THE PRE-CAMBRIAN ROCKS 103 
However, it may be said in summary that the Archean is a group 
dominantly composed of igneous rocks largely volcanic and for 
extensive areas submarine. Sediments are subordinate. The Algon- 
kian is a series of rocks which is mainly sedimentary. Volcanic 
rocks are subordinate. The Algonkian sediments where not too 
greatly metamorphosed are similar in all essential respects to those 
which occur in the Paleozoic and later periods. When the Algonkian 
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 like 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 localities 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 lithological, the former 
being mainly plutonic acid igneous rocks and the latter basic igneous 
rocks, largely volcanic. The Algonkian in many of the various 
