SUMMARY OF EARLY ORDOVICIC SECTIONS 



621 



ranean also seems to have been laid bare, though future studies in Sar- 

 dinia and elsewhere may show a similar continuous series. Erosion no 

 doubt occurred in many of the emerged areas, so that a part of the previ- 

 ously deposited ilrenig was again worn away. As a result, also, basal 

 conglomerates were formed and incorporated in the higher beds, when 

 the sea as-ain advanced. 







Figure 10. — Paleogeographic Map of eastern America and western Europe and Africa in 



early Ordovicic Time 



Showing the three principal oceans — the Atlantic or Arenig-Llandeilo sea in the lower 

 part, the Boreal or Beekmantown-Chazy sea in the upper center, and the Siberian sea on 

 the right. 



The readvance of the sea was apparently contemporaneous with the 

 whole of Llandeilo and at least a part of Caradoc time. As shown by the 

 different sections, Lower Llandeilo beds succeed the hiatus in some places 

 and Upper Llandeilo in many more. Again, some of the regions were 

 not covered until Caradoc time, as shown by the succession of these beds 

 next above the hiatus. As in previous transgressions, the Siberian trilo- 

 bite fauna and the Atlantic graptolite fauna came in contact in the Baltic 

 region, where more or less interfingering and overlap of the two series is 



