Geology of High Teesdale. 163 
will, I hope, have made the principal peculiarities in the struc- 
ture of Teesdale sufficiently understuod. In regard to the 
imbedded masses of trap (commonly called the great Whin-Sill) 
it appears : 
- (1.) That immediately below Caldron Snout, they are not summary. 
parallel to the strata between which they are interposed. 
(2.) That between Forcegarth Hill and Holwick, where the 
inferior surface of the trap appears to be nearly parallel to the 
lower strata, the country is intersected by numerous fractures 
which have greatly changed the level of the corresponding parts 
of the different formations. 
(3.) That a fracture passes down the valley, and produces 
a great down-cast which conceals the trap on the north side of 
the river. 
(4.) That the trap on the south side of the valley descends 
among the strata in the form of a great wedge, which diminishes 
in thickness from thirty or forty fathoms to about twelve feet. 
(5.) That, near the apparent termination of the Whin-Sill, 
a mass of trap breaks out on the opposite bank of the Tees, and 
is probably prolonged in the form of a dyke, or system of dykes, 
through the eastern moorlands. 
The importance of these facts cannot be properly under- 
stood, without first considermg the effects produced by the 
contact of the trap with the other rocks of the district. 
Secrion ITI. 
Composition of the Whin-Sill, &c.—Effects produced by its Contact 
with the other Rocks of the District. 
Tue trap rocks of High Teesdale do not exhibit any great Pl Cho. 
racter of the 
variety of external character or of structure. They are often Escarpmens 
x2 of Trap. 
