THE PRECAMBRIAN ROCKS 5 



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 insufiticient 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 

 series is intricately cut by igneous rocks, all of which stop at a definite 

 horizon, this is strong evidence that the adjacent rocks free from such 

 intrusives are later and probably belong to a different series. 



9. Amount of deformation. — The amount and nature of the 

 deformation are of assistance in correlation within limited areas. 

 Upon the whole, the older a series the greater and more intricate the 

 deformation. The difference in the amount of deformation in the 



