Invasion to Regional Metamorphism. 9 



province during the Cenozoic. The most probable, and 

 perhaps the only cause, which can be assigned to explain 

 this effect is a general rise of magma to higher levels, 

 the heat being carried by emanations and volcanic out- 

 break into the roof rocks. The result would be decrease 

 in density, because of thermal expansion as well as that 

 expansion dependent upon the passage from the solid 

 to the liquid state. 



In accord with the evidence that the Cordilleran prov- 

 ince has been uplifted by vertical forces, forces which 

 may be inferred as due to intrusion, is the fault block 

 character of the region. Mountain folding and over- 

 thrusting have not been absent, but the more dominating 

 feature is found in the presence of gravity faults which 

 show that large blocks of the crust have been elevated 

 by vertical forces. The process culminates in the high 

 and unfolded crust blocks of the Colorado plateaus, but 

 with a few exceptions even the basin blocks have been 

 elevated and the average elevation of the Great Basin 

 is now probably between 4000 and 5000 feet above the 

 sea. 



Although each broader step of inference leads to a 

 view of broader subcrustal igneous invasion, it does not 

 prove that a universal molten sea of rock underlies the 

 Cordillera at a moderate depth. The crust seems to 

 possess sufficient rigidity to show that the rise of 

 magmas never at any one time wholly broke up the 

 foundations of the older crust. It seems to have been 

 an intermittent process and to have gone forw^ard on 

 axial zones, as shown by the trend of the fault block 

 structures. 



Erosion to a depth of some miles, or even to the 

 present sea-level, would, however, show an enormous 

 extension of batholithic outcrops, and remind one of the 

 broad development of the Laurentian granites. That 

 which gives unique character to these earliest invasions 

 is the profound depth to which erosion has penetrated, 

 as well as a more universal development. 



Batholiths of New England. 



The outcrops of the pre-Cambrian are confined to 

 restricted areas in New England, occurring especially 

 in the anticlinorium of the Green Mountain axis and, 

 according to the geological map of North America issued 



