94 The Irish Naturalist. 



striking : traversing the Triassic strata of the district in a 

 general north-westerly direction, they appear as low wavy 

 walls of dark rock, very slightly raised above the surrounding 

 reefs. Macedon Point appears to have been in proximity to 

 a special centre of disturbance, for here we find two dykes 

 intersecting, thus forming a beautiful example of a cross dyke 



(fig- 1). 



Immediately opposite, in Cultra Bay, we find dykes striking 

 out to sea, in similar wavy lines, and traversing strata of very 

 varied geological age. 



Carrickfergus Castle stands upon one of the largest dykes 

 of the district ; the basalt of which it is composed is more 

 Coarsely crystalline than the basalt at Macedon Point, and is 

 well exposed to view below and between the castle walls. 

 This dyke forms a landmark of both geological and historic 

 interest ; it shows how the original line of weakness in the 

 earth's crust, now strengthened by the filling in of consoli- 

 dated lava, has offered more resistance to erosion, than the 

 surrounding strata ; and how man, seizing upon the position 

 thus prepared by nature, has raised his structure on the point 

 of strength due to the igneous action of bygone ages. 



The basaltic dykes of our northern coast do not always 

 strike out to sea in irregular branching lines ; they some- 

 times rather present the appearance of artificial masonry 

 composed of roughly rhomboidal blocks of basalt, such as 

 "North Star "dyke (fig. 2). This remarkable dyke traverses 

 the Carboniferous beds at Colliery Bay, and is known in the 

 neighbourhood as the " Black Ditch." 1 It forms one of a 

 series of interesting dykes close to Ballycastle, which attracted 

 the attention of Dr. Hamilton, more than one hundred years 

 ago; in 1784 he describes them " as extraordinary partitions 

 of basaltes, which like walls of iron intersect the strata 

 attendant on the coal of that place, and divide in twain the 

 solid precipice from its summit to the base." Thirty years 

 later Dr. Berger read an interesting paper " On the Dykes of 



1 " Ditch " is sometimes understoodin Ireland as equivalent to "fence " 

 or "bank." This recals the old term " whyn-dykes," which Dr. 

 Richardson tells us " obtained in Scotland the name of dykes from serving 

 often as fences, and from their material, that of lohyn, the Gaelic name 

 for basalt." Appendix to Dubourdieu's Statistical Survey of the County 

 of Antrim, p. 68. 



