TRAP DIKES AND VEINS. 
513 
of this nucleus have not been so much loosened by decompo¬ 
sition; but the application of a ruder blow will produce a 
still further exfoliation.”* 
Volcanic or Trap Dikes.—The leading varieties of the trap- 
pean rocks—basalt, greenstone, trachyte, and the rest—are 
found sometimes in dikes penetrating stratified and unstrat¬ 
ified formations, sometimes in shapeless masses protruding 
through or overlying themy>or in horizontal sheets interca¬ 
lated between strata. Fissures have already been spoken 
of as occurring in all kinds of rocks, some a few feet, others 
many yards in width, and often filled up with earth or an¬ 
gular pieces of stone, or with sand and pebbles. Instead of 
such materials, suppose a quantity of melted stone to be 
driven or injected into an open rent, and there consolidated, 
we have then a tabular mass resembling a wall, and called a 
trap dike. It is not uncommon to find such dikes passing- 
through strata of soft materials, such as tufi*, scoriae, or shale, 
which, being more perishable than the trap, are often washed 
away by the sea, rivers, 
or rain, in which case the 
dike stands prominently 
out in the face of preci¬ 
pices, or on the level sur¬ 
face of a country (see 
Fig. 592). 
In the islands of Ar¬ 
ran and Skye, and in 
other parts of Scotland, 
where sandstone, con¬ 
glomerate, and other 
hard rocks are traversed 
by dikes of trap, the in valley, near Brazen Head Ma^ 
converse oi the above 
phenomenon is seen. The dike, having decomposed more 
rapidly than the containing rock, has once more left open 
the original fissure, often for a distance of many yards 
inland from th^ sea-coast. There is yet another case, by 
no means uncommon in Arran and other parts of Scotland, 
where the strata in contact with the dike, and for a certain 
distance from it, have been hardened, so as to resist the ac¬ 
tion of the weather more than the dike itself, or the sur¬ 
rounding rocks. When this happens, two parallel walls of 
indurated strata are seen protruding above the general level 
of the country and following the course of the dike. In Fig. 
593, a ground plan is given of a ramifying dike of green- 
* Scrope, Geol. Trans., 2d series, vol. ii., p. 205. 
Fig. 592. 
