834 



ÏIIE BIUTISII ISLES. 



1,817 feet, is almost wboll}- buried beneath a sheet of lava of an average thiekness 

 of 100 feet. Its aspect possesses none of the picturcsqueness that distinguishes 

 the volcanic district of Auvergne, which is partly of the same tertiary age. There 

 are neither cones nor cup-shaped craters, for these have been swept away by 

 phming and levelling agents : wide tracts are idmost perfectly level, and covered in 

 many places with glacial drift. But the scenery is bold and striking wherever the 

 table-land is bounded by noble escarpments, with precipitous flanks rising above 

 the surrounding valleys or the sea. Along the shores of Lough Foyle, the lava 

 rests upon softer cretaceous and triassic strata, and as these are undermined by the 

 percolation of water from springs or by rains, the foundations give way, and 

 the superstructure slips down the hillside, and lies a shapeless mass till it has 

 been still further disintegrated by frost, rain, and streamlet, and carried away 

 particle by particle into the ocean.* But elsewhere the lava rises boldly from the 



Fiff. 193.— The Giaxis' Causeway. 



sea in a series of terraces of dark columnar basalt, separated from each other 

 by bands of reddish bole. At the bold promontory of Fair Head, or Benmore 

 (630 feet), huge columns of basalt descend from the top of the cliff in one or two 

 sheer vertical sweeps for several hundred feet, the base of the cliff being strewn 

 with broken columns of trap heaped up in wild confusion. 



The Giants' Causeway, a pavement formed of the tops of 40,000 columns of 

 basalt incessantly washed by the waves of the sea, is the most widely known 

 amono-st the natural curiosities of the coast of Antrim and of all Ireland. 

 Geologists account for the marvellous regularity of these prisms by the large 

 quantity of iron which they contain. About one-fourth of these crystallized 

 masses consist of this metal, and this accounts for the extreme hardness of the 

 basalt, the smoothness of its faces, its weight, its magnetic properties, and the 

 * Edward Hull, " The Physical Geology and Geography of Ireland." 



