264 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
BOOK V 
stones with their masses of coral and crowds of crinoids, there were now 
hud down, over these northern regions, vast piles of deep red sediment, from 
which traces of animal life are almost wholly absent. The shelving laud 
against which these ferruginous sands and gravels gathered can still in part 
be recognized. As the observer follows its margin, notes the varying local 
peculiarities of its sediment, and detects, sometimes in great abundance, 
remains of the vegetation which clothed it, the conviction grows in his 
mind that the remarkable contrast between these deposits, known as the 
Old Eed Sandstone, and those of the Silurian and Devonian systems is not 
to be accounted for by any mere rearrangement of the sea-bottom, or re- 
distribution of the land that supplied that sea-bottom with sediment. It 
has long been the general lielief among geologists that the subterranean 
inovements which, over the greater part of Dritain, brought the deposition 
01 the Upper Silurian formations to a close, led to a total alteration of the 
geography of the region affected, that the sea -floor was elevated, and 
that, over the upraised tract, large lakes or inland seas w^ere eventually 
formed, in wliicli the peculiar sediments of the Old Eed Sandstone were 
accumulated. 
The records of this series of geographical changes are too fragmentary 
to enable us to follow, except in a very general way, the sequence of events 
111 the transformation of the Silurian sea into the peculiar topographical 
conditions in which the Old Eed Sandstone was laid down. While there 
was a wide-spread elevation of the sea-floor, and of such ridges of insular 
land as may have risen above sea-level, the upheaval appears to have been 
of a somewhat complicated kind, and to have been combined with many 
local subsidences. The area of disturbance was probably thrown into a 
series of parallel ridges and troughs, the former continuing to lie pushed 
upw’ard, while the latter tended to subside. The ridges thus became; land 
surfaces, and them prolonged elevation may have more or less compensated 
for the denudation to which, on their emergence, they were necessarily 
exposed. The troughs, on the other hand, which sank down, may in many 
cases have subsided below the sea. But where the general upheaval of the 
crust was most pronounced, some of the depressions would be isolated above 
sea-level and become lake-basins in the terrestrial areas. 
Of some of these water-basins the outlines can still in some measure be 
defined. The rocks that rose into hills around them, and from which their 
enormous accumulations of detritus were derived, still partially survive. We 
can explore these piles of sediment, and from them can form some idea of the 
condition of the water in the lakes, and the nature of the vegetation on 
the surrounding land. The frequent occurrence and exceeding coarseness of 
t le conglomerates, which appear on many successive horizons throughout 
the deposits of these basins, probably indicate contemporaneous terrestrial 
disturbances. The same causes that led to the wrinkling of the crust into 
parallel ridges and troughs no doubt still continued in operation. Eroni 
time to time the ridges, much worn down by ]n’olonged denudation, were 
pushed upward, either by gradual uprise or by more rapid jerks. The 
