Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/236

 as is also that of the dyke in the quarries of Scrabo hill; but I am not able to state it with precision.

There is not that variety in the substance of the dykes, that their numbers, their distance from one another, and the varied nature of the rocks which they intersect would lead one to expect. I have found them composed of the following rocks, which are introduced in the order of their most frequent occurrence. Trap and greenstone, with their associates Lydian stone, flinty slate, greystone and wacke.

I have seen but one dyke of clay porphyry, viz. at Farland point in Donegal, and I conceive it to be altogether of a different class from those dykes to which my principal attention has been given in the present paper.

A dyke is formed either of a number of diminutive pillars aggregated together, or of square rhomboidal pieces piled one upon another like blocks of masonry, the long axes of these figures in either case lying transverse, and perpendicular to the walls of the dyke. These regular figures are often much disintegrated and rounded, and sometimes assume the coated form; the two appearances being often united in the same portion of rock.

Dykes are not, like metallic veins, divided into regular layers of different stony substances; nor do we find in them those drusy cavities which sometimes occupy the middle of metallic veins.

The more compact the trap, the more apt is it to assume the polyhedral form, to be homogeneous, and to be free from the porphyritic texture. In the hard variety I never found imbedded any detached mineral concretions, except a few small specks of soft green steatite. When less compact, it is often set more or less thickly with heterogeneous nodules, but seldom so abundantly as to