Cockfield Fell 
and Cleveland 
dykes. 
Dykes near 
Egglestone in 
Upper Tees- 
dale. 
24 Professor Sepewick on Trap Dykes 
may, however, admit of many exceptions. For no reason can be 
given @ priort, why the same forces, which produced the great 
fissures in our coal formations, should not again come into action 
in successive epochs in the natural history of the earth. Accord- 
ingly, it is found that basaltic dykes are not confined to any 
particular set of strata, but may occasionally appear among the 
newest secondary rocks. The facts exhibited by the north coast 
of Ireland have been already alluded to. The great dyke which 
starting from Cockfield Fell, in the county of Durham, crosses the 
plain of Cleveland, and terminates in the eastern moors of York- 
shire, leads us to a similar conclusion. 
This dyke, which preserves such an extraordinary continuity, 
forms a striking feature in all the geological maps of the 
district. Some good general descriptions have already been given 
of it*. My principal object in this paper will be, to place before 
the Society, in a connected point of view, those facts which appear 
to bear on the question of its origin. I shall afterwards notice 
some phenomena which are exhibited in High Teesdale, and seem 
to throw light on the same question. 
A mass of trap occupies the lower part of the left bank of 
the river Tees exactly opposite to the entrance of the Lune. It 
may be traced without difficulty for three or four hundred feet, 
close to the edge of the water; and it at length disappears under 
Egglestone bank; where it rests upon, or abuts against a bed 
of slate clay. The prolongation of the trap to the other side of 
the Tees is rendered highly probable by the appearance of a bed 
of similar character in the left bank of the Lune immediately 
under Lonton Chapel. But the accumulation of diluvium pre- 
vents this connexion from being established by direct evidence. 
The imperfect denudation on the left bank of the Tees did not 
* See the Geological Survey of the Yorkshire Coast by Young and Bird, p. 171. 
+ Geological Transactions, vol. IV. p- 76. 
