82 



Proceedings of the Royal Irish Academy, 



level with the highest points of the hills, the strata having since 

 been removed from the deep adjacent valleys, and from the neigh- 

 bouring lov;^ ground stretching away at foot from the hilly tracts. 

 The very low summits touched by the watershed between the Shannon 

 basin and that of the Woodford river — tributary of the Erne — have 

 already been referred to. The ground separating the Erne basin from 

 the general basin of the Blackwater, Eann, and Lagan, adjoining the 

 Mourne group, is almost equally low ; and from these facts we may 

 conclude that the differential lowering of the ground stretching 

 between the Donegal and Down hills synchronized with the lowering 

 of the Shannon basin, and indeed of the whole central plain : thus, 

 the formation of this basin involves the larger question of the 

 denudation of the entire island, l^o reason can be advanced to show 

 why the moulding of the features in the southern half of Ireland 

 has not been contemporaneous with that of features in the northern 

 half ; and it is a very noticeable fact that the summits of the highest 

 hills in the chief Irish groups lie approximately upon a plane 2500 

 to 3000 feet above present datum level, as may be seen from the 

 following list, viz. : — 



1. 



In 



the Donegal 



group, 



Errigal stands at 



2466 feet. 



2. 





>> 



J > 



Muckish 



5 » 



2197 



53 



3. 





Sperrin 



? y 



Sawel 



5 3 



2240 



3 > 



4. 



> > 



Mourne 



J > 



SHeve Donard 



3 3 



2796 





5. 



> > 



J 5 



3 3 



Slieve Bingian 



5 5 



2449 



33 



6. 





North Mayo 



3 3 



Nephin 



35 



2646 



3 5 



7. 







33 



Slieve Cor 



«3 



2369 



3 3 



8. 



> J 



South Mayo 



3 3 



Croagh Patrick 



1 5 



2510 



3 3 



9. 



5> 





33 



Mweelrea 



3 3 



2688 



3 3 



10. 





Galway 



3 3 



Formnamore 



3 3 



2239 



3 3 



11. 



12. 





)> 



3 3 



53 



Bennaheola 

 Benbawn 



33 



2336 

 2395 



33 

 33 



13. 





Leinster 





Lugnaquilla 





3039 



33 



14. 



5) 





3 3 



Kippure 





2473 



3> 



15. 



3> 



Magillicuddy 





Carrantuohill 



5 3 



3414 



33 



16. 



) ) 



»> 



3 3 



Mangerton 



3 3 



2756 



33 



17. 



5 > 



Galtee 



3 3 



Galtymore 



33 



3015 



3 3 



18. 



J> 



Knockmealdown 



The Summit 



3 3 



2609 





19. 



53 



Comeragh 



33 



Knockanaffrin 



33 



2478 



55 



When the circumstances of this coincidence are considered, it is even 

 more remarkable than at first sight appears ; 1, 2, 6, 7, 8, 11, and 12 

 being quartzite ; 3 mica schist, 13 cleaved felsite^ over granite; 4, 5, 



1 As recently ascertained by Mr. M'Henry of the Geological Survey. 



