434 CO. T. Clough—The Whin Sill of Teesdale. 
Sill is a sill deposited contemporaneously with the beds among which 
it lies, or it is a great intrusive mass breaking through and thrusting 
aside these beds like a wedge. They are on the horns of a dilemma. 
How can the Whin be a sill when it changes its horizon relatively 
to the other sills? How can it be an intrusive wedge when there 
is such a general absence of disturbance in the beds around ? 
William Hutton (Trans. Newcastle Nat. Hist. Soc. vol. ii. part 1., 
“On the Stratiform Basalt Associated with the Carboniferous 
Formation of the North of England”’) felt the same difficulty. The 
unconformity between the Whin Sill and the surrounding beds, 
which had been noticed by Sedgwick (Camb. Phil. Trans. 1824, 
“Geology of High Teesdale”), he thinks may be explained by the 
“nutting in” of new sedimentary beds. He says (p. 205, op. cit.), 
“There are many well-known instances of the ‘putting in’ and 
thickening of strata in the Coal-field, and Mr. Buddle, in his valuable 
sections published in the Transactions of the Natural History Society 
of Newcastle, has shown several: one of the most remarkable of 
these is mentioned at p. 201, where a bed of sandstone, twenty 
fathoms thick, is traced thinning out until it becomes a stone band 
in the Bensham Coal-seam in Wallsend Colliery, and is known 
finally to disappear.” 
If there really is such a general absence of accompanying 
mechanical disturbance, as I have indicated, then it seems to me 
that we shall be forced to conclude that the Whin consists in part 
of altered sedimentary beds, that it partly represents beds which 
were once in the position it now occupies, that it did not make room 
for itself simply by thrusting aside these beds, but also by in- 
corporating them into itself. 
As this conclusion is a very important one, I shall not content 
myself with taking the fact as proved on the authority of Hutton 
and the Dalesmen. merely, but shall proceed to mention in detail 
particular sections which show most conspicuously the absence of 
any disturbance. 
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Fie. 1.—Section on the Durham bank of the Tees, 180 yards from the foot of the 
Maize Beck. 
a Basalt. 6 Posts of saccharoid Limestone. ¢ Shale-bed; rather irregular. | 
Length of section, 20 yards. 
A few hundred yards below Caldron Snout, and on the Durham 
side of the Tees, the base of the Whin (or rather of a Whin) i is seen, 
