Structure of 
the dyke. 
32 Professor Sepawick on Trap Dykes 
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. 
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 
bluish colour. This colour is imdeed, with some unimportant 
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 irregular, sharp, angular 
fragments; and on a recently exposed surface there generally 
may be seen a number of minute brilliant facets: but the con- 
stituent parts are never sufficiently distinguished from each other 
to give it the appearance of a green-stone. The essential ingre- 
dients of the rock are, if I mistake not, pyroxéne and felspar, 
in which respect it agrees with the greater number of trap dykes 
which have been carefully examined, as well as with a great 
many varieties of recent lava. The principal modifications, of 
course, arise from the variable proportions of these essential in- 
gredients. Among the prevailing and nearly compact portions 
of the dyke, there are some larger crystals of felspar and car- 
bonate of lime; very rarely, however, in such abundance or 
order of arrangement, as to give any decided appearance of por- 
phyritic structure. Good specimens of amygdaloid are not 
common; where they do occur the nodules are chiefly composed 
of carbonate of lime. In one or two instances we found chal- 
cedony filling the hollows of an imperfect amygdaloid. Tron 
