PENGELLY — ON THE DEVONIAN AGE OF THE WORLD. 
333 
during wliicli tlie 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 JSTorth 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 uniformally sudden, 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 modem 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 
synonjTne 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 TJioper and Loicer 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 
thousand 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. 
