In 1898 I made a reconnaissance of the pre-Cambrian formations of the Belt mountains of Montana and published the results.* The section extended from the unconformable Cambrian above to Archean complex at the base. In 1900 I crossed the Belt mountains and endeavored to trace the connection, north of Helena, of the Belt terrane and the pre-Cambrian rocks of the Rocky Mountain “front” at Lewis and Clark pass and north on the south fork of the Dearborn river. It was evident that a great series of strata extended westward beneath the Cambrian that was in general similar to the Belt terrane, but quite different in detail. In 1901, at my request, Mr Bailey Willis studied the Front ranges of the Rocky mountains in northern Montana and found a great series of strata referred to the Algonkian.† In the survey of the Cœur d’Alene mining district of Idaho Mr F. . . .