GEOLOGICAL SOCIETY OE DUBLIN. 
21 
as it certainly is further north, where the little brook at the bottom of 
it falls over ledges of greenstone. 
It might at first sight, perhaps, seem unlikely that so hard and so 
tough a rock as this greenstone would he excavated rather than the 
cleaved slates which form the hill on one side of it, or the more brittle 
felstones, &c., which form the loftier hill on the other side. It is, 
however, a very common occurrence among these old Camhro-Silurian 
rocks, both in Ireland and in Wales, and elsewhere, that the greenstones 
have suffered degradation rather than the slates and felstones. The 
reason of this is, probably, that although very hard and very tough, they 
are in the first place more open, from their mineral constitution, to the 
slow action of the weather than the slates and felstones. The silicates 
of lime, &c., which they contain become converted into carbonates, as 
may he seen by their effervescing with acids along their cracks and cre¬ 
vices, and at the inner margin of their decomposed part; and these car¬ 
bonates are then dissolved, and the disintegration of the rock is the con¬ 
sequence. In the second place, greenstones, like basalts and some other 
igneous rocks, have a concealed internal spheroidal structure, which 
weathering develops by removing all the angular corners and promi¬ 
nences, and the weathered blocks are, therefore, more easily set in motion 
by the action of breakers and currents than the more permanently an¬ 
gular felstones, or the flat and shingly slate rocks. During the passage 
of the land, then, through the upper surface of the sea, at its various 
periods of slow elevation and depression, it is natural to suppose that 
the greenstones may have in many instances been more acted upon by 
denuding forces than the adjacent slates or felstones. 
The greenstone now described runs along the whole western side of 
Rock Big, stretching up the western flank of Arklow Rock, nearly to 
the summit. Near its boundary, and in some other places, it is fine¬ 
grained, and sometimes almost earthy in appearance, though still hard; 
but in its more central portion it is very crystalline, showing large glit¬ 
tering faces of the black lustrous mineral before mentioned. 
The eastern boundary of this greenstone runs in a nearly straight 
line, about N. N. E. and S. S. W., parallel to the general strike of the 
rocks, and it is very well defined, and determinable within five or six 
yards, at several points within the space of a mile. 
Parallel to this boundary a band of a very remarkable rock occurs, 
about 150 yards in width, and running right over the summit of Arklow 
Rock from one extremity of the district to the other. This rock would 
be generally called a feldspar, porphyry, or a porphyritic felstone, and 
over the greater part of its course it would be improper to give it any 
other name. It consists of a dark gray, or greenish-gray base, full of 
small white crystals of feldspar, about one-fourth of an inch long; the 
base or paste, which is quite smooth and compact, likewise exhibiting 
here and there brilliant facets of crystals of feldspar of the same colour 
as itself. The white opaque crystals of feldspar are sometimes rather 
irregular in form, though they do not exhibit any discoloured marks of 
weathering, nor much appearance of their angles having been worn or 
