TOPOGRAPHY AND GEOLOGY. iSf 



in so far as both masses were poured out during the same geological 

 epoch. 



At Pleaskin Head and the Causeway, a red layer among the basalts easily 

 attracts the eye. This is one of the bands of iron-ore which occur here and 

 there in the basaltic region. They mark an incident that occurred about 

 tlie middle of the volcanic epoch, when matters were calmer for a time, and 

 when lakes accumulated in the hollows of the lava-flows. The waters enter- 

 ing them were highly charged with iron salts, brought into solution from the 

 decomposing basalts round about, and the beds of more or less nodular ore 

 consist of the msoluble products, which were deposited as these salts broke 

 tip on oxidation. 



These stratified iron-ores are mined in Glenariff ; and another still more 

 valuable material is associated with them. The destruction of the lavas, 

 and especially of the rhyolites which are about to be described, set free 

 as occasionally happens, a certain 'amount of aluminium in the form of a 

 liydrate. This gives us a clayey substance, which is often mixed with true 

 clay (hydrous alummium silicate). This material accumulated in the lake- 

 basins as a fine grey mud, and is known as bauxite, an important commercial 

 source of aluminium. 



While the eruptions of basalt were quiescent, a completely different type- 

 of lava, the highly silicious rhyolite, welled up here and there, and produced 

 a white and almost granitic rock that is quarried in the dome-like hill of 

 Tardree. Near at hand, on Sandy Braes, natural glass (Obsidian) was 

 produced by the rapid cooling of these lavas. 



The main interest, however, of these sporadic outbursts of rhyolite lies 

 in their probable connexion with the Mourne Moun- 

 The Mourne tains. This handsome group of granite peaks, north 



Mountains. of Carlingford Lough, is known to be of later origin 



than the adjacent " Caledonian " granite of Newry. 

 The Mournes owe their boldness of detail, and their frequent craggy crests 

 and walls, to their comparative youth (Fig. lo). Yet, when viewed from a 

 distance, as from the Great Northern Railway above Newry, they show the 

 domed and rounded character which we associate with denuded granite 

 chains. The Mourne granite cuts across an earlier series of basalt dykes, 

 which abound upon the coast of Dov.^n ; it is itself traversed by a later 

 series. At Carlingford similar granite invades the black and rugged mass 

 of dolerite that forms the ridge between Dundalk and Greenore. This 

 dolerite cuts the Carboniferous Limestone. The granite of Mull and Skye, 

 again, is post-Cretaceous, and is of the same type as that of the Mourne 

 Mountains. The chemical composition of these granites corresponds to that 

 of the rhyolites of Tardree. Here are the facts that lead geologists to the 

 interesting conclusion that the Mourne granite was intruded as a molten 

 mass after the first basaltic eruptions had taken place in Ireland, but before 

 the outpouring of the later basaltic series. It is, as it were, the deep- 

 seated mass, the solidified caldron, of which the rhyolites of Tardree were 

 the surface-manifestations. 



What, then, was the age of these great eruptions, which have added, on 

 the one hand, the higli plateaux of Antrim and Londonderry, and on the 

 other the glorious summits of the Mournes, to the varied scenery of north- 

 east Ireland? During the lacustrine epoch, marked by the iron-ores, 

 numerous plants were washed down into the clays. Mr. Starkie Gardner 

 has determined these as belonging to the Eocene period, during which the 



