THE P RE-CAM BRIAN ROCKS 7 



rocks were laid down essentially the present conditions prevailed on 

 the earth. The Archean rocks on the other hand indicate that during 

 this era the dominant agencies were igneous. The physical condi- 

 tions had not yet become such as to lead widely to the orderly suc- 

 cession of sedimentary rocks Hke those being formed today. On 

 the whole the deformation and metamorphism of the Archean are 

 much farther advanced than the Algonkian. The two groups are 

 commonly separated by an unconformity which at many localities 

 is of a kind indicating that the physical break is of the first order of 

 importance. As evidence of this, at many places are the funda- 

 mental difference in the character of the rocks, the greater intricacy 

 of intrusion, greater deformation and metamorphism of the older 

 group, and deep intervening erosion. In some locaHties a part of 

 these phenomena are lacking, but the significance of an unconformity 

 is determined by the places where evidences of its magnitude occur 

 rather than where lacking. So profound are the contrasts between 

 the Archean and the Algonkian in each of the great regions of the 

 world in which the pre-Cambrian has been studied, and so similar 

 are each of these great groups with reference to the fundamental 

 principles discussed that it has been regarded as safe to correlate 

 these two groups even when in distant geological provinces. In mak- 

 ing this correlation it is not supposed that the formations of one 

 province are of exactly the same age as those of another province, 

 but that the formations assigned to the Archean and Algonkian 

 respectively in any given case belong to the two great eras of the pre- 

 Cambrian represented by the rocks of these groups. 



For extensive areas the Archean may be divided into Laurentian 

 and Keewatin. These divisions are purely hthological, the former 

 being mainly plutonic acid igneous rocks and the latter basic igneous 

 rocks, largely volcanic. The Algonkian in many of the various 

 geological provinces may be divided into two or more series separated 

 by unconformities. The formations of these series are commonly 

 sedimentary, although igneous rocks are often abundant. As a whole, 

 to the Archean group ordinary stratigraphical methods do not apply. 

 To the Algonkian such methods are as apphcable as to the Paleozoic 

 and later series. 



While the subdivisions of the Archean and of the Algonkian can 



