zn Yorkshire and Durham: 217 
them with the dykes in Upper Teesdale—(3) That it pro- 
bably gives out some lateral branches connecting it with other 
masses of trap in the same district. It may further be ob- 
served, that all this portion of the dyke, however 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 de- 
ees. 
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 con- 
jecture that a great fault, ranging along the line of demarca- 
tion, had thrown the magnesian limestone down below its na- 
tural level. But the supposition is not necessary; for the ap- 
pearance of the limestone below the level of the ridge may be 
only an indication of its unconformable position. 
Dyke in Lower Teesdale. 
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 excava- 
tions 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 tact, 
that the basaltic veins in the great coal-field do not generally 
pass up into the magnesian limestone; have led some to ima- 
gine, that the prolongation of the dyke of Cockfield Fell is for 
several miles concealed beneath the beds of that formation. 
These basaltic veins, which do not penetrate the magnesian 
limestone, prove one of two things. ‘Either that they took 
their present form before the deposition of the limestone; or 
that they were injected from below, but not with sufficient 
energy to break through the superincumbent limestone.—Nei- 
ther of these suppositions can apply to a great dyke intersecting 
an enormous mass of secondary strata which are newer than 
the magnesian limestone, and probably rest upon it. If there- 
* 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. 
Vol. 67. No. 335. March 1826. 2 fore 
