CHAP. XVI 
THE OLD RED SANDSTONE OF BRITAIN 
265 
troughs may in like niaiiiier have been still affected by theii old tendencj 
to subsidence. Hence, in spite of the effects of degradation and deposition, 
it is possible that the ridges may not, on the whole, have varied much in 
height, nor the basins in depth, during the time when thousands of feet were 
stripped off tlie land and strewn in detritus over the bottoms of the lakes. 
Let us try to realize a little more definitely the general aspect of the 
region in which the Old lied Sandstone water-basins lay. As the axes of the 
folds into which the crust of the earth was thrown ran in a north-east and 
south-west direction, they gave this trend to the lakes and to the tracts of 
land that separated them. These intervening ridges must in some instances 
have been hilly or even mountainous. Thus, the Scottish Highlands rose 
between two of the lakes, and poured into them an abundant tribute of 
gravel, sand and silt. The terrestrial vegetation of the time has been 
partially preserved. The hills seem to have been clothed with conifers, 
while the lower slopes and swamps were green with sigillarire, lepidodendia 
and calamites. One of the most characteristic plants was Psilopliyton, of 
which large matted sheets were drifted across the lakes and entombed in 
the silt of the bottom. A grass-like vegetation, with long linear leaves, 
seems to have grown thickly in some of the shallows ot the lakes. 
Of the land animals we have still less knowledge than of the vegetation. 
Doubtless various forms of insect life flitted through the woodlands, though 
no relics of their forms have yet been recovered. But the remains of 
myriapods have been found in Torfarshire.^ These early relics of the 
animal life of the land inhabited the woodlands, like our modern gally- 
worms, and were swept down into the lakes, together with large quantities 
of vegetation. 
Some of the lakes, especially in the earlier part of their history, 
abounded in eurypterid Crustacea. These animals inhabited the seas in 
Upper Silurian time, and appear to have been isolated in the water-basins 
of the Old Bed Sandstone. Certain species of Pterygotus, a Silurian genus 
found also in the Lower Old Bed Sandstone, reached a length of six feet. 
But the most abundant forms of animal life were fishes. These furnish 
additional evidence in favour of the lacustrine nature of the waters in which 
they lived. Such characteristically marine forms as the sharks and rays of 
the Silurian seas were replaced by genera of Acanthodians, Ostracodeims, 
Dipnoids, Teleostomes, Placoderms, and Palseoniscids, which abounded in 
the more northerly waters. The distinctive outward characters of many 
of these early vertebrates were their bony scales and plates. Some of them 
had their heads encased in an armature of bone, of large size and massive 
thickness. In several genera the bone was coated with a layer of glitter- 
ing enamel. Even now, after the vast lapse of time since their day, the 
cuirasses and scale-armour of these fishes keep their bright sheen in the 
hardened sand and mud from which they are disinterred. 
A difference is observable between the faunas of the different water- 
basins. Even where the same genus occurs in two adjacent areas, the 
1 Mr. B. jST. Peach, Proceedings of liogal Physical Society of Minliirgh , vol. vii. (1882). 
