618 A. W. GRABAU LOWER ORDOVICIC FORMATIONS 



Orthoceras limestone, while the next higher zone is that of Dicrano- 

 graptus clingani Carr. Here occur, besides the above, Dicellograptus 

 forchhammeri Gem., Climacograptus hicornis Hall, and Diplograptus 

 foliaceus Murch. The evidence from this section would seem to place the 

 Trinucleus coscinorMnus zone above the break. 



ORDOVICIC OF BORNHOLM 50 



This island forms the southeastern extension of the rock series exposed 

 in Scania. It shows a large area of pre-Cambric rocks on the northeast, 

 and an area of older Paleozoics on the south, and of Jurassic and younger 

 strata in the west. As in Scania, there are numerous dislocations which 

 complicate the succession. 



The series begins with the basal arkosic Nexo sandstone, 60 meters in 

 thickness, which corresponds to the Fucoidal sandstone of Sweden. This 

 is conformably succeeded by a green graywacke shale consisting of alter- 

 nating glauconitic clay shales and sandy shales with phosphate concre- 

 tions and occasional thin limestones, the total having a thickness of 57 

 meters. Only Hyoliihes (several species) and Torellella Icevigata Linnrs. 

 have been found so far. This, with the basal sandstone, represents a part 

 of the Lower Cambric. The upper part of the series is arenaceous and it 

 is disconformably succeeded by the Paradoxides tessini zone of the Middle 

 Cambric, the lower beds of which contain, according to Gronwall, worn 

 fragments of the underlying sandstone. The hiatus here represents the 

 lower part of the Middle Cambric below the Tessini zone. The total thick- 

 ness of the Middle Cambric, Avhich consists mainly of alum shales, is 4 

 meters, and it passes conformably into the Upper Cambric Olenus beds, 

 all of the higher Middle Cambric zones being represented. These Olenus 

 shales with the typical Swedish Upper Cambric fauna are 15 meters 

 thick. They are followed, apparently without break, by the Dictyonema 

 shales, with a thickness of 5 meters, which, however, appear to represent 

 only a part of the Dictyonema series. The Bryograptus and Tetragraptus 

 zones are absent, the Orthoceras limestone following directly on the 

 Dictyonema shale, with a thickness of 4 meters. The base of this forma- 

 tion consists of a thin, dark glauconitic limestone layer with much pyrite 

 and numerous phosphatic nodules containing sponge spicules. Desicca- 

 tion fissures are represented by their ridgelike fillings. . The main part 

 of the Orthoceras limestones consists of gray, somewhat nodular, limestone 

 with 10 to 15 per cent clay. It contains numerous trilobites, among them 

 Megalaspis limbata Buch., Ptychopyge applanata Ang., Symphysurus 



60 In this section I rely entirely on TJssing. Handlj. d, Keg. Geol., Bd. I, Abt. 2, Dane- 

 mark, 1910, as I have not visited this island. 



