510 Proceedings of the Royal Irish Academy. 
Contrasting this analysis with Miss Eobertson's, we have the fol- 
lowing : — 
Bray Head Serpentine. Dillenburg Serpentine. 
SiOo ....>. 39-863 41-70 
AI2O3 10-070 7-04 
CrA 1-299 — 
PeO 17-212^ ...... 26-95 \r^' 
CaO 6-380 Icq 3-34 g 
MgO 11-838 Is 10-26) ^ 
CuO 0-180 w — 
MO 0-112' — 
NaaO ...... 1-289 — 
CO2 4-198):::: ...... — 
Loss by ignition counted) g.^^g I g ^ ^ ^ ^^^g 
as HsO I ^ • • • • • • 
100-954 100-87 
In the Dillenburg analysis the sum of the protoxides, amounting 
to 40*55, represents about the normal amount of magnesia present in 
the noble serpentine. The same sum for the Bray Head mineral, 
representing 35-822, which is about the mean between the extreme 
amounts given by Dana in his list of analyses, for the MgO. It is 
therefore reasonable to assume that the protoxides replace magnesia. 
As to the nature of the rock from which the serpentine is an 
altered product, I have not yet had time to determine ; but, judging 
from the texture of the rock as seen in the spotted samples, it is pro- 
bably a diabase porphyry, of the group of which the Lambay porphyry 
is the highest type. 
If this rock be admitted to be a true serpentine, only now deter- 
mined to be such, and if it may present different textures and appear- 
ances, it is quite evident that many such beds may exist in the Co. 
Wicklow awaiting recognition and determination. That such is the 
case is my belief ; and from the examination that I have been making 
of the Bray Head rocks, I am convinced that some of the* hard, slaty 
bands and olive-green slates may on examination turn out to be altered 
intrusive rocks. In support of this proposition I hope to be able to 
submit a further Paper to the Academy, embodying the results of the 
analyses made by the aid of the grants received from the Academy. 
As regards the direction of the beds on the old road above the path, 
as given in the sketch of the section at that point, see page 508, I 
