96 



Froceedlnga of the Royal Irish Acadetny. 



such cases, intense crushing of the original rock mass is plainly 

 suggested : crushing, that is to say, by a thick mantle of moving ice, 

 We may, therefore, infer from these considerations that, prior to the 

 ice-period, the ground was probably honeycombed by streams, rivulets, 

 and underground vraters, particularly in limestone areas ; that much 

 clayey residue lay upon the surface after rock-solution by atmospheric 

 moisture, gases, and rain — the results of sub-aerial waste throughout 

 a prolonged period, possibly millions of years ; and that ice-erosion 

 operated upon rock strata affected as described, so that the features 

 still indicate differential effects of sub- aerial waste, while the action 

 of ice considerably diminished the total time apparently necessary for 

 a general lowering of the island's surface. 



CON-CLUSION. 



The various elements of this interesting subject, especially those 

 not previously considered in detail, may be summarized as follows, 

 viz. : — 



1. A post-Eocene plain of denudation probably existed in this 

 region, because the highest summits of the chief Irish mountain groups 

 lie upon an ideal plane, though formed of different kinds of rock, and 

 belong to five different formations and masses,, including granite of, 

 possibly, Miocene age. 



2. This plain was some 2500 feet above present limestone plain, 

 and was that upon which the Shannon originally commenced to flow. 



3. At the present rate of surface-waste, differential lowering may 

 have occupied 15,000,000 to 30,000,000 years. 



4. This period may have been greatly curtailed by glacial erosion 

 of the surface. 



5. The gouging action of a glacier accounts — apparently alone 

 can account — for the present form of the Shannon-bed above 

 Killaloe. 



6. Prior to the Glacial Period, the river was probably forced to 

 abandon the Killaloe gorge for a time, and flowed along the Scarriff 

 valley towards its estuary. 



7. Upon the melting of the glacier the bottom of the gorge had 

 become so modified that the river could resume its course there, and 

 southward as far as O'Priensbridge, though thereafter it became 

 deflected by moraine accumulations from its original course. 



