Dr. J. W. Evans — Devonian Rocks of North Devon. 547 



English Chalk. I have refrained from discussing in this paper 

 theories recently advanced to account for the origin of chert, partly 

 because of this lack of data as regards the formations in question, and 

 partly hecause the authors make no attempt to account for any 

 regularity in the recurrence of nodular lines. It may he that in the 

 formations discussed in these papers such periodicity is not apparent. 

 But so far as the English Chalk is concerned, this is one of the most 

 obvious features and an explanation of it may reasonably be required 

 of any hypothesis advanced as to flint origin. 



My thanks are due to Professor H. H. Swinnerton, D.Sc, id- 

 entically reading this paper in MS. 



IV. — The Correlation of the Devonian Hocks of North Devon 



WITH THOSE OF OTHER LOCALITIES. 



(Abstract of communication to Section C, British Association, 1919.) 

 By Dr. John W. Evans, F.E.S. 



THE Dartmouth Slates of South Devon and Cornwall, which 

 correspond, it would seem, to the Schistes d'Oignies of the 

 Ardennes, are not seen in North Devon, but may be concealed by 

 later rocks and be represented in South Wales and the Welsh Border 

 by the Bed Marls of the Lower Old Bed Sandstone. It is possible 

 that the Foreland Grits are a local facies of the upper portion of the 

 Dartmouth Slates, just as the arenaceous Cosheston Group is a local 

 development of the upper part of the Bed Marls. Both the Foreland 

 Grits and the Cosheston Group appear to have yielded the typical 

 Old Bed Sandstone plant Psilophyton. 1 



The usual correlation of the Lynton Beds with the Meadfoots of 

 South Devon seems well founded. The lower beds with Pteraspis 

 may be compared with the Schistes de Saint Hubert of the Ardennes 

 with Spirifer primavus and Pteraspis dunensis and the Schistes 

 a, Pteraspis dimensis in the Pas de Calais. The Senni Beds, which 

 overlie the Bed Marls on the north of the South Wales Coalfield and 

 contain Pteraspis and Cephalaspis, may be of the same age. The two 

 strata last mentioned have not, however, up to the present yielded any 

 marine forms. 



The Hangman Grits represent a great thickness of arenaceous beds 

 of the Old Bed Sandstone type overlying the Lynton Beds. Little is 

 known of the lower portion, but the upper beds include lacustrine or 

 fluviatile beds, with plant remains which are probably referable to 

 the Middle Devonian plant Ptilophyton. These are succeeded by 

 marine beds with several fossiliferous horizons, some of which have 

 yielded Stringocephalus. The upper part at least of the Hangman 

 Grits must therefore be considered to be of Givetian age, that is to 

 say, Upper-Middle Devonian, instead of Upper-Lower Devonian, 

 according to the usual correlation. This view is supported by the 

 discovery near Combe Martin (after the reading of the paper) in the 



1 The author is not inclined to accept the view that the Foreland Grits are 

 a repetition of the Hangman Grits, by faulting. 



