STRATIGRAPinC CLASSIFICATION DIASTROnilC CRITERIA 405 



is a horst, that the great medial region remained unmoved, while the 

 margins were often folded and elevated. The seas periodically flowed 

 over this medial land — in fact, were elevated over it — owing to the de- 

 trital materials unloaded into the oceanic areas, thus filling them and 

 causing them to spill over on to the lands/'-''® 



I cannot subscribe to this view. On the contrary, though accepting 

 the idea of permanent oceans and continents, it seems to me that the 

 crust of the lithosphere, not unlike the more mobile hydrosphere, was 

 subject to periodic movement away from the poles; that the surface of 

 the lands was exceedingly unstable and that this instability pertained, 

 though in an inferior degree, to the median areas as well as to those 

 along the borders of the continents. Schuchert's paleogeographic maps, 

 indeed, offer convincing proof of such instability. The general nature 

 and the reciprocal features of these "small movements'' are excellently 

 stated by Chamberlin and Salisbury. (Geology, vol. 1, 1906, pp. 540.) 



DIFFERENTIAL VERTICAL MOVEMENTS OF THE LITHOSPHERE 



Tilting of land areas. — The varying distribution of marine deposits 

 of successive ages naturally suggests differential upward and downward 

 movement of the lands as the immediate cause of such variation. If the 

 submergences had been occasioned solely by rise of the waters, whatever 

 the cause of the overflow, the successive submergences of the relatively 

 featureless lands would have been always similar in geographic pattern 

 and different only in lateral extent. In fact, a general similarity or 

 repetition of old patterns is recognizable, but there is also exceeding 

 diversity of expression, and the significant feature is that this diversity 

 is often greatest when directly succeeding stages are compared. One 

 stage may be very different from the next, but the third or fourth again 

 may be very much like the first. Only oscillatory movements of the land 

 surfaces could produce such results. The area affected by such move- 

 ments may be very large, as, for instance, during the middle Ordovician 

 and middle Silurian, when nearly half of the continent of North Amer- 

 ica was involved. During this period the Gulf waters seem at certain 

 times to have been completely withdrawn from the southern part of the 



3»Bull. Geol. Soc. Am., vol. 20, p. 438. 



It is to be remarked that this statement is inconsistent with others of this writer in 

 the same paper. Thus, on pp. 501 and 502, he speaks of the medial areas being buckled 

 and warped and of variable local loading and compensatory isostatic vertical movements. 

 Suess also noted conditions that are not in harmony with his general conclusion. Thus, 

 for instance, he says (Antlitz der Erde, English edition, pt. 3, p. 545) : "A close exami- 

 nation of the stratified s'eries often leads us to suspect the existence of numerous smaller 

 osclllatlong which are hard to reconcile with eustatic processes." 



