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graphical methods are not applicable to it. The only division which seems applicable is a bifold one, based on lithological characters and relations, viz.: 1, the more schistose rocks, generally dark colored, and 2, the more massive rocks (granites and granite-gneisses), generally light colored. To the latter class it is proposed to restrict the name Laurentian. For the former class, the coördinate name Mareniscan is proposed, the term being derived from the name of a township (Marenisco) in Michigan.

The necessity for a group between the Archean and Cambrian has come to be generally recognized during the last decade. But to all except those engaged in the study of pre-Cambrian rocks, the names which have been used to designate this group, or parts of it, have always been confusing, because of their multiplicity, their lack of definition, and the lack of uniformity in their use. This bulletin makes clear the nomenclature which has been adopted by the survey, and sets forth the relation of the various names which have been used to designate parts of the post-Archean (as here used) and pre-Cambrian group. Whether or not those not connected with the survey agree that the nomenclature officially agreed upon is the best possible, it is to be hoped that it may be uniformly adopted in the interest of intelligibility. It has the merit of simplicity and definiteness, and of avoiding disputed questions, so far as this is possible.

To the post-Archean pre-Cambrian group is given the name Agnotozoic, or, preferably, since its fossils are becoming known Proterozoic, a term coördinate with Archean, Paleozoic, etc. Since it is impossible to divide this group into systems coördinate with Cambrian, Silurian, etc., which can be correlated with each other throughout the various areas of Proterozoic rock, the term Algonkian is used for the present as a single system term to cover the whole Proterozoic group. In many areas the group is distinctly divisible into two or three systems comparable with the Cambrian, Silurian, etc. Thus in the original Huronian area there are probably two unconformable series of rocks, the lower unconformable on the Archean, and the upper unconformable below the Cambrian. These may be correlated with some degree of confidence with the Lower and Upper Huronian of the Lake Superior region. But here a third series, the Keweenawan, intervenes between the Upper Huronian and the Cambrian, and is unconformable with both. In the Grand Cañon region again, three series are recognized. But their relation to the three series of the Lake