12 Schuchert and Barrell — Revised Geologic 



which are compressive in character and lead to mountain-fold- 

 ing. Upon erosion and subsequent sea invasion, these angular 

 or structural unconformities are the most easily found and 

 those about which there can be the least doubt. The broad 

 and gentle flexures known as crustal warpings, on the contrary, 

 as a rule bring about the disconformities. The number and 

 importance of these, on account of their difficulty of detection, 

 are only now beginning to be appreciated. 



The crustal oscillations of the earth are not due to hetero- 

 geneous and unrelated movements, but are connected, in that 

 areas of elevation and depression remain positive or negative 



Fig. 2. 



\^^SM, 



rf'Vhe ourer cru*r or nrnospnere or me c/ose or TiiTiiilliT,' V, P*r 



i 1:11 rf-- k:-. A «L^l u~_:~ A -I — u: — fU«nM L.J Ail~ 



Pacific ocean TheOld-land Cordilleran Continental basin Appalachian TheOld-land Atlantic ocean 



basin Cascadia geosyncline emergenr stage geosyncline Appalachia basin 



Negative Positive Subposihve "Neutral Subposihve Positive Negative 



Fig. 2. Diagrauimatic cross section through North America in Paleozoic 

 time at about lat. 38°, to show effects of diastrophism. Vertical scale greatly 

 exaggerated. 



throughout the eras, or during more or less long stretches of 

 geologic time. According to Chamberlin (1910), " Deforma- 

 tions are inheritances, one of which follows another in due 

 dynamical kinship. The succession is therefore homogeneous 

 and the results co-ordinate. . . . Under this view, ocean basins 

 and continental elevations tended toward self-perpetuation." 



The major crustal deformations are periodic in appearance, 

 and their visible areas of movement are now in this continent, 

 now in another ; and it is this periodicity which conditions 

 geographic history and organic evolution. All of these active 

 and decisive, movements are of long duration, and their major 

 work is confined to the marginal areas of the continents. 

 Farther inland, new axes or depressed folds may rise or old 

 ones be accentuated, and so divide the continental basins into 

 a series of smaller water-ways. Isot only are the margins of 

 the continents elevated, but apparently at the same time the 

 oceanic basins are made either deeper or larger, or both. This 

 simultaneous movement of the oceanic bottoms and the con- 

 tinental margins is proven by the fact that the major crustal 

 deformations occur during the emergent times of the geologic 

 periods, and this is true not only for the continent undergoing 

 deformation, but as well for other land masses which have not 

 moved at all, but whose strand-lines have teen lowered in con- 

 sequence of the oceanic enlargement. 



