Geology of High Teesdale. 157 
forming im many places a magnificent escarpment, and not un- 
frequently putting on a rude columnar form. That the Whin- 
Sill is here imbedded among the other strata and nearly parallel to 
them cannot be doubted. Its upper and lower surfaces are not, 
however, exactly parallel, as it continually increases in thickness 
in its range up the valley towards Holwick. Near that place its 
whole thickness is, perhaps, more than thirty fathoms. 
As the mean inclination of the bed of the Tees, between Mid- Trp in the Bea 
dleton and the High Force, is considerably less than the dip Egle tpi 
the strata on the north side of the great longitudinal fault; it 
follows that in advancing up this part of Teesdale by the side of 
the river, we must necessarily cross a series of beds of the general 
section presented in a descending order. In this way (after pass- 
ing all the beds from No. 38 to No. 70.) we find the Whin-Sill 
(No. 71.) rising up in the bed of the river about half way between 
Bowlee Beck and Winch Bridge. Not far from the last-mentioned 
place it comes into immediate contact with the trap on the south 
side of the great fault—a fact which seems to prove, that the dis- 
location, produced by the rupture of the strata, is much less there, 
than it is farther down the valley. The collocation of the strata 
will be best understood by a reference to the accompanying map 
(Pl. vir.) and to a transverse section of the strata (PI. vii. Fig. 2.) 
From the place of its first appearance, between Bowlee Beck 
and Winch Bridge, the Whin-Sill extends on the north side of the 
fault for more than a mile through a low flat region, which, had it 
not been intersected by a deep chasm affording a passage to 
the river, would have been devoid of any geological interest. 
The only direct communication between the opposite sides of 
this portion of the dale, is over Winch Bridge, which is formed 
of a single plank of wood suspended by chains from the two 
escarpments of columnar rock which rise on the opposite sides 
of the river. 
