250 Prof. Sedgwick on some Trap Dykes 
computation falls below the truth; in consequence of the pro- 
bable extension of the dyke to the N. W. through the. Wood- 
land Fells and Egglestone Burn to the banks of the Tees. 
Should this supposition be admitted, we shall have an unin- 
terrupted dyke extending from High Teesdale to the confines 
of the eastern coast, a distance of more than sixty miles. 
The angle at which it cuts the strata is of course variable, 
and in many places cannot possibly be ascertained. At Bar- 
wick, near the Tees, its inclination to the horizontal beds of 
sandstone is more than eighty degrees ; and the angle at which 
it intersects the beds of shale and sandstone in the eastern 
moors is still greater; occasioned, perhaps, by the south- 
eastern dip, which generally prevails among the strata in that 
region *. 
Secondary formations, when interrupted in the manner 
above described, seldom preserve the same level on the oppo- 
site sides of their line of separation. Thus at Cockfield Fell, 
the coal-beds on the north side of the dyke are eighteen feet 
below the corresponding beds on the south side. In the ex- 
cayations at Preston and Barwick there is no indication of 
any great change having been produced in the relative level 
of the beds of sandstone; nor can any conclusive evidence be 
obtained on this subject from the obscure sections exhibited 
by the quarries in the eastern moorlands. Perhaps, as a 
general rule, the greatest dislocations are produced by those 
fissures into which trap is not intruded: such at least appears 
to be the case in the great coal-field of Northumberland and 
Durham. The injected masses of trap may be supposed to 
have acted as a kind of support, and to have partially hindered 
the broken ends of the strata from sliding past each other. 
Structure of the Dyke. 
Notwithstanding the great length of the Cleveland dyke, and 
the different nature of the rocks with which it is associated, it 
undergoes very little modification in its general structure. Its 
prevailing character is that of a fine granular trap rock of a 
dark blueish colour. This colour is indeed, with some unim- 
portant exceptions, so constant in all the sound specimens, that 
the dyke is provincially termed blue-stone by the men who 
are employed in working the quarries. It breaks into irre- 
gular, sharp, angular fragments; and on a recently exposed 
surface there generally may be seen a number of minute bril- 
liant facets: but the constituent parts are never sufficiently 
distinguished from each other to give it the appearance of a 
green-stone. ‘The essential ingredients of the rock are, if I 
* See the Survey of the Yorkshire coast by Young and Bird. 
mistake 
