SUMMARY OF EARLY ORDOVICIC SECTIONS 619 



palpebrosus Dalm., and Nileus armadillo Dalm. The formations thus 

 represent the M. Umbata horizon, but whether the M . planilimbata bed is 

 represented in the lower part of the limestone or whether this and the 

 Ceratopyge limestones are wholly wanting does not yet appear. 



The limestone is succeeded by the Dicranograptus shale of the lower 

 Upper Ordovicic, corresponding essentially to the Lower Hartfell shales 

 of Scotland. There is thus a pronounced hiatus here, representing the 

 Middle and Upper Arenig and the whole of the Llandeilo. Three zones 

 are represented in the bituminous shales of this series. These are: 



3. Zone with Climacograptus styloicles Lapw. 



2. Zone with Dicranograptus cling ant Carr. 



1. Zone with Climacograptus vasce Tullb. 



The lowest or oldest of these zones is not known in Scania, where the 

 upper series begins with the zone of Dicranograptus clingani Carr. This 

 lower zone, however, corresponds, according to Tullberg, to the zone with 

 Climacograptus wilsoni Lapw. in Scotland. 



There is thus clearly indicated a general overlap of the advancing 

 Upper Ordovicic series over Sweden, though this was a very irregular 

 one, some portions of Sweden becoming submerged before others. 



Summary of the Early Ordovicic Sections 



We may now summarize the early Ordovicic sections and deduce from 

 them the sequence of events. Throughout most of western Europe the 

 basal Ordovicic beds lie with a greater or less hiatus on the Upper Cam- 

 bric or older formations down to the Archean complex. This marks a 

 wide-spread transgression which was inaugurated with the beginning of 

 Ordovicic time, following a previous partial withdrawal of the sea. In 

 the English area alone the sedimentation seems to have been continuous, 

 the Upper Cambric beds being conformably succeeded by the Tremadoc. 

 But even here there may be proved a hiatus by future investigations. 



Two main areas of sedimentation are recognizable — the Baltic and the 

 Mediterranean. These were both open to the Atlantic of that period, but 

 were themselves separated by the Old land of Armorica, which extended 

 between them as a peninsula; while the southern or Mediterranean region 

 seems to have been merely an embayment of the Atlantic, the Baltic was 

 for part of the time at least a channel, connecting the Atlantic with the 

 sea then covering part of Siberia. 



A third center of deposition in Europe was the North Scottish one and 

 its possible extension in the western Scandinavian region. This district 

 appears to have been entirely separated from the Atlantic Ocean by the 



XLV — Bull. Gbol. Soc. Am., Vol. 27, 1915 



