944 



NEW YORK STATE MUSEUM 



As its characteristic species is cited Dictyonema f 1 a - 

 belliforme Eichwald. In some places the Dictyonema 

 slate shows also some other graptolites, which Schmidt believes 

 to be identical with Bryograptus kjerulfi Lapworth. 

 Also on the islands of Oeland (Moberg) and Bornholm (John- 

 strup) the Dictyonema zone has been found, and in Belgium 

 D. flabelliforme has long been known. It occurs there 

 in the eastern part according to Malaise, and forms, as 

 Dewalque asserts, as definite a horizon as it does in Wales and 

 Scandinavia, in the lower part of the systeme salmien above the 

 systeme revinien. 



The Dictyonema bed in Great Britain 



If we turn now to Great Britain, we find that there also the 

 Dictyonema zone is well developed, in north and south Wales 

 and in Shropshire, and that Dictyonema flabelli- 

 forme has long been known as Dictyonema sociale 

 Salter. But the zone is placed here at the base of the large 

 mass of dark gray slates, to which Sedgwick gave the name 

 Tremadoc slates, and which he inclosed in his Cambric system. 

 The bitter controversy between Murchison and Sedgwick in- 

 volved the position of this terrane, like that of the Lower Siluric 

 terranes, in much uncertainty, but it appears that the consensus 

 of the British geologists is now to consider the Tremadoc as a 

 part of the Cambric and thus to place the Dictyonema shale at 

 about the base of the Upper Cambric. This is clearly and ob- 

 jectively expressed in Geikie's Textbook of Geology, 1892, p. 729, 

 where we read : 



It is at the top of the Tremadoc strata that the upper limit 

 of the Cambrian or Primordial formations is now drawn in 

 Britain . . . There appears to be more satisfactory proof 

 of a distinct paleontological break at this stage of the geological 

 record in Britain, or at least between the lower and upper part 

 of the Tremadoc subgroup. 



This proceeding, which, as an incidental result, separates the 

 Dictyonema slate from the accepted base line of the Lower 

 Siluric by 1000 feet of strata, is at variance with the views of 

 the continental geologists, as is shown clearest by the publica- 

 tions of Professor Brogger and the new edition of the Lethaea 

 Palaeozoica. It finds its explanation, besides the historic reasons, 



