1848.) NICOL ON THE SILURIAN ROCKS OF THE TWEED. 195 
JANUARY 5, 1848. 
The following communication was read :— 
On the Geology of the Silurian Rocks in the Valley of the Twrrp. 
By James Nicot, Esq., F.R.S.E., Assistant Secretary Geol. Soc. 
THERE is perhaps no extensive formation in the British Islands of 
which we possess less certain geological knowledge, than of the rocks 
constituting the great mountain-chain which crosses the southern 
counties of Scotland from east to west. These deposits have indeed 
been long ascertained to be of far more ancient date than the old red 
sandstone and carboniferous rocks with which they are in contact ; 
and from this and their mineralogical characters have usually been 
- classed in the transition series of Werner, and more recently placed 
hypothetically on the parallel of one or other portion of the Silurian 
rocks of England. This very uncertain determination of their age 
has in a great measure arisen from the extreme rarity of their fossil 
remains, which till very lately were scarcely known to exist. Having 
in the course of last autumn succeeded in procuring a few specimens 
of organic remains from the central portion of this formation, in the 
upper part of the valley of the Tweed, I have thought that a notice 
of these may not be without interest to the Society. With this I 
shall combine some observations on the mineralogical character and 
position of the rocks in this district, and the indications which these 
afford of the geological age of the formation and the conditions under 
which it has been produced. Though chiefly limited to those districts, 
with whose structure I have had most opportunity of becoming per- 
sonally acquainted, these remarks will yet, I believe, be found appli- 
cable to the whole of this formation in the south of Scotland. 
The most remarkable physical feature of this district is the moun- 
tain-chain which extends from St. Abb’s Head on the east coast, in 
a W.S.W. direction, to the vicinity of Port Patrick on the opposite 
side of the island, a distance of 140 miles, with an average breadth 
of 25 to 30 miles. This high-land consists less of a smgle connected 
chain than of a group of smaller ridges, separated by longitudinal 
valleys, running parallel to the chief direction of the mass, or from 
E.N.E. to W.S.W. In these valleys the principal rivers, with the 
exception of the Tweed, have their course,—often nearly in a straight 
line. These ridges and valleys are crossed almost at right angles by 
another system of valleys, in which many of the secondary streams 
flow, and which is also followed by the Tweed in a considerable part 
of its upper course. By these two systems of valleys, the high-land 
is divided into huge oblong mountain-masses, often with steep decli- 
vities and flat, or almost tabular, summits. When seen from the 
low ground the mountains have thus apparently a very complex or - 
irregular arrangement ; but wherever a proper view is obtained from 
the high grounds, their disposal in parallel ridges is distinct. : 
The whole eastern portion of this extended mass of mountains is 
composed of greywacke and clay-slate, with a few beds or veins of 
felspar porphyry and trap rocks. In the south of Scotland these 
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