LAKES IN RELATION TO GEOLOGICAL FEATURES 453 
ascertaining how far these rocks may have extended over the northern 
land barrier. 
Reference must now be made to the intrusive sheets of igneous 
materials occurring on various horizons, which give rise to conspicuous 
features in the landscape. One group is composed of olivine-dolerite 
and teschenite, which, in the Lothians, seem to have been injected 
into the Lower Carboniferous rocks before the Coal Measures were 
deposited. xAnother series, consisting of quartz-dolerite, occurs both 
as sills and east and west dykes, and belongs in all probability to the 
close of the Carboniferous period. 
Powerful subterranean movements again ensued after the deposi- 
tion of the highest members of the Carboniferous system. The great 
succession of sediments that had accun]ulated during this period were 
upheaved and subjected to erosion. 
PEKMIAN AND TRIASSIC 
The series of deposits next in order consist of red and grey sand- 
stones and marls that have been referred partly to Permian and partly 
to Triassic time. In the present stage of inquiry it is difficult to 
define the precise position in the geological sequence of some of these 
sediments owing to the absence of definite palaeontological evidence. 
Partly from the lithological characters of the strata and partly from 
the nature of the organic remains, Ramsay inferred that these rocks 
were laid down in inland lakes or enclosed basins, thus implying 
continental conditions. 
In the south of Scotland they occupy various isolated areas, as, for 
example, (1) in the south of Arran, (2) in the centre of the Ayrshire 
coal-field, (S) at Thornhill, Dumfriesshire, (4) on the shores of the 
Solway, (5) at Lockerbie, (6) at Mofiat, (7) at Loch Ryan. Of 
special interest are the contemporaneous volcanic rocks underlying 
these sandstones in Ayrshire and at Thornhill, regarded by Sir A. 
Geikie as of Permian ao:e. In the resrion surroundine; these volcanic 
rocks, from Muirkirk to Dalmellington, numerous vents or necks pierce 
the strata including the highest Carboniferous rocks. Formerly the 
sediments at these various localities were regarded as of Permian 
age, but this classification has been modified by more recent research. 
The discovery of fossils characteristic of the Avicula contorta zone 
(Rhaetic) and of Lower Liassic age in Arran led to the grouping of 
the sandstones and marls in south Arran with the Trias. The red 
sandstones overlying the volcanic rocks of Ayrshire and those at 
Thornhill have been ranged with the Trias of south Arran on litho- 
logical grounds. Again, the sediments stretching along the coastal 
belt from Annan to Canonbie have been grouped with the Runter 
sandstones of Cumberland, which in the latter region are succeeded by 
