It is not uncommon to
find such dikes passing through strata of soft materials, such as tuff, 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 precipices, or on the level surface of a country (see Figure 592).
(FIGURE 593. Ground-plan of greenstone dikes traversing sandstone. Arran.)
In the islands of Arran and Skye, and in other parts of Scotland, where
sandstone, conglomerate, and other hard rocks are traversed by dikes of trap,
the converse of 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 the 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 action of the weather more than the dike
itself, or the surrounding 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.
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