Vol.51.] RA.DIOLAJUAN ROCKS IN THE LOWER CULM MEA8URES. 613 



Swimbridge. He divides the Culm group of Central Devon as 

 below in descending sequence (op. tit. p. 194) : — 



Upper or Grit Group of Central Devon. 



Upper Shale Group. — A mass chiefly of dark shales. Carbonaceous grits 

 and shales, the lowest part being the Codden Hill chert series. 



Middle or Calcareous Group. — Limestone, mostly black, irregularly b;dded, 

 associated with shales, often resting on trap rock, and fossiliferous. 



Lower Shale Group. — Black argillaceous plate or very laminar shale, not 

 subject to slaty cleavage, and scarcely fossiliferous. 



It may also be mentioned that practically the same sequence of 

 the Culm Measures is given by Phillips in the ' Manual of Geology,' 

 1855. 1 The Codden Hill cherts and shales are stated to be of 

 considerable but variable thickness, and they are placed with a 

 query below the anthracitiferous gritstone series with plants of the 

 coal-formation and above the black limestone and shale with 

 Posidoniw. In the subsequent edition of Phillips's 'Manual' by 

 Mr. B. Etheridge in 1885, 2 the Codden Hill grits, cherts, and shales 

 are referred to the age of the Millstone Grit. 



Dr. Harvey B. Holl in 1868 3 traced out the succession of the 

 Culm Measures in South Devon and East Cornwall, and states that 

 the beds on both sides of the trough are symmetrical, the limestones 

 near the base being precisely similar to those near Bampton in 

 North Devon, and the succeeding grits and cherts resembling those 

 of Codden Hill. The direction of these chert-beds is followed in 

 the Launceston district and also on the east of Dartmoor, between 

 Bickington and East Down, where the chert is to some extent 

 replaced by a siliceous but less flinty rock. Volcanic rocks are 

 associated with the chert and other beds. Dr. Holl likewise 

 describes the slates at Waddon Barton, near Chudleigh, with 

 Posidonomya and Goniatites as resting on Devonian limestones and 

 overlain by typical carbonaceous sandstones. 



In the 5th edition of ' Siluria ' (1872) Sir B. Murchison gives a 

 section across North Devon 4 and states that the Pilton and Pether- 

 win Beds of the Devonian are surmounted by true Carboniferous 

 schists with the Posidonomya-limestone of Swimbridge and Venn, 

 which is a thin representative of the Carboniferous Limestone and 

 is followed by the Goniatite-grits of Codden Hill, and beds of Culm 

 representing the Millstone Grit series of other parts of England. 



In a sketch of the Geology of Devonshire (1878) Mr. Townshend 

 M. Hall 5 refers to the black limestones and shales with Posidonomya 

 and Goniatites at Venn and Swimbridge, near Barnstaple, as repre- 

 sentative of the Mountain Limestone of Derbyshire, Yorkshire, and 

 the North of England, and states that in close proximity to the 

 limestones are beds of whitish shale passing occasionally into a 

 substance resembling porcellanite, with beds and veins of a hard 

 white or grey chert, which are well shown at St. Stephen's Hill 



1 P. 171. 2 Pt. ii. p. 220. 



3 Quart. Journ. Geol. Soc. vol. xxiv. (1868) p. 400. 



4 P. 272. 5 White's ' History, etc., of the County,' reprint, p. 9. 



Q. J. G. S. No. 204. " 2 x 



