GENERAL PRINCIPLES — CONCLUDED 



25 



York are highly metamorphosed and folded, with indurated, folded 

 (Ordovician) strata resting upon their north side, and indurated, 

 non-folded, and slightly tilted (Triassic) strata coming against 

 them on the south side. Each of these groups of rocks represents 

 a distinctly different geologic age. This method cannot be used 

 over wide areas such as different parts of a continent, because for 

 instance, certain strata (Cretaceous) in the eastern part of the 

 United States may be unconsolidated and horizontal, while rocks 

 of the same age are highly folded in the western United States. 

 5. Study of adjacent lands. Examination of the materials of 



Fig. 17 



Diagram to illustrate correlation of rock formations by 



degree of change or structure. (W. J. M.) 



the Coastal Plain of our Atlantic sea-board clearly shows them to 

 have been largely derived from the rocks of the Piedmont Plateau 

 and Appalachians, and hence these Coastal Plain materials are 

 the younger. Also the peneplain character of the surface of the 

 Piedmont Plateau proves the greater age of this region because the 

 peneplain was being produced by wearing off the very materials 

 which were deposited in the adjacent ocean to produce what are 

 now called the Coastal Plain deposits. 



6. By diastrophisin. According to Chamberlin, 1 the great 

 deformations of the earth's crust have been of periodic occurrence. 

 Each great movement has " tended toward the rejuvenation of 

 the continents and toward the firmer establishment of the great 

 (oceanic) basins." Between any two great diastrophic movements 

 there has been a time of quiescence when the base-leveling 

 processes have more or less lowered the continents. Such " base- 

 leveling of the land means contemporaneous filling of the sea 



1 T. C. Chamberlin: Diastrophism the Ultimate Basis of Correlation, in 

 Jour. GcoL, Vol. 17, 1909, pp. 685-693. 



