BANKS OF THE TWEED AND SOME OF ITS TRIBUTARIES. 549 
(1.) Thus below Preston, on the River Whitadder, there is a high bank 
which gives a good section of mar] strata, covered by boulder clay. The strata 
have been evidently pushed out of their position by some force pressing against 
them from the westward. Boulder clay lies over these disturbed strata. It is 
therefore not unreasonable to suppose, that this boulder clay may have been the 
means of dislocating the strata. The locality is about 300 feet above the sea. 
(2.) At Langton Quarry, about 250 feet above the sea, near Dunse, the 
sandstone strata have been broken up, and the fragments lie in a confused 
manner. Over the fragments, there are beds of sand and gravel, evidently 
deposited by water. (Pointed out by Mr SreveEnson of Dunse.) 
(3.) At Letham, not far from Berwick, about 120 feet above the sea, there 
is a quarry where the sandstone strata have been passed over, and planed 
down to a horizontal surface; and over them is a bed of clay full of pebbles 
and small boulders. 
_ (4) A whinstone dyke runs E. by N. and W. by S. through the districts 
south of the Tweed, called the Mattilees Dyke. It stands up above the general 
surface of the land in many places. Mr Carr of Felkington pointed out to me 
when I lately examined this dyke, how in several parts of its course, large 
blocks of the dyke lie on the south side of the dyke, and very seldom on the 
north side; showing that some agent of great power had passed over the dyke 
from the north, pushing masses of it over to the south. 
I1L—Theoretical Views. 
1. Under this head, I may first raise the question, Has the River Tweed 
formed or excavated the valley in which it flows, or does the river run in a 
valley previously formed ? 
Probably several causes have contributed to form the valley. 
In the first place, there has been considerable dislocation and rupture of the 
earth’s crust in the district now occupied by the river. 
In this part of Great Britain, it will be remembered, that all the principal 
rivers run east and west. The formation of the Forth river and its estuary, 
is owing to dislocations, which occurred along the valley now occupied by the 
estuary and river—dislocations* forming lines whose average direction is nearly 
east and west. 
With regard to the estuary of the Tay, though not so well acquainted 
with that district, I know that there are eight or ten whinstone dykes which 
run through Forfarshire, each in a direction nearly east and west. Many years 
ago, I traced their course and mapped them. 
Professor RAMSAY, in a recent paper, explains that the River Eden, which 
* These dislocations are explained in a little treatise, entituled “The Estuary of the Forth.” Black- 
wood: 1871. 
VOL. XXVII. PART Iv. 7 *F 
