On the Physical Geology of the Neighbourhood of Dublin, 145 
phystown, the outer part of a roundish projection’ of granite 
seems to bea 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 Mourne 
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; a neces 
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 andthe 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 
