Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/240



It is very evident from these observations that some of the dykes at least were formed at a later period than the metallic veins; an inference that might be extended to all, could we show them to be all contemporaneous. That they are so is rendered probable by their parallelism, and by the nearly uniform texture of the trap, in whatever rock the dyke is found. The simple minerals too contained in the trap favour this opinion. Calcareous and heavy spar, and lime intimately mixed with the trap abound most in the dykes of secondary rocks; iron pyrites is common to those both of primitive and secondary districts; and radiated zeolite, olivine and augite are common to the dykes of primitive rocks and to the beds of trap of secondary formation.

From this inference, however, I must except a class of dykes which run parallel to the metallic veins, and are probably intersected by that class of dykes which I have been describing in the present paper. An example of this new class of dykes is found at Farland point in the county of Donegal, where alternating strata of slaty quartz and sienite are traversed by a dyke of clay porphyry hearing east of north 21°: a dyke of trap is found at the same place bearing west of north 49°, the angle between the two being 100°. There can be no doubt that they meet one another, but the spot being covered by the sea, I could not discover the point of intersection.

Whatever date and whatever agents we are disposed to assign to the origin of dykes, their uniformly vertical and nearly parallel positions evince that both they and the mountains which they intersect have not undergone any modern disturbance beyond superficial abrasion, but that they remain in the same situation as at the remote period at which they were formed.