LAKES IN RELATION TO GEOLOGICAL FEATURES 467 
sediments, whose remains are traceable across parts of the plateau. 
Remnants of the old consequent river system established before the 
isolation of the three blocks are still preserved in the southern region, 
of which the Nith is the finest example. 
On the south side of the plateau the subsequent or longitudinal 
system of drainage has been set up by rivers attacking the weak 
sediments in flank. Thus the Tweed, working from the east, along 
the less resistant Carboniferous and Upper Old Red Sandstone strata, 
eventually followed the grain of the Silurian rocks, and by these 
means was able to intercept all the old consequent drainage west- 
wards to beyond the centre of the plateau. South of the plain of the 
Merse, remnants of the old transverse streams again appear ; for the 
Coquet, Rede, and Tyne, rising on the north side of the Cheviot 
range, cross it in well-marked hollows. 
The tributary of the North Channel River, flowing along what is 
now the plain of the Solway Firth, cut its way backwards along the 
younger sediments. In the higher reaches of this stream, the Liddel, 
by following the grain of the Lower Carboniferous rocks, captured 
some of the smaller streams belonging to the Tweed system. The 
Solway-Liddle tributary deflected, from Luce Bay eastwards to the 
Esk, the old consequent rivers that crossed the Southern Block. 
On the north side the development of the Central Plain has 
obliterated most of the courses of the old consequent streams, but 
the Doon on the west and the Clyde in the middle have maintained 
the old direction of their valleys by becoming obsequent streams. 
Frequent reference has been made in geological literature, and 
particularly by Sir A. Geikie, to the remarkable course of the river 
Nith. The infant stream rises on the north slope of the Southern 
Uplands, and flows northwards from the Silurian plateau to the plain 
of Carboniferous strata, along which it runs in an easterly direction 
for five miles to New Cumnock, where it changes its course to the south- 
east in the direction of the Solway. The easterly course of the 
stream above New Cumnock was doubtless determined by a subsequent 
tributary of the old consequent river that crossed the plateau before 
the isolation of the three blocks. 
As in the northern and central regions already described, there 
is evidence to show that, in the Southern Block, the eastward-flowing 
streams extended farther to the west than at present, and that portion 
of their territory has been captured by rivers draining to the west 
and south-west. The Tweed may be instanced as an excellent 
example of these mutations. By means of its tributary, now repre- 
sented by the Biggar Water, it cut backwards till it captured the 
old consequent Clyde, a large part of which it rendered obsequent. 
It also appears to have receded far to the west by appropriating the 
