BIRI LIMESTONE OP NORWAY 573 



differentiates it markedly from the early portion of the Durness limestone 

 series, and, if the correlation here suggested be correct, we may confidently 

 expect the finding at some future time of the true Olenellus fauna along 

 the contact of the arenaceous and siliceous beds, and probably in the 

 lower beds of the limestone as well, where Salterella and Hyolithes should 

 occur. Nor is it impossible that some future favorable exposures of the 

 higher parts of the limestone Avill reveal the presence in it of a scattered 

 Beekmantown fauna such as has been obtained from the Durness lime- 

 stone. 



The alternative correlation favored by Eothpletz is that the limestones 

 represent a local calcareous accumulation during Cambric time, while the 

 darker muds of Sweden were deposited farther east, both, however, be- 

 longing to the same province. Such an interpretation presents some for- 

 midable difficulties, not the least of which is the determination of the 

 source of the material in a region characterized everywhere else by mud 

 deposition. One might, perhaps, refer the deposit to a. supermarine 

 origin, either in a playa lake or a river floodplain. But while this would 

 explain the peculiar characters of the deposit and its lack of organic 

 remains, it does not solve the mystery of the' origin of the calcareous sedi- 

 ment, nor of the absence of clastic siliceous material in it. 



Lower Ordovicic of the Atlantic Region 

 t rem ad og 



The Lower Ordovicic of Britain begins with the Tremadoc series. This 

 is a purely clastic series of terrigenous origin, typically seen in Carnarvon- 

 shire, North Wales, and extending into Merionethshire. It consists mainly 

 of dark gray shales, which have a total thickness of about 1,000 feet and 

 carry an abundant fauna. Two divisions are recognizable — a lower, with 

 Dictyonema socialis (D. flabelliforme), and an upper, with Asaphellus 

 liomfrayi. Besides the Dictyonema, the lower division carries a trilobite 

 fauna, in which Niobe liomfrayi, N. menapiensis, Psiloceplialus innolatus, 

 Angelina sedgwicki, and Asaphellus affinis predominate. These shales 

 are also found in the Malvern Hills, where they are 1,300 feet thick 

 (Bronsil gray shales), but include about 300 feet of diabases and basalts. 

 Since these beds carry the Dictyonema fauna throughout, they are believed 

 to represent the Lower Tremadoc only. 



In the Lake District of North England the . Tremadoc rocks are in- 

 cluded in the great shale series forming Skiddaw Mountain, as shown by 

 the occurrence of Bryograptus in a part of this series. In Scotland these 

 strata appear not to be developed, and we may infer that an old Ordovicic 



