LAKES IN RELATION TO GEOLOGICAL FEATURES 447 
with that of the underlying Ludlow rocks — led to a change in the 
classification of these strata by the Geological Survey. Though 
formerly grouped with the Old Red Sandstone, the Downtonian strata 
are now reg-arded as forming the hi£>:hest subdivision of the Silurian 
system in the south of Scotland. 
The Silurian strata are pierced by various igneous masses, com- 
prising granite, quartz-diorite, and hyperite, together with numerous 
dykes of porphyrite, diorite, and mica-trap. From the relations 
which they bear to the Silurian strata and to the Upper Old Red 
Sandstone, it is evident that they are of Lower Old Red Sandstone age. 
Other igneous intrusions of later date traverse the tableland, to which 
reference will be made in the sequel. 
Towards the close of Downtonian time the Silurian strata were 
elevated and subjected to prolonged denudation ; for, both in Ayr- 
shire and the Pentland Hills, the basal conglomerates of the Lower 
Old Red Sandstone, containing greywacke pebbles derived from the 
old tableland, rest unconformably on the folded and eroded edges of 
the Silurian rocks. In Lanarkshire this unconformability has not 
been detected, as there seems to be a passage in that district from the 
one formation to the other. At that period also the crystalline 
schists of the Highlands must have formed a prominent land barrier 
towards the north before the deposition of the Lower Old Red Sand- 
stone sediments. 
THE OLD RED SANDSTONE 
The series of deposits belonging to the Old Red Sandstone are 
generally supposed to have been laid down in inland lakes, owing to 
their distinctive lithological characters and the nature of their 
organic remains. Instead of a profusion of marine forms we find 
abundant remains of land plants, ganoid fishes whose living represen- 
tatives are now found in rivers and lakes, eurypterids, bivalve crus- 
taceans, and myriapods. But, whether lacustrine or marine, it is clear 
that the whole series presents different lithological characters from 
those of the underlying Silurian and overlying Carboniferous rocks. 
The sediments of the Old Red Sandstone may be grouped in three 
great divisions — the lower, middle, and upper — a classification based 
partly on the fish remains and partly on the land plants found so 
abundantly on certain horizons. 
The representatives of the lower division occur to the south of 
the Grampian chain, and in the Central Lowlands form parallel 
belts — the one extending from the coast of Kincardineshire and 
Forfarshire to the mouth of Loch Lomond and the Firth of Clyde, 
the other from the Pentland Hills south-west to Ayrshire. The 
centre of this basin is occupied by Carboniferous and Triassic strata. 
