The Cretaceous-Eocene boundary, or Laramie problem, is one of the outstanding questions of North American geology, and one in which the writers have become deeply interested as the result of nine and six field seasons, respectively, spent as members of the U. S. Geological Survey parties in Montana, Dakota, and Wyoming areas. The intimate knowledge of local stratigraphy thus gained, through coal or oil and gas work in a number of fields, has afforded a background for extensive reconnaissance studies carried out in the spring and autumn of 1923, during which critical sections in Montana and Dakota areas were reexamined, and the results obtained by earlier workers in local and more or less detached fields were interpreted and integrated. In this work an earnest effort has been made to apply all practicable checks, to the end that the conclusions reached may be as free as possible from coloring by . . .