56 



The Irish Natuj'alist. 



surface; it cooled down through, not daj^s or months, but 

 centuries; and the crest of the long Leinster ridge was no 

 doubt originally formed of upheaved Ordovicians, giving a 

 wilder and more broken aspect to the mountains than they 

 wear at present. The last phases of earth-movement and 

 "igneous" action are to be seen in the veins of pure quartz 

 which occasionally cut the granite; these no doubt represent 

 the courses of hot waters containing silica in solution, as in 

 modern geysers, which circulated, decomposing some minerals 

 and constructing others, during the final consolidation of the 

 chain. 



The broad tabular joints of granite, and still more its 

 uniform powdery mode of disintegration, give us to-day the 

 familiar long sweeping outlines of the Dublin hills. Occa- 

 sionally, as on Three Rock Mountain, more resisting blocks 

 protrude above the general surface of decay, like the ** tors" 

 of Devonshire ; and the slopes are strewn with masses broken 



Fig. 6. 



Granite in the Dingle, near Carrickmlnes, showing how the rock be- 

 comes broken up into loose blocks ])y the widening of its joints. The 

 face exposed, 30 ft. high, is formed by one of a series of joints parallel to 

 the trend of the valley. (From a photograph by Prof. Haddon and 

 Mr. J. McNab.) 



