walcott.] NOMENCLATURE CAMBRIAN. 241 



The view of Dr. Hunt was concurred in by Sir J. W. Dawson, who in 

 1872 wrote as follows : 



I concur in the view that it may well have the name Siluro- Cambrian, while the 

 name Cambrian or Primordial will remain for those great and important fossiliferous 

 deposits extending downward from the Potsdam in America and the Tremadoc in 

 England, and constituting au imperishable mouumeut to the labors of Sedgwick aud 

 Barrande. 1 



A notice of the proposed change from u Potsdam or Primordial" of 

 the first edition of Dana's Manual of Geology to " Primordial or Cam- 

 brian" in the second edition, was published iu 1874 j 2 and the term was 

 used in a subordinate sense in auother edition of the Manual of Geology 

 (r876) as a subdivision of the Lower Silurian (p. 163). 



The term " Cambrian n came into general use iu the Canadian and For- 

 tieth Parallel Surveys and iu the U. S. Geological Survey in 1882. 

 With the exception of a few that prefer to use Taconic, the term is gen- 

 erally employed by geologists of all countries. 



Most of the English geologists of the present time draw the line be- 

 tween the Cambrian and Silurian at the summit of the Tremadoc and 

 the base of the Arenig terranes. From a study of the list of genera and 

 species of the fauna of the Lower and Upper Tremadoc slates, I think 

 that the line of demarkation between the two systems would naturally 

 fall between these two subdivisions of the.Tremadoc. The fauna of the 

 Lower Tremadoc, in its general facies, is essentially the type of the 

 Cambrian fauna, and there are only a few genera and species more char- 

 acteristic of the Silurian. The large fauna of the Upper Tremadoc slate 

 is characteristic of the Lower Silurian and the presence of a few genera 

 and species of essentially Cambrian type are not sufficient to include it 

 in the Cambrian. The fact that there is a slight unconformity at the 

 summit of the Tremadoc and the base of the Arenig is not of sufficient 

 general importance to warrant the placing of the liue of demarkation 

 between the groups at this horizon. 



In a table showing the classification of the British sedimentary rocks, 

 by Prof. A. J. Jukes Brown, 3 the Tremadoc slates are placed iu a stage 

 between the Cambrian and Ordovician. 



This view of the Tremadoc slates was held by Mr. J. W. Salter, 4 who 

 wrote : 



But a few words must he said ahout this hand of slate, which so intimately con- 

 nects the Lingnla flags below with the rocks ahove, and which is nevertheless so dis- 

 tinct in fossil characters from either. 



'Canad. Nat., new ser., vol. 7, 1873, p. 7. 



2 Dana, J. D. Reasons for some of the changes in the subdivisions of geological time in tbe new 

 e»lilion of Dana's Manual of Geology. Am. Jour. Sci., 3d ser., vol. 8, 1874, p. 213. 



3 The classification of stratified rocks. Geol. Mag., new ser., dec. in, vol. 2, 1885, p. 297. 



4 Memoirs of the Geological Surveys of Great Britain. Geology of North Wales, second edition, 

 1881, p. 355. 



£ull f 81- 16 



