408 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



in ulterior causes and effects, there is still some difference in tlie progress 

 and final results of the movements in the continental and oceanic parts of 

 the crust. These differences are due to a secondarily contributing causal 

 factor operating in the former but absent in the latter. On the conti- 

 nents, namely, the reduction of the inequalities of the surface is produced 

 as much by erosion of the mountains as by filling of the hollows, whereas 

 the undulations of the oceanic bottoms are lessened, if at all, chiefly and 

 perhaps solely, by greater accumulation of deposit in the depressions. 

 The doubt expressed by the words "if at all" is based on the suggestion 

 that subsidence, due to depositional loading and to inherent differences 

 in specific gravity, may have maintained topographical relations pending 

 the periodic revivals of diastrophic activities when the inequalities must 

 have been accentuated. Accordingly the aggregate vertical displacement 

 is much greater on the continents than within the oceanic basins. 



The movements of the second group consist chiefly of gradual displace- 

 ments occasioned by temporary accommodation to slowly accumulating 

 stresses. These stresses are produced by suboceanic spreading on the one 

 side and continental creep on the other, by depositional loading, and by all 

 other causes tending to disturb isostatic equilibrium. Ulteriorly, then, 

 the causes of the movements are essentially the same in both groups. 

 They differ in that the movements of the second group are connected with 

 the early accumulation phases of the stresses, in that they are relatively 

 continuous in operation and comparatively local in their contributions to 

 orogenesis. Those of the first group, on the other hand, are associated 

 with the saturated stages of accumulation of stresses, are likely to be rela- 

 tively impulsive in operation, and their results in the way of mountain- 

 building are geographically more extensive. Their dynamic effects differ 

 further in that those of the first group are of a higher order, and that the 

 structural readjustments within the lithosphere caused by them are as a 

 rule more permanent. 



Varying effects on the strandline. — From the standpoint of stratig- 

 raphy the greatest difference lies in their respective effects on the same 

 parts of the strandline, namely, the impulsive movements of the first 

 group tended always to land elevation and sea withdrawal from the east- 

 ern, western, and southern sides of the continents of the northern hemi- 

 sphere ; hence they resulted in negative displacements of these parts of the 

 shoreline. The gentle movements of the second group, on the contrary, 

 tended in the main to gradually lower these parts of the continents and 

 consequently induced sea transgression and partial submergence of their 

 flanks. Coming to the northern side of more particularly the North Amer- 



