THE EOCENE SKI; IKS ~>\~t 



of liquid lava beneath it Whether as cause or coincidence, it seems 

 certain that great horizontal tension arose which started a series of 

 fissures, these running gem-rally from north-west to south-east, and 

 sometimes extending for 50, 60, or even 100 miles. Into these 

 fi.-.-uivs \\vlled up the liquid rock which has consolidated into 

 l>;i-.-ilt, and where they reached the surface the lava welled out 

 in vast streams like those of modern Iceland, forming the great 

 basalt plateaux of Antrim and the west of Scotland. 



This process of dyke-making and the outpouring of lava -floods 

 was repeated again and again, till on the great plain or plateau which 

 bordered the west of Scotland the lavas accumulated to a depth of 

 several thousand feet. In some places 3000 feet of them still 

 remain, and we know not how great a thickness has been removed. 

 Here and there they filled up and obliterated river-channels like 

 those in the island of Mull. 



After the building up of these plateaus another episode took 

 place. Large bodies of a basic magma were pushed into and 

 through the plateau basalts and consolidated into huge domes of 

 gabbro, troctoUte, and other such coarse-grained rocks, but whether 

 they ever extruded matter at the surface cannot be known, because 

 the summits of the bosses are removed. 



The next phase was the protrusion of material from a different 

 magma, which has consolidated into acid rocks of various kinds, 

 from glassy obsidians and rhyolites to granophyres and a rock of 

 granitic character. These now form conical hills which resemble 

 in some respects the trachytic hills of Auvergne. 



The last phase was a second formation of fissures, producing 

 another system of basic dykes which traverse all the previously 

 formed rocks, cutting not only the older dykes but the great 

 masses of gabbro and granophyre. Whether they poured lava to the 

 surface is not known, and whether the formation was the final 

 episode is not quite certain, for there is also a series of acid dykes 

 which traverse all the older volcanics and do not appear to be 

 cut by the later basic dykes. The age of these later basic and acid 

 dykes is also uncertain ; they may be Oligocene, or even Miocene ; 

 but whenever vulcanism did cease the whole region began to sink 

 down again, and at the present time great parts lie below the sea. 



Ireland. The largest remaining portion of the great lava 

 plains is the basaltic plateau of Ulster (Antrim and Derry) (see 

 Fig. 186), which has an area of about 2000 square miles. This 

 plateau presents steep escarpments on all sides, but the ground 

 falls towards a central trough in which lie Lough Neagh and the 

 valley of the river Bann. The component lavas are almost entirely 

 basaltic, but they are divisible into a lower and upper series. The 



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