50 REPORT OF MEETINGS FOR 1901 



makes its way upward through the vent of the volcano, 

 sometimes tearing off great masses of rock as it does so, at 

 other times bursting its way through the basin of fluid rock 

 seething in the crater, just as bubbles of steam make their 

 way upward through boiling porridge. Every explosive effort 

 drives off quantities of rock material. The more violent ones 

 send rock fragments of all sizes flying into the atmosphere 

 above the vent, often to great elevations above the volcano. 

 The finer materials, after being blown about by the wind, fall 

 back again to the surface of the Earth, and what does not 

 happen to fall into the crater usually remains on the flanks 

 of the volcano, or, at any rate, falls at no great distance 

 from it. This fragmentary material, shot out by the explosive 

 eruptions of a volcano, and accumulated outside of the crater, 

 is what is here called tuff. No better examples of an ancient 

 deposit* of this nature can be seen anywhere than in the 

 cliffs and on the shore to the east and west of Canty Bay. 

 The term agglomerate is commonly restricted to the materials 

 which fall into the crater. 



If the eruptions are of a quieter nature, and not characterised 

 by violent explosive outbursts, the fluid rock may be forced 

 upward by the high pressure steam below, it may reach the 

 lip of the crater and it may thence flow down the side of 

 the cone, cooling and slacking in speed as it goes. The 

 term lava is restricted by geologists to rock which has flowed 

 from a volcano and cooled above ground. There are no true 

 lava streams quite clo?e to Canty Bay at present, though 

 some were there in former times. 



If the fluid rock is propelled in amongst the other rocks 

 underground, or if it eats its way there by other means, and 

 there consolidates without reaching the surface, the solidified 

 result is generally spoken of under the name of a trappean 

 rock, if the depth at which consolidation ensues is not very 

 great ; according to its mode of occurrence such a rock may 

 be either a sill, which runs nearly coincidentally with the 

 bedding ; or, if it cuts as a wall-like mass across the rocks 

 with which it is associated, it is called a dyke.* 



* It may be remarked here that a " dyke," etymologically repfarded, 

 is simply a something— a wall, a hedge, or a ditch— which marks ttfe 

 }itt)its between two particBlar pieoeg of gronndi 



