528 THE JOURNAL OF GEOLOGY. 



graphical methods are not applicable to it. The only division which 

 seems applicable is a bifold one, based on lithological characters and 

 relations, viz.: i, 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 coordinate name Mareniscan is 

 proposed, the term being derived from the name of a township (Mare- 

 nisco) 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 defi- 

 nition, 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 desig- 

 nate 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 intelligibil- 

 ity. 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 Pro- 

 terozoic, a term coordinate with Archean, Paleozoic, etc. Since it is 

 impossible to divide this group into systems coordinate 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 sys- 

 tems comparable with the Cambrian, Silurian, etc. Thus in the orig- 

 inal Huronian area there are probably two unconformable series of 

 rocks, the lower unconformable on the Archean, and the upper uncon- 

 formable 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, inter- 

 venes between the Upper Huronian and the Cambrian, and is uncon- 

 formable with both. In the Grand Canon region again, three series 

 are recognized. But their relation to the three series of the Lake 



