18 



Gustaf T. Troedsson 



Summary. 



Contribution to the knowledge of the uppermost Ordovician of 

 Västergötland, together with an attempt at comparing the Ordovician- 

 Silurian boundary-layers of Sweden with those of North America. 



1. In the first part there is given a stratigraphical and paleontological review 

 of the Dalnianites heds — i. e. the late Braehiopod schists — of Västergötland. 

 Their stratigraphical position and classification appear from the table on p. 5. 

 Since the fauna of the zone of Staurocephalus clavifrons is little known, we cannot 

 yet state whether this zone will belong to its overlying strata — the Dalnianites 

 beds — or to its underlying strata — the Trinucleus beds. The former are imme- 

 diately overlaid by the Silurian (Upper Graptolite shales of Västergötland) and are 

 only a few metres thick. Their rock varies extremely, being now a dark well- 

 stratified shale, now a compact coral limestone or — most typically — a light 

 thick-leaved arenaceous shale. The latter is more or less plentifully mixed with 

 quartz sand and at some places it shows discordant stratification, so that it must 

 have been deposited in shallow water. The fauna was studied mainly for purposes 

 of comparison with the Dalmanites beds of Skåne. The fossil list, shown on p. 12, 

 is complete only as regards this comparison, but is in other respects very frag- 

 mentary. Of the known 46 species of the Dalmanites beds of Skåne, only 11 (12) 

 are met with in the same strata of Västergötland, but 8 of them are found also in 

 the Staurocephalus beds in the two provinces. The endemic feature of the Dalma- 

 nites fauna of Skåne is striking. 



2. The Dalmanites beds of Sweden are equivalent to the zone of Bicello- 

 graptus anceps (Upper Hartfell) and its equivalents in England and Scotland. Fur- 

 ther they are contemporaneous with etage 5 b of Norway and the Borkholm beds of 

 Esthonia. Since Twenhofel has stated that the fauna of the latter is very similar to 

 that of the Bichmondian part of the Anticosti Section, it is evident that the Dalmanites 

 shale of Sweden must also be equivalent to the Richmondian of North America. 



The Upper Hartfell beds are generally looked upon as the latest beds of Or- 

 dovician, because the overlying strata contain the earliest Monograptidse. The latest 

 Ordovician graptolite fauna of Europe is also met with in Sylvan shale (Arbuckle 

 Mountains, Oklahama) of the Richmond age and in Polk Creek shale (Oachita Moun- 

 tains, Arkansas). Therefore, in opposition to Ulrich, we must include the last- 

 named in Richmondian, but, on account of the graptolite fauna, we must place 

 Richmondian at the top of Ordovician. 



^~s^-> 



(Tryckt den BO februari 1921.) 



