PENGELLY ON THE DEVONIAN AGE OF THE WORLD. 833 



during which the limestones, slates, and associated sandstones of 

 North and South Devon were deposited. As nearly as can be 

 determined, contemporary rocks occur in Cornwall, Herefordshire, 

 Wales, Scotland, Ireland, France, Belgium, Germany, Russia, Turkey, 

 Siberia, Tartary, China, Central and South Africa, Australia, 

 Tasmania, Falkland Isles, Brazils, and various parts of North America. 



The history of the period has been largely and ably illustrated by 

 Hugh Miller, De la Beche, Lonsdale, Sedgwick, Murchison, Austen, 

 Phillips, Rogers, Bigsby, and many others. 



It appears to have been a period in which red deposits prevailed, 

 the colour being due to the presence of the peroxide of iron. In this 

 respect it is contrasted strongly with the Silurian beds below and 

 the Carboniferous limestone above ; the change, however, is in 

 neither case uhiformally suclder., so that by the test of colour alone 

 it is not easy to draw a sharp line of separation between the 

 Devonian and the more ancient or more modern system. The red 

 colour is less prevalent in Devonshire, — this is especially the case in 

 South Devon, where the deposits are chiefly clay-slates, and lime- 

 stones, commonly grey or more or less blue. The characteristic red 

 rocks are well developed in Herefordshire and many parts of Scotland, 

 where they have been carefully studied under the name of " Old Red 

 Sandstone," a term now generally regarded as a chronological 

 synonyme for " Devonian." 



Red colours, however, are by no means confined to the period now 

 under notice ; this, indeed, is implied by the epithet Old Red," 

 used for the purpose of distinguishing the deposits to which it is 

 applied from others of the same colour above, and therefore more 

 modern than, the Carboniferous formation ; and which were formerly 

 known, as they are still occasionally, as the New Red Sandstone. Here, 

 again, it was necessary to speak of the Upper and Lower New Red, 

 now the Triassic and Permian systems. 



Nor are still more modern deposits destitute of this, hue, as has 

 been pointed out by Sir C. Lyell, when speaking of the Upper Eocene 

 formation of Auvergne.* 



The thickness of the Devonian rocks has been estimated at ten 

 Ihousand feet in Herefordshire ; at least twelve thousand feet in 

 Ireland, and eleven thousand nine hundred and fifty feet in North 

 America. 



Considerable variety of opinion has prevailed respecting the age 

 of the rocks of North and South Devon and Cornwall ; nor is this 

 surprising, since they are completely isolated, frequently display 

 great metamorphism and mechanical violence, and have very few, 

 if any, fossils in common with rocks, now known to be, of the same 

 age elsewhere in the British Isles. Thanks, however, to the labours 

 of Mr. Lonsdale, Professor Sedgwick, Sir R. I. Murchison, and 

 others, they have been determined to be, as has been already stated, 



Manual, 5th Edition, page 109. 



