1848.] NICOL ON THE SILURIAN ROCKS OF THE TWEED. 199 
boniferous group. Its relation to this deposit is very distinctly seen 
both on the north and south. On the former it is well-exhibited in 
a section in the western part of the Pentland Hills, exposed along 
the channel of the Lyne*, one of the tributaries of the Tweed. 
Highly-inclined strata of greywacke and clay-slate form the bed of 
the stream for a considerable distance. On the broken ends of these 
slate rocks a formation of grey or very light red sandstone rests in a 
gentle curve, corresponding to the ridge of the mountains, which in 
this place appear to have been formed by the upheaval of the grey- 
wacke and superincumbent sandstones. Similar sections are common 
in the eastern part of the Pentland chain, but more complex from 
the intrusion of the clay-stone porphyries and other igneous rocks. 
Fig. 2. 
Red = 
Sandstone. = oa = — 
ES = ——— 
: 
Section Fig. 2 of Southdean hill on the Jed is on the south side of 
the formation, and forty-five miles distant in a direct line from that 
just described. In it the greywacke appears in the channel of the 
river in highly inclined beds covered round the base of the hill by 
nearly horizontal strata of red sandstone, which is again overlaid by 
amass of dark greenstone forming the summit. Lower down the 
Jed there are several similar sections, one of which near Jedburgh, 
first described by Hutton, is now well known, having been copied 
into many popular works on geology. I shall only notice another 
section in this vicinity, which is interesting as showing the relation 
of the igneous rocks of the Cheviot mountains to the red sandstone 
Greywacke. 
* This river rises near the ridge of the Pentlands in the old red sandstone, it 
then passes into the greywacke exposed by the denudation of the sandstone, and 
lower down again enters the sandstone, which is succeeded by a band of porphyry 
rocks forming the exterior portion of the Pentlands. Beyond this it runs through 
a low plane of the old red sandstone, and near Newlands Bridge enters a gorge in 
the greywacke hills by which it is conducted tothe Tweed. Viewing this river 
as its continuation, it crosses the whole chain chiefly in the transverse valleys, till 
at Melrose it enters the Roxburghshire red sandstones, and near Kelso the carbo- 
niferous formations of Northumberland and Berwick: thus exposing in its course 
a complete section of all the formations in the south of Scotland. 
