PHYSICAL AND FAUNAL EVOLUTION 65 



to northern Europe (see Fig. 8). Within the protected interior 

 sea, limestones (Upper Stones River and Black River) accumulated. 

 Limestones accumulated also along the shores of Laurentia (Canadian 

 shield) in the St. Lawrence channel, the two types of sediment and 

 faunas thus occurring side by side. There is no need for postulating 

 a dividing ridge in this channel, for the faunas and sedimentation 

 would remain different as long as the different physical conditions 

 persisted. 



In Great Britain and elsewhere in Europe the zone of Coeno- 

 graptus gracilis forms the summit of the Middle Ordovicic. The 

 next succeeding zone (Hartfell shales of the Moffat district) is of 

 Upper Ordovicic or Caradocian age. This begins with the zone of 

 Dicranograptus dingani, which in North America is represented by 

 the Magog shales or Diplograptus amplexicaulis zone, which succeeds 

 the Normanskill beds. 



The diastrophic movement, which in North America resulted in 

 the emergence of most of the continent at the end of Lower Ordo- 

 vicic time, was likewise marked, though to a less extent, in Europe. 

 Lamansky has recently shown' that between Baltic Port and the 

 banks of the river Volkov, the Lower Ordovicic beds (Etage B) 

 show the progressive off-lapping structure characteristic of a retreatal 

 or beveled-off series of sediments. At Baltic Port only the Mega- 

 laspis planilimbata zone (Blla) occurs. Farther east, at Reval, 

 the higher Asaphus broggeri zone (BII^) and a part of the Asaphus 

 lepidurus zone (BII7) have appeared. In the extreme east of the 

 gouvernement of St. Petersburg, on the Volkov, the whole of BII7, 

 and the Asaphus expansus zone (Bllla) have come in above the 

 others. The line of disconformity and erosion is marked by slight 

 irregularities, by glauconite, iron oxide, and phosphate concretions, 

 rarely by siliceous sediments. Above the erosion plane, the beds 

 of BIII/3 and BIII7 (zones with Asaphus raniceps and Asaphus 

 eichwaldi) show progressive overlapping, the latter being repre- 

 sented only by clastic material at Baltic Port. Above these lies the 

 Echinosphaerites limestone C, which shows continued westward 

 overlapping. 



I W. Lamansky, "Die altesten silurischen Schichten Russlands," Mem. du 

 Comite GeoL, N. S., Livr. XX, 1905. 



