geological explorations in that region by Marcou, Newberry, Le Conte, and others. It was not, however, until systematic examination of large areas, both topographical and geological, had been instituted, which permitted the construction of geological maps of a substantial degree of accuracy, that any attempt could be made to determine the number and comparative importance of these movements and their relative position in the structural history of the region. Even then the conditions under which such examinations were conducted, necessitating the covering of large areas in a given time, which time was dependent more upon the geographical . . .