~ Parr II. Sxcr. iii. (7) §1.] OLD RED SANDSTONE. 707 
prolific fauna of the Wenlock and Ludlow rocks was driven away 
from Western Europe by the geographical revolutions which, among 
other changes, produced the lake-basins of the Old Red Sandstone. 
When a marine population—crinoids, corals, and shells—once more 
_ overspread that area, it was a completely different one. So thorough 
a change must have demanded a long interval of time. 
Rocks.—<As shown by the name of the type, red sandstone is the 
predominant rock. The colour varies from a light brick-red toa 
deep chocolate-brown, and occasionally passes into green, yellow, or 
mottled tints. The sandstones are for the most part granular 
siliceous rocks, where the component grains of clear quartz are coated 
and held together by a crust of earthy ferric oxide. Scattered 
pebbles of quartz or of various crystalline rocks are frequently 
noticeable among the sandstones, and this character affords a passage 
into conglomerate. The latter rock forms a conspicuous feature in 
many Old Red Sandstone districts. It varies in thickness from a 
_ mere thin bed up to successive massive beds, having a united thick- 
ness of several thousand feet. ‘The pebbles vary much in com- 
position. In some beds they are chiefly of quartz, in others of 
granite, syenite, quartz-porphyry, gneiss, greywacke, or other crystal- 
line or compact rocks. ‘They are sometimes tolerably angular, 
particularly where the conglomerate rests upon schists or other rocks 
which weather into angular blocks. In the upper Old Red Sand- 
stone, thick accumulations of subangular conglomerate or breccia 
recall some glacial deposits of modern times. Yor the most part 
the stones in the conglomerates are well rounded, sometimes indeed 
remarkably so, even when they are a foot or more in diameter. 
Their size ranges up to blocks five feet or more in length; but these 
larger masses are usually angular fragments that have been derived 
from rocks in the immediate neighbourhood. The smaller rounded 
blocks must often have come from some distance; at least it is impos- 
sible to discover any near source for them. Bands of red and green 
clay or marlite occur, in which seams and nodules of cornstone may 
not infrequentiy be observed. Here and there, too, the sandstones 
assume a flaggy character, and sometimes pass into fine grey or olive- 
eoloured shales and flagstones. Organic remains occur in some of 
these grey beds, but are usually absent from the red strata, though 
in some of the conglomerates teeth, scales, and broken bones of fishes 
are not uncommon. In the north of Scotland peculiar very hard 
calcareous and bituminous flagstones are largely developed, and have 
_ yielded the chief part of the remarkable ichthyic fauna of the 

system. In Scotland, also, contemporaneously erupted porphyrites, 
felsites, and tuffs play an important part in the petrography of the 
Old Red Sandstone, seeing that they attain a thickness in some places 
of more than 6000 feet, and form important ranges of hills. 
Life.—No greater contrast is to be found between the organic 
contents of any two successive groups of rock than that which is 
presented by a comparison of the Upper Silurian and ae ne Sand- 
Z 
