88 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



varies from less than an inch up to 18 inches in thickness, and consists of pisolitic 

 concretions of hrematite, from the size of a pea to that of a hazel nut, wrapped up in a 

 soft ochreous clayey matrix. Where it is absent, its place is sometimes taken by an 

 aluminous clay, worked as "beauxite," which has yielded stumps of trees and numerous 

 leaves and cones. Beneath the iron-ore, or its representative, lies what is called the 

 " pavement," — a ferruginous tuff, 8 to 10 feet thick, resting on " lithomarge," — a lilac or 

 violet mottled aluminous earth sometimes full of rounded blocks or bombs of basalt. 

 The well-known horizon for fossil plants at Ballypallidy is a red tuff in this zone. 



This intercalated band of ferruginous deposits forms one of the most persistent and 

 interesting features in the Antrim plateau. The actual area now occupied by it has 

 been so reduced by denudation into mere scattered patches that it probably does not 

 exceed 170 square miles. But the zone can be traced from Divis Hill, near Belfast, to 

 Rathlin Island, a distance of 50 miles, and from the valley of the Bann to the coast above 

 Glen arm, more than 20 miles. There can be little doubt that it was once continuous over 

 all that area, and that it probably extended some way further on all sides. Hence, the 

 original area over which the iron-ore and its accompanying tuffs and clays were laid 

 down can hardly have been less than 1000 square miles. This extensive tract was 

 evidently the site of a lake during the volcanic period, formed by a subsidence of the 

 floor of lower basalts. The salts of iron contained in solution in the water, whether 

 derived from the decay of the surrounding lavas or from the discharges of chalybeate 

 springs, were precipitated as peroxide in pisolitic form, as similar ores are now being 

 formed on lake-bottoms in Sweden. For a long interval, quiet sedimentation went on 

 in this lake, the only sign of volcanic energy during that time being the dust and stones 

 that were thrown out and fell over the water-basin, or were washed into it by rains from 

 the slopes around. 



Immediately above the iron-ore, or separated from it in places by only a few inches 

 of tuff, comes the group of Upper Basalts, which varies up to 600 feet in thickness, 

 though, as the upper portion has been everywhere removed by denudation, no measure 

 remains of what may have been the original depth of the group. The general character 

 of these basalts is more frequently columnar, black and compact, and with fewer examples 

 of the strongly amygdaloidal structure so conspicuous in the lower group. But this 

 distinction is less marked in the south than in the north of Antrim, so that where the 

 intervening zone of tuffs and iron-ore disappears, no satisfactory line of division can be 

 traced between the two groups of basalt. The occurrence of that zone, however, by 

 giving rise to a hollow or slope, from which the upper basalts rise as a steep bank or 

 cliff, furnishes a convenient topographical feature for mapping the boundary of these 

 rocks. Among the upper basalts, also, there is perhaps a less frequent occurrence of 

 those thin red partings of bole between the successive flows, so conspicuous in the lower 

 group. But the flows are not less distinctly marked off from each other. Nowhere can 

 their characteristic features be better seen than along the magnificent range of cliffs from 

 the Giant's Causeway eastwards. The columnar bed that forms the Causeway is the 



