268 Palao-Geological and Geographical Maps of the British Islands. 
land with the northern Highlands of Scotland and Ireland. The western and 
northern limits of this land area are incapable of definition; it may have 
included isolated basins besides those we are able to identify in North Britain. At 
the commencement of the period we are now dealing with, land prevailed to a 
much greater extent than that shown in the map, but as time went on, the areas of 
the sea and inland lakes were extended down to the close of the Devono-Silurian 
period. 
Puate XXVI. 
The Devonian Period. 
The epoch represented in Figures 1 and 2, Plate XXVLI., is that ranging through 
the Lower and Middle Devonian stages, embracing the beds of the “ Lynton,” 
«‘ Hangman,” “ Ilfracombe,’ and “ Morthoe” divisions of Devonshire, and those 
lying between the Systéme Gedinnien, and the Calcaire de Frasne of Belgium. 
This series, several thousand feet in thickness, is entirely marine. It is laid open 
in North and South Devon in England, passes below the Cretaceous rocks of the 
Thames valley, and re-appears in numerous sections along the river valleys of 
Belgium, such as those of the Sambre, the Meuse, and the Ourthe, as well as along 
the valley of the Rhine and its tributaries. In a somewhat altered form it occu- 
pies a large tract of country bordering the valleys of the Usk and the Wye, in 
Monmouthshire and Herefordshire, and is generally, but erroneously as I believe, 
called by the name of “ Old Red Sandstone.” These last-named beds, I consider 
to have been deposited in an estuary, bounded towards the north-west and north- 
east by Silurian lands, but opening southwards into the sea, in which the Devonian 
beds were being contemporaneously formed. I have, therefore, called these 
beds “‘Estuarine Devonian.”* From the remainder of the British Islands, including 
the whole of Ireland and Scotland, the Lower and Middle Devonian beds are 
absent, owing to causes which I shall presently endeavour to explain.t 
Nature of the Devonian Beds.—From what has been said, it will be inferred that 
the Devonian beds south of the Severn differ in some characters from their repre- 
sentatives north of that river. 
South of the Severn the formation consists of beds of grit, shales, and limestone, in 
alternating masses, highly fossiliferous, and yielding remains of molluscs, corals, 
and crinoids, and some plants.{ North of the Severn the beds consist of red and 
* «“ On the Relations of the Rocks of the South of Ireland to those of North Devon, &c.”—Quart. 
Journ. Geol. Soc., May, 1880, p. 268. The term used in this paper is “lacustrine.” I have since 
preferred the term estuarine. 
} As I have shown in the paper above referred to.—Zbid., pp. 264 and 270-3. Mr. Etheridge in his 
Presidential address expresses his concurrence in my views.—Quart. Journ. Geol. Soc., May, 1881, p. 
193, e& seq. 
{ Mr. Etheridge has given a complete account of the fauna of Devonshire in his Presidential address, 
supra cit. He enumerates no less than 235 species as occurring in the Middle Devonian beds of South 
Devon. 
