MAJOR DIVISIONS OF THE PRE-CAMBRIAN 15 



nents is worked out, when we know more about paleogeography, when we 

 learn how extensive are the great unconformities, we may be able to 

 correlate pre-Cambrian rock series from province to province and even 

 from continent to continent, but as yet this can not be safely done. Be- 

 cause of this fact, it was decided that only a single period should be recog- 

 nized as equivalent to the Proterozoic, and for this place the term Algon- 

 kian was proposed and accepted. Who proposed the term Algonkian the 

 record does not show. It is not a competitor of Keweenawan or Huro- 

 nian, for it includes them both ; and it has never been proposed to define 

 either of these terms for general use to cover the essentially sedimentary 

 pre-Cambrian series of the entire earth. While Algonkian is given a 

 systemic position upon the atlas of the United States, its definition enti- 

 tles it to rank as a group, and this rank is recognized in this paper. 



At the same conference at which the term Algonkian was adopted for 

 llie upper group of tlie pre-Cambrian, it was proposed to use the term 

 Archean for the basement complex below. On the atlas sheets of the 

 Geological Survey this term was assigned a systemic value, but like Algon- 

 kian it is here recognized as representing a group. This definition of the 

 term Archean has been objected to by a number of geologists, because 

 before it was made the term had been very generally used to indicate all 

 pre-Cambrian rocks. The difficulty with this use of the term is that 

 under it two rock groups are placed together which are fundamentall}' 

 different. The Algonkian group is much more nearly allied to the Pale- 

 ozoic than to the Archean, as the term is here used. Indeed, the only 

 essential difference between the Algonkian and the-Paleozoic is the almost 

 complete absence of definite fossil forms in the Algonkian. To attempt 

 to include both pre-Cambrian groups under one name would result in 

 making two groups of rocks to appear to be alike when as a matter of fact 

 they are radically different. -When this point is appreciated it is clear 

 that the term Archean is especially adapted to designate the older group 

 of the pre-Cambrian. In confining the term to the oldest group of rocks, 

 the essential intent of Dana in proposing the term is carried out, so far 

 as this can be done and still make the nomenclature used represent the 

 facts of geology. 



In short, the same plan has been followed with reference to the term 

 Archean that has been applied to the terms Keewatin and Laurentian — 

 that is, to define them so as to make them correspond as nearly as possible 

 to published maps on which they have been placed, and at the same time 

 make them mean a definite thing rather than two or more different things. 



Those who have commonly used the term Proterozoic rather than 

 Algonkian have naturally substituted Archeozoic for Archean, and thus 



