506 Proceedings of the Royal Irish Academy. 
Anyone examining tlie maps of that country would see minute red 
lines, indicating the occurrence of narrow veins of greenstone. In the 
neighbouring district were lower Silurian beds, in which there were 
trap rocks of all kinds, in great quantities, and of great thickness. It 
was clear that the trap in the lower Silurian must have come up 
through Cambrian ; and therefore it was at first sight odd that there 
should be so little trap in the Cambrian, where that only was exposed 
to view, and such an immense development of it in the Silurian. But 
this might be accounted for by the supposition that the trap had passed 
through channels or pipes of communication. Therefore it was quite 
possible that, in the county of "Wicklow, the trap might have passed 
through such channels as were described by Mr. Westropp. '^o 
matter how much a district was worked, it was impossible to exhaust 
the facts of it. Bray Head was twice examined by most careful work- 
men, who yet never saw this bed of greenstone. Mr. Harkness did 
discover it near the old road, and reported the fact to him (Mr. Jukes), 
but he did not credit it at first." 
I have given the whole of these details because of their importance 
in estimating what has been done for the exact determination of the 
Bray Head Bocks. 
In the first place, it may be observed that Messrs. Harkness, King, 
Jukes, and Westropp all speak of the dyke as a greenstone, without 
further determination apparently than that afforded by mere in- 
spection. 
Secondly — while the survey memoir states that the dyke traverses 
''the usual greenish and reddish purple grits and hard slate layers" 
somewhat in their line of strike, i.e. from JS'.E. to S.W., Mr. West- 
ropp's careful description (accepted by Mr. Jukes) shows that the dyke 
from the top of the hill to the old road above the path is regularly 
interstratified with the beds. 
Thirdly — that it branches into these beds between which come in 
grit beds. 
Eourthly — that the so-called greenstone presents different textures 
in different parts : in one place, a hard finely crystalline texture, of a 
dark grayish colour, and then very durable ; in another, where split 
up, it is a rotten brownish green rock, with numerous black specks of 
a mineral which probably is hornblende. 
Fifthly — some of these thin beds have quite the look of an ash. 
Bringing out the significance of these details, we have Mr. Jukes 
stating that he, too, had sometimes been perplexed as to whether parti- 
cular masses of trap were ash or were crystalline rock decomposed ; 
