NEW YORK STATE MUSEUM 



The Dictyonema bed in Scandinavia 



If we wish to arrive at a proper valuation of the importance 

 of the Dictyonema horizon for the determination of the upper 

 boundary of the Cambric, we must turn to northern Europe, 

 where the subdivisions of the Cambric and Lower Siluric ter- 

 ranes have been studied most extensively, and the Dictyonema 

 slate has long been known. A lucid discussion of the relations 

 of the Upper Cambric to the Lower Siluric has there been fur- 

 nished by Professor Brogger in his paper, " Ueber die Verbrei- 

 tung der Euloma-Niobe-Fauna (der Ceratopygenkalk fauna) in 

 Europe." 1 Freeh 2 has also discussed the problem of the upper 

 limitation of the Cambric in an able and concise manner. 



We learn from these treatises that in Sweden, and still more 

 in the Baltic provinces of Russia, the boundary between the 

 Cambric and Siluric is most sharply defined; that there is a 

 break in the deposition, and that an entire change of the faunal 

 associations leaves little doubt as to the boundary line. In 

 Norway, however, as Brogger has well shown in his " Die siluri- 

 schen Etagen 2 und 3 im Kristianiagebiet etc. 1882," a gradual 

 transition takes place between the beds assigned to the Cambric 

 in Sweden and to the Lower Siluric Ceratopyge limestone. Here 

 the boundary, as Brogger states, has to be drawn by " charac- 

 teristic peculiarities in faunistic features." The same distin- 

 guished author states [loc. cit. p. 79] in regard to the region of 

 Christiania : 



It can not be denied, that in the beds below the Dictyograptus 

 shale the fauna bears throughout a primordial (Cambric) char- 

 acter ; no graptolites, no cephalopods, no trilobites of the type of 

 the characteristic Ordovicic genera (Asaphidae, Trinucleidae, 

 Cheiruridae, etc., etc.). Immediately above the Dictyograptus 

 shale itself we find simultaneously the first richer graptolite 

 fauna (Bryograptus zone), the first Asaphidae (Symphy- 

 surus incipiens Brogger) and soon also the first cephalo- 

 pods (Orthoceras atavus Brogger) , in the Ceratopy- 

 genkalk, in the region of Christiania and correspondingly also in 

 the fauna of Hof, of the Shineton shales, etc. There seems then 

 to remain only the possibility of placing the boundary at the 

 Dictyograptus shale itself. I have hence united this with the 

 Cambric. 



1 Nyt. Mag. for Naturvidensk. 1896. B. 35, S. 164-240. 



2 Lethaea Palaeozoica. 4897. 2:30-34. 



