Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/234



The conclusion to be drawn from this table is that the bearing of the dykes is from S.E. to N.W., and that the dykes all cut the planes of the strata through which they pass, at very considerable angles.

A shift in the direction of a dyke is an accident of rare occurrence; it would probably however be more frequently observed, did not the soil, which covers the surface, prevent us from tracing the dyke to any distance.

The most considerable shift that I ever observed was at Rannagh Point in the Isle of Arran-mor, where it was not less than 47° in 126 feet. At Muir-a-Bole on the N.W. side of Arragh, I observed two other considerable shifts; the one of 22°, the other of 27°.

Dykes differ greatly from one another in their widths, which measure from a few inches to several hundred feet. The latter dimensions are of rare occurrence, and I have only met with three cases of the kind, all in secondary strata; and in two of these the enlargement took place at the bottom of the dyke. One is found in the red bay of Cushendall; the other at Pool-a-Phuca in the county of Fermanagh. The third case occurs between Portrush and Dunbar Castle on the coast of Antrim, and has been noticed by Dr. Richardson. I think I have ascertained that there is a remarkable difference in the average widths of the dykes, according as they are found in primitive or secondary rocks. Out of sixty-two dykes that I have measured, the average width of thirty-eight in the primitive districts is 9 feet; that of twenty-four in the secondary is 24 feet.

I have already noticed some of the more remarkable elevations at which dykes have been found. The height to which a dyke rises above the surface of the stratum, which it intersects, is sometimes