226 Br. Berger on the Dykes 



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 occur- 

 rence ; 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 ob- 

 served two other considerable shifts ; the on« 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 j 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 some- 



