622 A. W. GRABAU LOWER ORDOVICIC FORMATIONS 



recorded. The slow westward transgression of the Siberian Sea in the 

 Baltic is shown by the progressive overlap of the lower by the higher 

 members of the series. 



The period of initial transgression corresponds to the Potsdam and 

 early Beekmantown (Little Falls) transgression in North America. The 

 first retreat, in late Arenig time, corresponds to the great Beekmantown 

 retreat and the period during which the St. Peter sands were widely 

 spread over central North America. The readvance during Chazy time 

 corresponds to the readvance in the Llandcilo, and as in that case differ- 

 ent members of the Chazy rest on the erosion plane or the reworked St. 

 Peter, the successively higher members of the Chazy overlapping the 

 earlier ones. Finally, the Black Eiver beds overlapped the Chazy and the 

 early Trenton beds in turn overlapped the Black River, as in various por- 

 tions of the Canadian and the Rocky Mountain region. 



It thus appears that these movements were simultaneous in Europe and 

 North America, and that hence they belong to the changes of level due to 

 diastrophism, expressed in the lowering and the raising of the sealevel all 

 over the earth. Accordingly, breaks in the series as here described should 

 occur between the corresponding formations in many other parts of the 

 world. The paleogeographic conditions for western Europe are expressed 

 in the map (figure 10) on the previous page. 



