558 A. W. GRABAU — LOWER ORDOVICIC FORMATIONS 



1865 likewise included them in the Lower Silurian, thus leaving only the 

 Harlech grits to represent the Cambrian. Ramsay in 1878 went even 

 further, including all the formations in the Silurian — a proceeding in 

 which he was anticipated by Barrancle 2 and followed by some recent 

 writers, notably Bernard (1895). Lapworth' s classification of 1879 has 

 now become the standard for British as well as American geologists, 

 (hough continental geologists still retain Silurian for the two upper divi- 

 sions, referring to them either as Lower and Upper Silurian, TJnter him! 

 Ober Silur (Kayser, etcetera), or as Ordovicic and Gotlandic (Hang and 

 some Swedish geologists). 3 



The Tremadoc rocks of Wales Mere considered by Barrande as transi- 

 tion beds between those carrying the first (Cambric 4 ) and those holding 

 the second (Ordovicic) faunas. They were included in the Cambrian 

 by Lapworth, and have there been generally retained by British geologists. 

 On the continent of Europe, however, the rocks of this age are now being 

 generally classed as Lower Ordovicic (tieferes Untersilur), and the divid- 

 ing line is drawn at the base of the Diciyonema flabelliforme shales. 



The British subdivisions of the Ordovicic, as now used, fall into the 

 following five groups, in descending order : 



5. Ashgillian or Upper Bala. 



4. Caradocian or Middle Bala. 



3. Llandeilan or Lower Bala. 



2. Arenigian or Arenig. 



1. Tremadocian or Tremadoc. 



From the point of view of continuous sedimentation, these five groups 

 may be combined into three divisions, separated from one another and 

 from the succeeding and sometimes the preceding ones by disconformities 

 (more rarely unconformities). These combinations are (a) the Trema- 

 docian and Arenigian, (5) the Llandeilan and Caradocian, and (c) the 

 Ashgillian. In a general way these correspond to (a) our Beekman- 

 townian (including the Dictyonema beds, Potsdam, and Little Falls hori- 

 zons), (&) our Chazyan and Trentonian (to the top of the Lorraine), 

 and (c) our Eichmondian. With us, too, these formations are separated 



2 See especially "Du Maintien de la Nomenclature etablie par M. Murchison, par M. J. 

 Barrande." Extrait du Compte Rendu Stenographique du Cong-res International de 

 Geologie, tenu a Paris du 29 au 31 aout et du 2 au 4 Septembre, 1878. 



3 For a summary of the early history of classification see .1. E. Marr : The classifica- 

 tion of the Cambrian and Silurian rocks. Geological Magazine, decade ii, vol. viii, pp. 

 245-250. June, 1881. 



4 The terms Cambric, Ordovicic, and Siluric are used in the sense generally employed 

 in this country, while Cambrian of Sedgwick refers to the series from the Harlech to the 

 Bala, inclusive. Silurian has a different value according to the author employing it, 

 from Barrande and Ramsay, who include the Cambric, Ordovicic, and Siluric in it, to 

 Sedgwick and Lapworth, who use it in the sense of our Siluric only. 



