Dyke in Lower 
Teesdale. 
28 Professor Sepewick on Trap Dykes 
same line of direction, and through them with the dykes in Upper 
Teesdale —(3) That it probably gives out some Jateral branches 
connecting it with other masses of trap in the same district. It 
may farther be observed, that all this portion of the dyke, how- 
ever modified by local circumstances, dips towards a point on 
the north-eastern side of its general line of direction, so as to 
make with the horizon an angle perhaps in no instance less than 
eighty degrees. 
The high ridge of coal strata, extending from Bolam to 
Houghton-le-side, forms a kind of abutment which encroaches 
considerably on the line of the magnesian limestone. The 
present collocation of the two formations might lead to a 
conjecture that a great fault, ranging along the line of demarca- 
tion, had thrown the magnesian limestone down below its natural 
level. But the supposition is not necessary ; for the appearance 
of the limestone below the level of the ridge may be only an 
indication of its unconformable position. 
In the low region of the magnesian limestone we lose all 
traces of the basalt from Houghton-le-side to Coatham Stob. 
From the last mentioned place it may be traced through 
the quarries of Preston across the Tees; and very large ex- 
cayations have been made in a corresponding quarry at Barwick 
on the right bank of the river. The mineralogical character of 
this dyke, its direction, and its dip, agree so well with the one 
which ranges through Cockfield Fell ; that no one has, I believe, 
denied the probability of their being continuous*. The great 
distance between Houghton-le-side and Coatham Stob in which 
no trap has been discovered; and still more the fact, that the 
basaltic veins in the great coal-field do not generally pass up 
* Should any one maintain that the dykes of Cockfield Fell and the plain of Cleveland 
have a distinct origin; he may, perhaps, draw an argument in favour of his own opinion, 
from the great thickness of the vein of trap in the quarries of Preston, Barwick, Langbargh, 
&c. In this one respect there is undoubtedly a considerable difference between them. 
