THE PRE-CAMBRIAN ROCKS LOD 
a very variable extent and significance. It is now realized that a 
sharp orogenic movement may take place resulting in uplift, erosion, 
subsidence, and therefore discordance of strata, which may not affect 
an adjacent area. Thusit should clearly be understood that it cannot 
be assumed that unconformities due to orogenic movements are more 
than of district extent. There are, however, great movements of 
uplift and subsidence which are continental and may be even inter- 
continental. Unconformities due to movements of this kind may 
have a very wide extent, and may thus be used for correlation from 
province to province, or possibly even from continent to continent. 
But in order that this may be fully done, it is necessary to show that 
the unconformity upon which correlation is based is an extensive one. 
As yet insufficient careful study has been made of known uncon- 
formities from this point of view. Here is a great and fundamental 
field for investigation. If the known unconformities of the world 
_ were broadly studied, it is probable that many can be determined to 
be local, others to be provincial, others continental, and a few inter- 
continental. No more important determination than this remains 
to be made in geology. So far as I can see until this work is done 
there will be no very close correlation of pre-Cambrian formations 
from province to province and from continent to continent. 
7. Relations to series of known age.—The relations of a formation, 
series, or group, to other formations, series, and groups of known age 
are of very great assistance in correlation. Frequently a formation, 
series, or group may be continuous or recognizable in the different 
districts of a geological province when other formations, series, or 
groups are not continuous. The position of the latter with relation 
to the former, whether above or below, and if above or below, con- 
formable or unconformable, are valuable helps in correlation. Thus 
the Keweenawan is practically continuous about the entire Lake 
Superior basin. This is the only series of which this is true. The 
position of the series called Upper Huronian immediately but un- 
conformably below the Keweenawan in different districts in con- 
nection with other facts is of great significance. 
8. Relations with intrusive rocks.—The older is a series the more 
intricately is it likely to be cut by intrusive rocks, and this relation 
is of assistance in correlation in connection with other criteria. If a 
