County Dublin, Past and Present. 35 



But one of the most interesting features of the Ordovician 

 of Co. Dublin is the abundant evidence of volcanic action. In 

 the upturned conglomeratic strata, north of the coral limxcstone 

 of Portraine, one can see, at low water, blocks of greenish rock 

 containing crystals of felspar. Beds of fine-grained purple 

 and green volcanic ash also occur, showing that cr>^stalline 

 and glassy materials were being throvv^n out from some neigh- 

 bouring vent into the Ordovician sea. The microscope aids 

 us in recognising these materials (fig. 5) ; but the visitor can 

 easily detect the fragments lying in the ordinary sediments. 

 At the south end of the promontory, a massive development 

 of igneous rock comes up below the sandy shales. Part of this 

 shows stratification and an ash-like character; part shows the 

 steam-vCvSicles of a lava-flow, now infilled with calcite. The 

 greater portion of Ivambay Island is formed of a "porphyry" 

 that has broken through the Ordovicians in a molten state ; 

 this handsome rock, which deserves to be widely known as an 

 ornamental stone, is little inferior in effect to the famous 

 antique green porph3ay, quarried by the Romans in the Morea. 

 It was for a time unable to rise through the rocks above it, 

 and large cr5'Stals of felspar' developed under conditions of 

 pressure and slow consolidation. Then relief from pressure 

 came, through some local movement of the rocks ; the mass, 

 with its contained water, began to flow upward, bearing the 

 "porphyritic" crystals; and at the surface, as in all volcanos, 

 the water of the molten mass burst into steam, scattering the 

 fragments of lava far and wide. At times more gentle action 

 caused lava- flows to move outward from the crater ; the accu- 

 mulation of these and of the tuffs and ashes would build a 

 conical mountain above the level of the sea; and hence prob- 

 abl}^ on the site of I^ambay Island, in Ordovician times, a 

 volcanic isle was reared, fringed with coral banks as in the 

 present Pacific Ocean. Away in the south one could have 

 seen a similar cone constructed about Tallaght ; while farther 

 still huge clouds of dust and steam would hang above the 

 great volcanos of Rathdrum and eastern Wicklow. The 

 activity seems to have been less intense than in corresponding 

 times in Wales ; but all across the open Ordovician sea, isle 

 must have been answering isle with frequent explosions and 

 outpourings of dark fluid lava or viscid natural glass. The 

 materials in Co. Dublin were andesites and aphanites, rocks 

 with some sixt}^ per cent, of silica ; but farther south highly- 

 silicated lava-flows of obsidian were poured out, comparable 

 to the black glassy masses of the Yellowstone Park. 



The glass has cr>^stallised through slow subsequent alter- 

 ation ; but the original structures of fine curving shrinkage- 



' These have been incorrectly described as Orthoclase {G. S. /. iy., p. 44) ; 

 though much altered, they may safely be referred to the lime-soda 

 felspars. 



