100 Scientific Proceedings, Royal Dublin Society. 
naturally suggestive of a rapid rate of cooling in the outpoured 
lava. 
Phenocrysts, much corroded at the time of effusion by the 
surrounding glass, are also met with, frequently collected together 
in groups. They now consist of chlorite and other secondary 
minerals, but were originally composed, in some cases at least, of 
olivine. 
The slaty character of the rock results from a kind of “ auswei- 
chungs-clivage.” In hand specimens the rock is seen to be tra- 
versed by numerous irregularly curved planes of cleavage, which 
are usually coated with chlorite and more or less regularly 
striated. In thin slices, cut transversely to the cleavage, numerous 
undulating dark lines running in a general direction are seen to 
divide the slice into lenticular areas. These lines correspond to 
the cleavage planes, and that gliding has taken place along them 
is shown very clearly in one instance where a crystal of felspar 
happening to lie transverse to the cleavage planes has been 
repeatedly broken and displaced along them (fig. 2). How slight 
the dislocation has been may be judged from the figure; measure- 
ments give a maximum displacement for each shear of 0:06 m.m., 
and a total displacement of twice this (0.12 mm.). ‘The two ends 
of the crystal have scarcely moved relatively to each other, the 
greatest displacement being reached in the middle of the erystal. 
On the other hand, the number of glide-planes is very great. The 
length of this single crystal (1°2 mm.) is crossed by no less than 
eight planes, so that the mean distance between them is 0-14 mm. 
The cleavage thus has evidently been produced by a great number 
of internal shears, each of very triflimg amount, and, as our crystal 
of felspar shows, the sum total of the whole, reckoning displace- 
ments in one direction as negative and in the other as positive, 
might amount to zero. The whole appearance points to packing 
under earth-pressure. Save for the presence of minute pheno- 
crysts the prevalent rock on the right-hand side of the road 
strongly reminds us of the “Spilite’”’ of Rosenbusch. 
Near the little cottage opposite the lane leading to Raheen 
church variolitic streaks appear in the rock running more or less 
parallel to concentric lines of jointing. They are rendered very 
distinct by the green colour which many of them present, due to 
a copious development of epidote. The varioles are seen in hand 
