Geology of High Teesdale. 161 
a great bed of trap, which, in some places, appears to be 
more than two hundred feet thick. The two escarpments which 
are thus formed opposite to each other, are often broken into 
the finest forms, and marked by clusters of columns, which are 
grouped with much more regularity than in any other part of 
Teesdale. The trap can on neither side of the valley be con- 
sidered as an overlying formation; for the higher portion of the 
metalliferous series rests immediately upon its upper surface. 
At first, also, it appears to partake of the general inclination of 
the other strata. We might, therefore, conclude, that it was 
a regular bed inter-stratified with the metalliferous limestone. 
But, if we follow the part of the valley above-mentioned, from 
the commencement of Widdy Bank to Caldron Snout, a dis- 
tance of about two miles, we have repeated opportunities. of 
verifying its true relations to the subjacent strata: and on the 
north side of the river, not more than two or three hundred 
yards below Caldron Snout, we find the base of the trap gra- 
dually sweeping over the broken ends of the stratified rocks, 
and descending into the bed of the river. 
These phenomena will be better understood, by referring to Natural 
the accompanying sections (PI. 1x. Fig. 4, 5.) than by any verbal ““”” 
description. The first section (Pl. rx. Fig. 4.) is transverse to 
the valley, and shews the apparent relation of the Whin-Sill 
to the whole formation of metalliferous limestone. The second 
is longitudinal, and shews the true relation of the trap to the 
inferior strata, as it is exhibited in the natural sections below 
Caldron Snout. These two sections completely demonstrate, that 
the Whin-Sill (at least in this region) has not been regularly 
deposited with the other stratified rocks, and that it is not limited 
to a given place in the general section of the Teesdale strata. 
There is, indeed, an evident impropriety in designating these 
masses of trap by the name of Whin- Sill: for the word Sill, 
Vol. II. Part I. X 
