GEOLOGICAL SOCIETY OP DUBLIN". 
25 
masses of granite are really intrusive into Cambro-Silurian rocks, they 
yet belong in reality to the Cambro-Silurian period, being older than 
the beds which lie above the felstones, though, of course, newer than 
the parts in which they themselves lie. These are conclusions which I 
drew from my examination of parts of the county of Wicklow and Wex¬ 
ford, three or four years ago, and which every subsequent examination 
has tended to confirm. 
In the present instance I should look upon the slates of Bock Little 
as the lowest beds of the district, and, therefore, older than the traps of 
Bock Big ; and I believe, the larger masses of these trappean rocks to have 
been formed in the order of their succession from west to east,-—the por- 
phyritic ash having been first formed; then the sheets of felstone, and 
their accompaning ashes, then the very regular band of greenstone of the 
Hanging Stone, followed by the deposition of several beds of greenstone 
ash, those by the formation of beds of argillaceous mud, and afterwards by 
the thick beds of conglomerate, derived probably from a portion of some 
of the previously consolidated traps that had become exposed to the action 
of a current. Other beds of ash and other flows of felstone then took 
place, as indicated by the highest beds of the section. If the metamor¬ 
phism that has Subsequently produced the innate crystalline structure 
in the porphyritic ash be attributable to the greenstone immediately 
alongside of it, then, probably, the whole mass of the greenstone on 
the west flank of Bock Big, and all that of Bock Little, is of intrusive 
origin, and is newer than the felstones, &c., to the east of it. It is 
nearly certain that the intrusion of the elvan (or granite) dykes on the 
west side of Bock Little took place subsequently to the production of the 
greenstone, both because the greenstone nearest to them is more highly 
crystalline than the rest, as if it had been remelted; and because in 
one corner of one of the lower quarries of Bock Little I found a small 
vein of Elvanite cutting through a mass of greenstone, apparently part of 
the general mass of the neighbourhood. The elvan, then, must be looked 
on as the newest rock of the district. 
It would follow, however, both from the highly crystalline structure 
of the elvans and greenstones of Bock Little, and from their boundaries 
being parallel to the general strike of the country, that their intrusion 
took place while the beds above them were yet horizontal and un¬ 
disturbed, either by elevation or denudation; and that their present situ¬ 
ation is due to their having partaken in the general movement and 
general erosion that has effected the whole country, and impressed 
upon it the general strike of its rocks, and the general outline of its sur¬ 
face. 
It has been already stated that the metamorphic effect of any of 
these rocks is very slight, and confined strictly to their immediate 
neighbourhood, disappearing at a distance of a few feet from them in 
the aqneons rocks, and at that of a few yards in those igneons rocks 
which have been altered. 
I believe any disturbing effect consequent on the intrusion of these 
igneous rocks to have been as restricted as their metamorphic action; 
VOL. V.-PEOC. SOC. E 
