652 ELIOT BLACKW ELDER 



suggest that the erogenic forces are not local but planetary. 

 Chamberhn^ regards the crumplings on two sides of an ocean as a 

 result of the subsidence of the earth segment beneath that ocean. 



The folding along the relatively narrow belts was nearly always 

 accompanied by more or less warping and volcanic activity over 

 adjacent, much wider areas. The warpings have not infrequently 

 disturbed stream activities and strand-lines over large portions of 

 continents, as at the close of the Permian and in the Miocene. 

 The transitions from the folded to the merely warped areas are in 

 some cases abrupt and in others gradual, through gently folded or 

 faulted tracts. Where the demarcation is sharp, as the east side of 

 the Rocky Mountains, there may have been some original line of 

 weakness such as a decisive initial dip; whereas in the cases of 

 gradual transition the underlying mass may be tolerably uniform 

 in resistance, and may therefore permit the force to be transmitted 

 through it and die away gradually. 



In several instances observed relations indicate that the most 

 effective compression was accomplished on the side toward the 

 ocean basins. Thus we have the closely appressed folds and over- 

 thrusts of the Laramide system in western Montana and Idaho 

 changing into gentle open folds farther east. Again, in the Appa- 

 lachian orogeny, the Rhode Island district was more closely 

 crumpled and metamorphosed than Pennsylvania on the northwest 

 side of the deformed zone; and a like comparison may be made 

 between the region of great overthrusts and isoclinal folds in North 

 Carolina on the southeast and the open arches of northern Alabama 

 on the northwest. For at least some of the orogenic systems there 

 seems to be a significant arrangement of the batholithic intrusions 

 on the seaward side of the zone, with superficial volcanoes and lac- 

 coliths along the landward side. This is perhaps only another way 

 of saying that the batholiths are characteristic of the more intensely 

 folded parts of the zone, while the superficial volcanic features occur 

 where the rocks have been less deformed. It is admitted, however, 

 that this distribution of intensities and phases of igneous activity 

 cannot be demonstrated for many of the epochs and may be 

 accidental rather than significant. 



' T. C. Chamberlin and R. D. Salisbury, Geology, I (1904). 



