122 



DEVONIAN. 



DEVONIAN. 



The term Devonian was adopted by Sedgwick and Murchison 

 in 1838. This determination was based on the researches of 

 William Lonsdale, who, from a study of the fossils found in the 

 Plymouth and Torbay Limestones, had in the previous year 

 concluded that the strata constitute a natural history group 

 intermediate between the Silurian rocks and the Carboniferous 

 Limestone.^ Hence it was that the greater portion of the rocks 

 of North and South Devon and of Cornwall came to be re- 

 garded as the marine equivalents of the Old Red Sandstone. 



The Devonian rocks of West Somerset, Devon, and Cornwall 

 consist of a series of slaty rocks, grits, sandstones, and lime- 

 stones, with interbedded eruptive rocks, together attaining a 

 thickness estimated at from 10,000 to 12,000 feet. 



Apart from the labours of Sedgwick and INIurchison, we are 

 largely indebted to Sir Henry T. De la Beche and John Phillips for 

 our knowledge of the strata and fossils of Devon and Cornwall. 

 To De la Beche we owe the establishment of the Geological Survey, 

 the earliest publications of which institution were the maps and 

 reports on Cornwall, Devon, and West Somerset." The labours of 

 other observers will be noted further on. But, as before mentioned, 

 recent researches tend to the belief that in the Devonian rocks we 

 have not only representatives of Upper and Lower Old Red Sand- 

 stone, but of the strata which bridge over the unconformity which 

 separates these divisions. Hence in the uppermost Devonian beds 

 we find strata of Lower Carboniferous age, while the lowermost 

 beds may perhaps be linked with the Silurian rocks. Under such 

 circumstances we cannot do better than consider the beds separately 

 as Devonian, while at the same time pointing out some of the ascer- 

 tained facts and some of the conclusions which they foreshadow. 



The following general grouping may be adopted : — 



Upper 

 Devonian. 



Middle 

 Devonian. 



Pilton, Baggy, and Mar- 

 wood Beds. 



Pickwell Down and 

 Cockington Sandstones. 



Morte, Berry Park, and 

 Dartmouth Slates. 



Ilfracombe, Torquay, and 

 Plymouth Limestones 

 (Great Devon Limestone) 

 and Lower Slates. 



) Lower 



i Carboniferous. 



) Upper 



i Old Red Sandstone. 



Devonian (Eifelian).^ 



1 T. G. S. (2), V. 724, 727 ; Q. J. viii. 3. 



- See also Mem. Geol. Survey, vol. i. 



■* This is a general term applicable to the Middle Devonian series, being derived 

 from the Eifel district near Coblentz in Germany. The Lower Devonian beds are 

 sometimes grouped under the name Coblentzian. 



