478 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



in all parts of the world. This would have been impossible if the limits of the 

 formations had not been drawn by natural processes simultaneously in opera- 

 tion over the widest areas. 



"It has been fortunate for stratigraphical geology that its earliest develop- 

 ment took place in Eng/and, a region where the frequency of gaps in the strati- 

 fied series neither riser above nor falls below the mean, a region which has at 

 times been submerged beneath the sea, at others covered by fresh-water lakes 

 or left exposed as dry land. . . . The limits of the formations established 

 by William Smith and his successors correspond for the most part with nega- 

 tive phases. . . . 



"In this also we find an explanation of the difficulties which were encoun 

 tered in correlating the stratified series where it attains its complete marine 

 development, as in the eastern Alps, with the succession established in Eng- 

 land. In this again we recognize the source of the opinion expressed by many 

 eminent investigators to the effect that this succession stands in relation to cer- 

 tain cycles, i. e., a perpetually recurring alternation produces a periodic return 

 of similar conditions. 



"As to the precise nature of these phenomena we can only hazard conjec- 

 tures. . . . The analysis of the Rhaetic series in the Alps shows that the 

 positive movement, which carries the Rhaetic shore further and further out- 

 ward till it finally extended across a large part of central Europe into the 

 north of Scotland, must have been oscillatory. . . . There can be no doubt 

 that the evidence afforded by the Rhaetic series and by the Purbeck is strongly 

 in favor of numerous subsidiary oscillations. Oscillations of greater importance 

 may be recognized with certainty, such, for instance, as those which caused the 

 stages of the Lias to extend in transgression, some to a greater, others to a less 

 distance. [Described in detail in II : 260-277. See also Ulrich's paper in this 

 volume, which describes the many oscillations of the American Ordovicic] 

 . . . These represent in other words secondary cycles within the primary. 

 The principal phases or the primary cycles reveal themselves with even greater 

 definiteness. 



"But it is a striking fact that in the best known of these primary cycles the 

 positive phase is of much greater duration than the negative phase which fol- 

 lows it. . . . 



"Finally, the regular and uniform character of the movements may be recog- 

 nized from the concordant superposition of the more recent beds on those of 

 much greater age. Of this there are numerous examples. Murchison, in de- 

 scribing the recent marine beds with Arctic shells at Ust-Waga, on the Dwina, 

 has pointed out their absolutely conformable superposition on the horizontal 

 Permian sediments ; he has also shown how at other places the latter sediments 

 rest in perfect concordance on much older beds, so that the stratigraphical rela- 

 tions offer no hint of the great gap which occurs at the line of contact [— dis- 

 conformity]. That this should be the case may well be cause for astonishment, 

 for some degree of erosion, weathering, or other alteration of the surface must 

 have occurred in the interval, and I can scarcely help thinking that even in 

 this case some kind of erosion, though feeble perhaps in its effect, must at one 

 time have been active. 



"The question now presents itself as to whether these positive movements 

 were likewise eustatic. 



