474 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



RELATIVE IMPORTANCE OF DEVONIAN DEFORM ATIVE MOVEMENTS 



Was this Devonian period of diastrophic activity comparable in taxo- 

 nomic importance to the preceding late pre-Cambrian and late Ordovician- 

 early Silurian periods, and succeeding Chester-Pottsville and the late 

 Mesozoic-early Cenozoic movements? I think not. If there is any 

 truth in the suggested theory of inland migration of the continental 

 belts of active folding, then the importance of the Devonian movements, 

 as indicated by the predominance of clastic deposits of this age in the 

 Appalachian Valley, is more apparent than real. It is doubtless a fact 

 that the balance of vertical movements, at any rate for the North Ameri- 

 can continent, has favored elevation rather than subsidence. The average 

 height of the continents and even more the inequalities of their surfaces 

 have been greater since the beginning of the Pleistocene than at any 

 previous Mesozoic or Paleozoic time. This is shown by the notable 

 decrease in limestone deposition in the interior areas since the Ordovician. 

 Subsequent to the Silurian continuous deposition of limestone to thick- 

 nesses of 500 feet or more are confined to the southern Appalachian and 

 southern Cordilleran basins and besides these to areas south of the 35th 

 parallel. However, most of this change from limestone to clastic sedi- 

 mentation, in so far as it affected the interior areas, came after the 

 Tennessean. The average relief of the median areas, as shown by the 

 composition, distribution, and volume of deposits, remained essentially 

 the same during the Devonian, Waverlyan and Tennessean periods as it 

 had been in the preceding Ordovician, Canadian, and Ozarkian. During 

 all these periods the interior lands were low and, as a rule, almost feature- 

 less ; and the clastic matter derived from them, therefore, was commonly 

 not only very scant but also very fine. But, although the general relief 

 of these lands was not greatly augmented, the growing frequency of 

 change in character of deposits shows corresponding increase in diversity 

 of surface features. 



There are occasional exceptions to the rule of fine and scant elastics 

 derived from the median lands during these relatively quiescent periods. 

 Thus they provide reasonably coarse quartz sands when the Saint Peter, 

 Sylvania, and Berea sandstones were spread southward from the Canadian 

 areas of pre-Cambrian crystalline rocks. But of the instances mentioned 

 the first two suggest eolian means of distribution — a mode of transpor- 

 tation that is effective when Streams, on account of the slight grade, 

 must fail. Even in the case of the thin sandstone beds on the flanks of 

 Ozarkia (Oriskany, late Devonian and Sylamore), which were carried 

 from exposed areas of Saint Peter and older sandstones on this land^ 



