620 A. W. GRABATJ LOWER ORDOVICIC FORMATIONS 



old Caledonian land-mass which extended across the Atlantic to New- 

 foundland. 



The sections discussed clearly indicate that the early Ordovicic trans- 

 gression of the sea marked an eastward advance of the Atlantic waters 

 along both channels, and with this also a lateral spreading and inunda- 

 tion of the low shores of the bounding lands, namely, Caledonia on the 

 north, Armorica in the center, and probably North Africa on the south, 

 though satisfactory sections of the African region are not available. In 

 the Baltic region the .Siberian Sea also transgressed from the northeasi. 

 and so a commingling of the Ceratopyge fauna from that region and of 

 the Dictyonema fauna from the Atlantic region took place. The Atlantic 

 fauna seems to have advanced eastward earlier — no doubt owing to the 

 mud deposits which kept out the purer water fauna from the east. Thus 

 when the Ceratopyge fauna in turn advanced with the clearing of the 

 waters, the beds in which it occurs came to overlie the Dictyonema shales. 

 On the margin of the old land the progress of the transgression is indi- 

 cated by the overlap of the Arenig beds over the Tremadoc, the latter 

 being absent in southern Scotland — the southern border of Caledonia — 

 and in Brittany and Normandy — the northern border of Armorica. In 

 like manner the Ceratopyge beds are wanting or represented only by shore 

 sands in the Baltic provinces of Russia, and they are similarly overlapped 

 by the later Ordovicic beds in Jamtland (Sweden). The Phyllograptus 

 shales of the Atlantic came in contact with the Megalaspis limestones of 

 the Siberian sea and the two likewise interfinger in the Baltic region. 

 In the southern embaymcnt the transgression included northeastern 

 Spain and southeastern France before the end of Tremadoc time, but did 

 not reach Bohemia until well along in Arenig time. When the northern 

 Scottish region and a part of the northern border of Caledonia were sub- 

 merged cannot be determined with precision, since the Beekmantown 

 fauna, which alone existed there, can not be correlated in detail with the 

 Atlantic or Siberian phases. Most probably, however, the early part of 

 the Durness limestone (exclusive of the Cambric portion below the hia- 

 tus) belongs to the period of the Tremadoc and Ceratopyge sediments. 



The second great event indicated by the sections is the retreat of the 

 late Arenig Sea, until the Baltic region apparently became entirely dry 

 and the Scandinavian lands were joined to Armorica. The withdrawal 

 was westward to the Atlantic and eastward in the Siberian end of the 

 channel. In southern Wales alone deposition appears to have continued, 

 the Llanvirn series marking a transition from the Arenig to the Llan- 

 deilo, but also having distinctive characters, owing to the much contracted 

 character of the embaymcnt in which they were deposited. The Mediter- 



