

320 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



the continents with respect to the level of the sea and the consequent 

 restriction of marine habitats favoring the development of shallow-water 

 faunas. Both factors tend to extraordinary diversification in environ- 

 ment, hence in distribution and specialization of faunas and floras. Both, 

 again, but more especially the second, tend to increase the normal irregu- 

 larity in production and distribution of clastic matter, thus causing fur- 

 ther localization in faunas. The oceanic currents, too, probably follow 

 very different paths today from those which carried certain Paleozoic 

 pelagic faunas (especially graptolites) almost if not entirely around the 

 globe. At certain times the two American continents must have been 

 separated, permitting the representative of the Gulf Stream to pass 

 through the opening. It is not my intention to discuss the oceanic cur- 

 rents of past ages. Too little is known about them. For present pur- 

 poses it suffices to say that since the form and intercommunications of 

 the Paleozoic oceans were often very different from their present deriva- 

 tives, it is reasonable to infer that the currents which traversed them 

 were correspondingly different. 



In view of the known and probable differences between conditions pre- 

 vailing now and in the past, I maintain it is unreasonable to insist on 

 agreement with the present standard in determining Paleozoic conditions. 



Oscillatory Character of Continental Seas 



GENERAL DTf^CU^'^SION 



Any one who will undertake a course of critical comparison of numer- 

 ous sections of rocks laid down in the various continental seas must in- 

 evitably reach the conclusion that the old surface was exceedingly 

 unstable with respect to sealevel and subject to oft-repeated differential 

 movements and warping. The causes and character of these movements 

 are to be discussed in a later part of this work. Here it will suffice to 

 say that the differential movements are indicated by abrupt local or gen- 

 eral changes in the character of sediments, by imperfections in the strati- 

 fied record at one locality which are partly or wholly supplied by the 

 record in another place, by the sudden extinction of, say, an Atlantic 

 fauna in a given area and the subsequent occupancy of the same area by 

 a Pacific or a. Gulf fauna, and by other more or less competent criteria. 

 Simple vertical movements, if there ever were such, are indicated by more 

 equable general advances and retreats of the- strandline. The mass of 

 the continent as a whole always bore a definite and constant relation to 

 the oceanic areas, and in so far the land areas may be called stable ; but 

 in the respective attitudes of different ])arts of its surface to sealevel and 



