GEOLOGICAL SOCIETY OF DUBLIN". 
17 
not easy to see, either, how the mechanical theory can be proved, 
for there is both cleavage of rocks and distortion of fossils. When a 
pebble of conglomerate was split, the whole rock was hard. When a 
fossil was distorted, the rock was soft. Were there two periods of this 
kind; or was distortion of fossils connected with cleavage of rocks at all? 
In fact, are they parts of the same thing ? 
Professor Haughton said that he was not unaware of the difficulty 
suggested by Mr. Kelly; it had occurred to himself at the very outset of 
his investigations, and he felt disposed to cut the Gordian knot by de¬ 
nying that the Conglomerate was hard when cleaved and jointed. Ac¬ 
cording to his idea, the cleavage took place in the planes of greatest 
pressure, and this pressure, acting even upon a soft bed of wet, loose 
gravel, would develop a latent structure in this mass, including its 
quartz pebbles, predisposing them to divide cleanly along the planes of 
maximum pressure. 
OX THE IGXEOUS BOOKS OF ABKLOW HEAD, BY J. BEETE JUKES, M. A., F. B. S. 
The headland immediately south of the small town of Arklow, near 
the borders of Wicklow and Wexford, has long been pointed out by 
our old and respected friend, Mr. Griffith, and others, as containing 
a very interesting assemblage of igneous rocks, forming, as it were, an 
epitome of those generally found in the Cambro-Silurian district of the 
S. E. of Ireland. In June last Mr. Griffith read a paper to this Society, 
containing his notes, written ten years before, on a traverse which he 
had then just made across this district, and explaining his views of its 
structure. It had been the mutual hope and expectation of Mr. Griffith 
and myself to have gone over the ground together, in company with 
Professor Haughton, and thus to have discussed our different views on 
the spot, and combined our information, in order to have arrived at a de¬ 
finite conclusion. 
This meeting has unfortunately been deferred, and I hope only de¬ 
ferred; but, finding myself able to run down for two or three days during 
the last week of November, I took the opportunity of doing so, and will 
now lay before you the result of my own observations, together with a 
series of specimens collected by myself on the spot. 
I may at once say that there is little or no difference between Mr. 
Griffith and myself as regards the facts of the case, nor is there room for 
much difference. The principal varieties of igneous rock are well cha¬ 
racterized and clearly exhibited. Our views seem to differ chiefly in 
our notions as to the origin of some of the more obscure and indefinite 
varieties. Mr. Griffith, I believe, looked upon all the igneous rocks as 
intrusive, and on the varieties above alluded to as metamorphic; 
while I, in accordance with the views first clearly put forth by the late 
Sir H. De la Beche, and afterwards abundantly elucidated by the work 
done by the officers of the Geological Survey, both in Wales and Ireland, 
— allowing fully the intrusive character of some of the igneous rock,— 
believe that intrusion to have taken place during the Cambro-Silurian 
VOL. V.—PBOC. SOC. D 
