224 [JPi'oc. B.N.F.C, 



was the cause of all modifications of surface configuration. 

 These surface features were of great importance to the geolo- 

 gical surveyor, even trivial details in the relief of a district 

 being often of help, and as different kinds of rocks might be 

 sculptured in characteristic ways, the trained geologist was 

 often able to recognise at a distance the material which con- 

 stituted some hill or mountain. The agencies which had 

 given to this country its present form might be divided into 

 subaerial and subterranean. The latter included the volcano 

 and earthquake, which, however terrifying locally when they 

 were in action, had comparatively little effect on the earth's 

 crust as a whole. Most of the work was done by the sub- 

 aerial or atmospheric agencies. These, which included rain, 

 wind, and air, were quiet and long-continued, and, acting 

 through immense periods of time, had been able to carve out 

 their mountains and excavate their valleys. Variations in 

 temperature had caused masses of rock to split off, which 

 washed by rain into streams, had scooped out hollows in the 

 beds of the rivers (" pot-iioles ") as well as helping to erode 

 their beds and lower their levels. Rain, with carbon dioxide 

 in solution, dissolved limestones, forming fissures and caverns, 

 and also by dissolving the cement which bound other rocks 

 together, disintegrated them. The wind helped by carrying 

 sand, which carved and polished the rocks over which it passed. 

 The sea around their coasts flung huge boulders at the cliffs, 

 gradually undermining them, and eventually bringing them 

 down, thus supplying it with fresh material to continue its 

 work. All these agencies had been wearing down the country 

 for countless centuries, and the question arose, " How is ic 

 that there is any land left ?" The answer to that question 

 was that molten rock was being poured out on the surface of 

 the earth, forming what were termed igneous rocks; that the 

 land was rising gradually in some places, and that all the 

 material which was worn off the land was carried to the sea 

 and laid down there in sheets and layers, which eventually 

 might appear above the sea as sedimentary rocks. In past 

 ages Ireland had many times been partly or wholly beneath 



