Chap. XL]                      THE OLD RED SANDSTONE.                                243
 

CHAPTER XI.

THE  OLD  EED  SANDSTONE, OR DEVONIAN EOCKS, AS  EXHIBITED

IN THE BEITISH ISLES.

Having 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 aU 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 SUurian 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 SUu¬
rian rocks into such red deposits of England, Wales, and a part of Scotland,
has already been alluded to f, and iUustrated by several diagrams.

In the lower junction-beds, as seen within the SUurian region, it is only
by the detection of certain Upper Ludlow fossils in thin beds or tUestones,
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. StiU the change is, on the whole, hthologicaUy weU marked in
that region, from the underlying grey Silurians to the overlying red and
yeUow rocks.

Good examples of this succession are to be seen near Ludlow, be¬
tween that town and the Clee HiUs, and thence all along the eastern edge
of the Upper SUurian rocks in Hereford, Radnor, and Brecon, as well as on
the west flank of the Malvern and May HiUs, and around the vaUey of
Woolhope. The grandest exhibitions, however, of the Old Red 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. Geol. Surv. of Great Britain and Ireland, vol. i. p. 51.
t Pp. 55, 56, 58, 64, 87,106, 110, 124 & 134-144.

e2