in Yorkshire and Durham. 253 
which have been already described, the trap passes through 
horizontal beds of slate-clay, and the changes produced by its 
presence are in all these cases strikingly similar. At Nunthorp 
and Langbargh these beds of slate-clay belong to the great 
alum-shale formation (lias), and are easily identified by the 
belemnites, pectinites and other characteristic fossils which are 
imbedded in them. On approaching the dyke they become 
much indurated, and are divided bya great many vertical 
fissures, which, when combined with the ordinary cleavage, 
separate the strata into rhomboidal fragments. In all such 
cases the rifts and fissures are coated over with oxide of iron. 
In other instances, the true horizontal cleavage entirely disap- 
pears; and the indurated masses might then be easily mistaken 
for beds which had been tilted out of their original position. 
The alteration produced in the coal-shale at Gaundlass Mill 
is exactly analogous to what has been described, though not 
so strikingly exhibited. 
In the quarry at Barwick, on the right bank of the Tees, 
the vein of trap is well denuded, and the south side of the sec- 
tion exposes a great many horizontal beds of sandstone, which 
are separated into prismatic blocks by a number of natural 
transverse fissures. Close to the dyke this structure disappears ; 
the sandstone is much more compact, and breaks into amor- 
phous fragments. 
It must however be allowed that in some other localities the 
sandstone did not, under similar circumstances, appear to have 
undergone any modification. 
Perhaps, as a general rule, none of the changes above de- 
scribed are well exhibited, where the portion of the dyke, in 
contact with the horizontal beds, assumes the appearance of a 
wacké. Should this observation be sufficiently verified, it 
would seem to indicate, that the earthy texture of the dyke is, 
in some cases, rather due to its original mode of agerégation, 
than to any subsequent decomposition. 1 may, however, as- 
sert unequivocally, that I neyer saw any beds which are easily 
susceptible of modification (such as coal or carbonaceous shale) 
in immediate contact with the trap, without having undergone 
a remarkable change. 
The overlying trap at Bolam bears no resemblance to a sub- 
stance which has been tranquilly deposited on the inferior 
strata; for it is separated from them by a broken indented su- 
perficies which has exposed many distinct beds to its immediate 
action. Some of the massy columns rest on a bed of shale 
partially converted into a substance resembling Lydian stone, 
which rings under the hammer, or flies in all directions into 
a number of sharp splinters. Others are supported by a bed 
of 
