392 PEOCEEDIKaS OF THE GEOIOGHCAL SOCIETY. [JuEG 18, 



the vertical cutting and grooving of ice in glaciers, and of running 

 water in rills, brooks, and rivers. 



AU glens, ravines, and narrow winding vaUeys, then, except 

 passes on the crests of hiU-ranges, have been formed either by the 

 grooving action of glaciers, where they have existed, or by the erosive 

 action of rivers, whether great or small. 



5. The present surface of the ground*, where it differs from the 

 original surface of deposition of the immediately subjacent rock, is 

 in all cases the direct result of denudation, either atmospheric or 

 marine, the internal forces of disturbance having only an indirect 

 effect upon it, and having ceased to act long before the present sur- 

 face was formed. 



It is perhaps necessary to support the latter proposition by a few 

 considerations referring to our present district. In the South of 

 Ireland the internal forces of disturbance have thrown the rocks into 

 long parallel folds, to which the external features of the present 

 surface show a certain degree of conformity. There never is an 

 absolute conformity, however, between them, inasmuch as the pre- 

 sent surface of the ground never exactly coincides with the surface of 

 a bed of rock, except perhaps for a few feet on a steep bank, but is 

 always formed across the edges of the beds. 



The limestone of the vaUeys lies in a hollow of the Old Eed Sand- 

 stone ; but the horizontal surfaces of the vaUey-grounds have often 

 highly inclined beds of limestone below them. The Old Eed Sand- 

 stone of the hills rises into long ridges ; but the ground never rises 

 so steeply as the beds do, since in ascending the steepest hiUs we walk 

 across the edges of successive beds, and generally find the lowest 

 beds at the surface near the summits of the hills. 



The rocks were certainly not first denuded, so that their .present 

 surface-terminations should be exposed while they were still hori- 

 zontal, and then roUed into their present inclined position. Let us 

 look at section fig. 5, PL XX., the lowest and darkest-coloured part 

 of which represents the form and structure of the ground from 

 Youghal Bay to the Knockmealdown Mountains, and suppose the 

 rocks to be all extended into a horizontal position. In such a case 

 the limestone would form long tabular hiUs and ridges, and the Old 

 Eed Sandstone longitudinal valleys. But, as the present hill-tops are 

 formed of the ends of beds that lie 2000 or 3000 feet deep in the Old 

 Eed Sandstone, they must, if horizontal, be great irregular hollows or 

 rock-basins of that depth, excavated by some inexplicable process out 

 of the Old Eed Sandstone; and the subsequent movements of disturb- 

 ance must have inverted the surface of these basius by some process 

 stiU more inconceivable, so that the bottoms of the basins became the 

 summits of the hiUs. 



The hypothesis of the surface being first denuded and then dis- 

 turbed thus lands us in utter absurdity and confusion, and is there- 

 fore quite inadmissible. The simple and natural hypothesis is evi- 

 dently that which supposes that, some time after the formation of the 



* Volcanic districts and those now subject to earthquakes are here excepted, 

 as to include them would require some slight change in the phraseology. 



