MAJOR DIVISIONS OF THE PRE-CAMBRIAN 19 



have prevailed, the secondary minerals — epidote, chlorite, and calcite — 

 are very conspicuous and characteristic. Where anamorphic changes 

 have taken place, often due to abundant intrusives or powerful dynamic 

 action, or both, these minerals are likely to be less important. 



4. Martin and Leith have noted that the Archean and Algonkian con- 

 trast in their surface expression, often making it possible to separate 

 areas of the two groups on a physiographic basis. Thus, in the Lake 

 Superior region the Archean is characterized by a peneplain with small 

 elevations, for the most part without regular outlines, sometimes con- 

 trolled by the prevailing northeast-southwest structures of the Lake 

 Superior region. The Algonkian, as a rule, has bolder relief and its ele- 

 vations are strongly linear, in general parallel to the structure of the 

 Lake Superior basin. The distinction in the physiography of the 

 Archean and the Algonkian is of course dependent on the features men- 

 tioned above, and especially on the fact that the former is essentially 

 igneous and the latter essentially sedimentary. Erosion working upon 

 diverse materials gave different expressions. 



In consequence of all of the above differences it follows that ordinary 

 stratigraphic methods may be applied to the Algonkian, Avhile such 

 methods are not applicable at all, or only with extreme difficulty, to the 

 Archean. The discriminations for the few areas of Archean mapped in 

 detail are mainly lithological. This fundamental distinction has been 

 recognized in practice from the days of Logan and Murray. Every strati- 

 graphical geologist who has been willing to do detailed work has had 

 little difficiilty in mapping the Algonkian, unless subsequent intrusions 

 have been vast in amount and the metamorphism profound, whereas in 

 only a few areas has it been practicable to map the Archean rocks on a 

 structural basis, except as to the divisions Keewatin and Laurentian and 

 the occasional subordinate masses of sediments. 



To the dual division of the pre-Cambrian emphasized it has been 'ob- 

 jected that it can not everywhere be applied. To this objection instant 

 agreement is made. It may be equally well said that the Paleozoic can 

 not everywhere be separated from the pre-Paleozoic. In the Sierra 

 Nevada these groups are not yet satisfactorily discriminated. For the 

 coast ranges the same point might be made as to the difficiilty of sepa- 

 rating the Mesozoic from the pre-Mesozoic. The fact is, the classifica- 

 tion of rocks must be made from areas where the physical conditions are 

 favorable for making discriminations and where the succession is full — 

 not in areas where the conditions are- unfavorable and the succession im- 

 perfect. Wherever it is not practicable to separate the Algonkian and 

 the Archean precisely, the same plan may be followed as that used where 



