Dykes in Antrim and Down. 95 



the North of Ireland" before the Geological Society. 

 Although he found dykes at various altitudes, more than half 

 of those he observed were situated on the sea-shore. From 

 a careful determination of their compass bearings, he found 

 their general uniform direction to be from south-east to north- 

 west. 



In the County of Down, we have a very interesting outlier 

 of the Basaltic Plateau of Antrim, Scrabo Hill, which may 

 possibly have been connected in the Tertiary Period with the 

 lava-sheets north of Belfast Lough. It contains very striking 

 examples of dykes, laid open in the faces of its various quarries. 

 The dolerite of Scrabo not only overlies the Triassic sandstone 

 of the hill, but has intruded along lines of weakness, in great 

 horizontal sheets, the edges of which appear in the exposed 

 sections as horizontal wavy dykes. In one quany, the whole 

 strata are traversed by a great vertical dyke, which not only 

 cuts through the horizontal intrusive sheets, and intervening 

 beds of sandstone, but pierces the capping of dolerite above. 

 This vertical dyke stands out as a great leaning pillar against 

 the hill ; it is of looser texture than the intrusive sheets, and 

 shows a tendency to spheroidal weathering. 



In the present paper, I have only dwelt on a few out of the 

 many characteristic dykes of our district ; examples could 

 easily be multiplied, and their study offers an interesting field 

 of research. 



Some geologists refer the basalt sheets of our plateau to 

 " fissure" eruptions, others refer them to eruptions from 

 volcanic cones, long since obliterated by glacial action. .It is 

 not easy to obtain direct proof where so many of the original 

 fissures must be concealed by the superincumbent masses. 

 The thought indeed sometimes forces itself, may not the one 

 form of eruption merge into the other, without any special 

 surface of demarcation ? To trace, where it is possible, the 

 connection between the dykes, and the supposed old volcanic 

 "necks" of the district might lead to interesting results. 



To these sheets of basalt we owe the preservation of much 

 of our land-surfaces; they have suffered denudation through the 

 ages, but the denudation would have been much greater, had 

 the softer strata been exposed to it unprotected. 



It is in the balancing between the external and internal 

 energies of the globe, that we find the conditions necessary 



