On the Physical Geology of the Neighbourhood of Dublin. 145 



phystown, the outer part of a roundish projection of granite 

 seems to be a slightly separated coat or shell of uniform thickness, 

 which might be removed by wedges. This, also, might be 

 supposed at first sight to be an instance of concentric structure ; 

 but it appears to be really in some way the effect of the atmo- 

 sphere, although the granite is quite sound and undecomposed. 

 (This is well seen on a much larger scale on one of the Mourn e 

 granite mountains.) 



In some places, owing to the decomposition of the felspar, the 

 granite has, for a depth of several feet, become so rotten that it 

 can be chopped out with a spade ; it is then used as " freestone " 

 for sanding kitchen floors. This phenomenon may be sometimes 

 observed at some height on a steep hillside, e.g., above Ticknock, 

 on the west side of the Three-Rock Mountain, as well as on low 

 ground. It seems to be the effect of atmospheric action; aneces 

 sary condition being some local peculiarity in the felspar ; but 

 what this may be does not seem to have been ascertained. 



Disturbance and Denudation. — There is a wide unconforma 

 bility between the next succeeding formation and the Lower 

 Silurian on which it rests ; showing that there was great distur- 

 bance and denudation between the completion of the Lower 

 Silurian strata and the commencement of the deposition of the 

 others ; this being the second of which we have evidence in this 

 district. Mr. Jukes thought it probable that the Lower Silurians 

 were already disturbed, to some extent, when the granite came 

 up into them ; the intrusion of the granite, if it did not directly 

 cause, was, at least, accompanied by further disturbance of those 

 rocks. At any rate, the great disturbance that actually took 

 place would afford opportunity to the denuding agencies of work- 

 ing very unequally on different parts of the- great stratified mass. 

 Whilst in some areas of this district several thousand feet in 

 thickness of the Lower Silurians still remain, in others the ground 

 was bared of all such rocks before the Old Red Sandstone 

 was laid down. This was, almost doubtless, the case 

 where the Limestone lies directly on the Cambrian, as at 

 Howth, and it was inevitably so where the Limestone stretches 

 over the Silurian on to the granite, as it does a few miles 

 S.E. of Dublin. But as the Cambrian floor of the Silurian sea 

 was doubtless irregular in this district, and as the granite 



