O'E-EiLLY — On the Occurrence of Serpentine at Bray Head, 505 
2 inclies to 1 foot ; there are also some curious fine veins of 0*25 to 
0*5 incli thick. 
"The rock is greenstone, having different textures in different 
parts; up the hill, where it occurs in one bed, it is a hard finely 
crystalline rock, of a dark grayish colour and very durable, for it is 
scarcely at all affected by weathering ; but where it becomes split up 
into a number of beds, it is a rotten brownish green rock, with nume- 
rous black specks of a mineral, which probably is hornblende ; some 
of these beds have quite the look of ash. 
The trap is so regularly interstratified with the sedimentary rocks 
that at first sight it might be supposed to be contemporaneous ; how- 
ever I believe it to be intrusive, for the fine veins, before alluded to, 
may be seen entering the adjacent beds of slate, some dying away, 
while others cut across the laminse of the slate in a manner that I 
cannot think that an ash would behave ; nevertheless, anyone examin- 
ing a hand specimen of one of these veins would be inclined to call it 
an ash. In conclusion, I beg to observe that, while there is such a 
profusion of igneous rocks associated with the lower Silurian deposits 
of "Wicklow, it is rather remarkable that one small greenstone dyke at 
Greystones, and the trap I have just described, are the only ones 
which have yet been found in the Cambrian rocks of that county." 
In the discussion on this Paper (given on p. 179 of the same Jour- 
nal) Professor Jukes is reported to have said : — " He could understand 
his (Mr. "Westropp's) having been perplexed at the ashy appearance of 
some of the branching veins which he had described, because he him- 
self had been occasionally perplexed as to whether particular masses 
of trap were ash, or were crystalline rock decomposed. But the fact 
that the geeenstone which he described, as running persistently 
between two beds in the upper part of the mountain, split into two 
or three beds below (a circumstance of which he had not been pre- 
viously aware), proved that the greenstone was intrusive. The small 
veins issuing from it were an additional proof to the same effect. 
Such intrusions were not uncommon. He knew of beds of intrusive 
greenstone running evenly between other beds for miles of length and 
breadth, preserving almost the same thickness throughout, and not 
producing any appreciable alteration in the beds above or Lclow. 
When he first surveyed JS". "Wales in conjunction with Mr. Selwyn, 
Mr. Selwyn was obliged, after he had mapped out the country, to 
spend some months in going over the whole ground again, and ham- 
mering every suspicious-looking rock ; for it was only in that way that 
crystalline greenstone could be known from green siliceous grits. 
H.I.A. PEOC., SEE. III., VOL. I. 2 N 
