Whin-Sill, 
156 Professor Sepewick on the 
In the other direction, to the S.W. of Saddle-bow, it passes 
into the form of a regular dyke, the range and extent of which I 
had, however, no opportunity of examining. 
I think it quite impossible to convey a correct notion of the 
physical structure of this part of Lunedale by mere verbal descrip- 
tion. I have, therefore, endeavoured to shew the manner in which 
the intrusive rocks appear at the surface by help of a section 
(Pl. viit. Fig. 3.) passing through Saddle-bow in a direction from 
N.E. to S.W. The filling up of this section is partly hypothetical. 
It is, however, sufficiently founded on direct observation to prove, 
that the trap can not have been originally interstratified with the 
other beds which form a regular part of the Teesdale series 
(Section 1. p. 8). 
(3.) I next proceed toe consider the manner in which the great 
Whin-Sill appears among the strata of Teesdale. It is first seen in 
the bed of the Lune immediately under Lonton Chapel, where its 
whole thickness does not amount to more than eleven or twelve 
feet. In my former paper (p. 24.) I remarked the probable con- 
nexion between this bed of trap, and the mass, of the same 
mineralogical character, which appears in the north bank of the 
Tees. I still think this connexion very probable ; but I must at 
the same time observe, that the trap on the north bank of the river 
has little appearance of being a regular bed, and is in contact with 
rocks which belong to a higher part of the general Teesdale section. 
The trap under Lonton Chapel is exactly parallel to the beds which 
are above it and below it; and by the gradual rise of the whole 
system of strata it is brought to the top of the escarpment on the left 
bank of the river, about two hundred yards from the former place 
(Pl. rx. Fig. 1.). Beyond this place it is lost under a great mass of 
gravel, but it again breaks out in the hill side, on the road leading 
from Lonton te Middleton. From thence it is continued, as has 
already been stated, for several miles on the south side of Teesdale, 
