Chap. XL] 
THE OLD EED SANDSTONE. 
243 
CHAPTER XI. 
THE OLD EED SANDSTONE, OR DEVONIAN ROCE!S, AS EXHIBITED 
IN THE BRITISH ISLES. 
Hiving described at some length the lowest known receptacles of former 
beings, a shorter consideration only can be devoted to the younger races 
which were successively enclosed in the higher tiers of the vast primeval 
burying-places. 
During the accumulation of nearly all the Silurian deposits of Britain, 
which were characterized by a certain fauna, the bottom of the sea was, to 
a wide extent, occupied by dark and grey-coloured sediments. At the 
close of that period a great change occurred over large areas in the nature 
and colour of the submarine detritus. In and around the Silurian region, 
for example, the dark-grey mud was succeeded by red silt and sand ; the 
colour being chiefly caused by the diffusion of iron-oxides in the waters *. 
These physical changes were accompanied by the disappearance of those 
tenants of the deep, the records of which we have been tracing, and by 
the appearance of other animals. 
The gradual passage upwards, from the highest strata of the grey Silu- 
rian rocks into such red deposits of England, Wales, and a part of Scotland, 
has already been alluded to f, and illustrated by several diagrams. 
In the lower junction-beds, as seen within the Silurian region, it is only 
by the detection of certain Upper Ludlow fossils in thin beds or tilestones, 
partially also of reddish colours, that the limit can be defined, so gradual 
is the mineral transition from the strata of the one era into those of the 
other. Still the change is, on the whole, lithologically well marked in 
that region, from the underlying grey Silurians to the overlying red and 
yellow rocks. 
Good examples of this succession are to be seen near Ludlow, be- 
tween that town and the Clee Hills, and thence all along the eastern edge 
of the Upper Silurian rocks in Hereford, Eadnor, and Brecon, as well as on 
the west flank of the Malvern and May Hills, and around the valley of 
Woolhope. The grandest exhibitions, however, of the Old Eed Sandstone 
in England and Wales appear in the escarpments of the Black Mountain of 
Herefordshire, and in those of the loftiest mountains of South Wales, the 
Fans of Brecon and Carmarthen, — the one 2860, the other 2590 feet above 
the sea. (See Map.) In no other tract of the world, visited by me, have 
* See some excellent observations on the influence of iron-oxides on marine life, by the late Sir 
Henry De la Beche, Mem. G-eol. Surv. of Great Britain and Ireland, vol. i. p. 51. 
t Pp. 55, 56, 58, 64, 87, 106, 110, 124 & 134-144. 
r2 
