246 Scientific Proceedings, Royal Dublin Seciety. 
XXXVIL—ON THE OCCURRENCE OF MICROCLINE FELD- 
SPAR IN THE DALKEY GRANITES, sy J. P. O'REILLY, 
C.E.,M.R.1A. PLATE 15. 
[Read June 16th, 1879. ] 
WHILE engaged last summer in taking the directions of the main 
lines of jointing which traverse the Granites of Dalkey Island, 
my attention was attracted to the peculiar manner in which 
the Feldspar crystals have withstood the action of the weather, 
standing out from the rock mass in strong relief, while the ac- 
companying quartz seems to have been eaten away with relative 
facility, contrary to what might be expected to occur, considering 
the composition of these constituent minerals, their relative hard- 
nesses and resistances to solvents. 
On examining some of those crystals of Feldspar, of which 
the faces were sufficiently complete and even, 1 was unable to 
recognise the usual physical characteristics indicative of an orthose 
erystal. 
Furthermore, I was able to recognise in the erystal, when cut 
transversely, a peculiar transversely banded chequered structure 
which led me to imagine that the crystals might possibly be twin 
forms, according to the Baveno law. 
Asthis form is not usual, at least in the Granites of this country, 
T thought it might prove interesting’ to further examine the 
erystals under the microscope, 1 accordingly had these slices 
prepared from the two crystals, by Mr. Emile Bertrand, of Paris, 
whose skill as a mineralogist, and whose experience in mineral 
preparations, is well established. 
He returned me the samples prepared, declaring them to be 
very fine specimens of Microcline Feldspar, the properties of which 
mineral they markedly present under polarized light. 
T have since had occasion toexamine crystals from the Graniteson 
Killiney Hill, and in the quarried stones now being furnished to the 
