376 FRANCIS PARKER SEEPARD 



the same level, metamorphism would tend to be somewhat equal 

 in effects on each. Before that time the more deeply buried pre- 

 Cambrian would tend to be more severely metamorphosed unless 

 it was below the zone in which the most effective compression was 

 being concentrated. Further confusion in regard to differential 

 metamorphism may be produced by the cutting of great valleys in 

 the rising mountain ranges. Along the lines of these valleys the 

 overburden is less, and therefore there would be less severe meta- 

 morphism. Thus the amount of metamorphism of a local series of 

 rocks in this general region can only be used to tell its relative age 

 in exceptional cases. 



The foregoing consideration of the varying character of the 

 lithology and metamorphism is intended to show how extremely 

 difficult it is to correlate unfossilif erous formations in a mountainous 

 region. Attempts at such correlations are of course suggestive, 

 but they should be duly qualified. 



