12 TOPOGRAPHY AND GEOLOGY. 



the chalk strata of Yorkshire became contorted, and those of Dorsetshire 



were in places set vertically on end. The Irish region was cracked across 



by numerous fissures, mostly running north-west and south-east, and molten 



lava oozed up these passages, and established a multitude of volcanic cones 



upon the surface. Sheet after sheet of lava was 



The Plateaux of poured out across the undulating downs, filling up the 



Co. Antrim. hollows, burying the beds of gravel, and uniting with 



one another to form continuous and stratified layers. 



Little occurred in the way of explosive action. Here and there, as at 



Carrick-a-rede, a volcanic neck remains to us, choked with fragments of lava 



and chalk, torn off by the more violent eruptions ; but on the whole the 



action was continuous and steady, until the broad land-area, from the Faroe 



Isles to Fermanagh, was covered with basalt, and was converted into a 



region of plateaux. 



The cracks up which the lava welled are seen as dykes at the present day, 

 the " whinstone dykes " of the northern peasantry, and stand out conspicu- 

 ously across the white quarries of the chalk. The chalk is baked and 

 rendered crystalline by contact with them, and is also compacted by the 

 pressure of the mass of lava above ; hence it has been justly styled the 

 " White Limestone," in opposition to the soft English Chalk. The gravels 

 above are reddened, and form a marked zone along the irregular surface of 

 contact between the lava and the limestone (Fig. 7). 



Occasionally, a more massive intrusion has taken place, and the great 

 knot of lava has had its effect upon the modern lands- 

 Slemish. cape. The huge crag of Slemish, where St. Patrick 



tended his master's sheep, is the one true mountain of 

 Co. Antrim, and towers above the plateaux by reason of its toughness and 

 resistance. It is formed of dolerite, a completely crystalline type of basalt, 

 and was doubtless the neck of one of the later volca- 

 Fair Head. noes. Fair Head is similarly made of intrusive do- 



lerite, and the crystals of augite and felspar in some of 

 the veins traversing it are an inch or more in length. This coarse mass has 

 given rise to a superb cliff that faces the northern ocean, and its vertical 

 joints, produced as it cooled, enable the frost and other agents to throw 

 down enormous blocks on to the talus at its foot, and to keep the main crag 

 sheer and imposing. 



The jointing is here, indeed, actually columnar ; and these regular shrink- 

 age cracks, so characteristic of cooling lava, impart in 

 -, , IS 1+ many places an effect of titanic architecture to the 



tolumnar Basalts, ^liff-walls of the Antrim coast. In successive tiers, 

 the columns stand above one another, like those of 

 some Roman amphitheatre. The bottom of each lava-flow cooled slowly, 

 and the columns are there regular and well formed ; the upper part cooled 

 more rapidly, in contact with the variable currents of the air ; and thus each 

 great flow became divided into two layers, a basal one with well developed 

 columns, and an upper one more rubbly and irregular. The next lava-flow 

 spread over the older one, and the process of cooling was repeated. Here 

 we have the secret of the alternation of columnar layers and duller 

 bands at Pleaskin Head, and of the beautiful structure 

 'The Giant's of the Giant's Causeway, which is the basal portion 



Causeway. of a flow that is traceable at a far higher level up the 



cliff. The connexion between Stciffa, an offshoot of 

 the Mull volcano, and the Giant's Causeway, is, of course, mythical, except 



