O'Eeilly — On the Occurrence of Serpentine at Bray Read. 507 
part. The colour is only faintly green ; on the fresh fracture rather 
and admitting that, from the fact of the rock being split up as described, 
it must be intrusive. 
"We have, also, his important remarks as to the survey of N^. "Wales 
by Mr. Selwyn, and his hammering of every suspicious-looking rock, 
for it was only in that way that crystalline greenstone could be known 
from green silicious grit ;" his endeavours to explain the seeming- 
anomaly of an abundance of eruptive rocks in the lower Silurian, and 
their absence in the Cambrian through which they must have passed. 
Finally, his very significant statements, " that no matter how 
much a district was worked, it was impossible to exhaust the facts of 
it" — " that Eray Head was twice examined by most careful workmen, 
who yet never saw this bed of greenstone" — and lastly, his admitted 
incredulity as to its existence when it was first reported to him. 
All this points to the clear conclusion that nearly everything has 
yet to be done for the accurate determination of the true nature of 
these rocks and of their stratigraphical relations. 
I submit the results of my own observations as a contribution 
towards these ends. I accept Mr. Westropp's description of the dyke, 
as it appears on the old road and on the railway cutting, as practically 
correct and give sketches of the dyke as it appears in these two places, 
see page 508 and Plate XXI, As to the splitting up into branches on 
the shore, I have been unable to verify this, as the rocks are difficultly 
accessible, so far as I am aware only by boat ; but there is visible from 
the path a series of dark- grey beds projecting into the sea, which may 
be the continuation of this dyke, but may possibly be a different set 
separated from the dyke observed by one of the many faults which 
influence so markedly the outline of the coast here (as a matter of fact 
the beds here are both markedly faulted and much disturbed). As to 
the true nature of the rock I had samples, taken from the outcrop on 
the railway, analysed ; and the results, taken in connexion with the 
hardness, density, and the appearance of the rock in thin section under 
the microscope, prove it to be a serpentine, but evidently a product of 
alteration of a basic rock, as most serpentines are. The density deter- 
mined on two specimens was found to be 2*803. This agrees well 
with that mentioned in Dana's ^'Mineralogy" (5th ed., 1874), p. 804, 
for a dark-green serpentine of J^ewbury, Mass., U. S., given as 2 '804. 
The hardness is about 3*5, the rock being easily cut by fluor spar ; 
while the toughness so characteristic of certain magnesian rocks, 
and which shows itself by the difficulty met with in breaking up a 
sample into pieces is remarkable : it crushes, but does not readily 
2 N 2 
