On the Microcline Feldspur in the Dalkey Granites, 247 
contractor of the Rathmines and Pembroke Main Drainage Works, 
and which are being piled along the wall between the Pigeon-house 
Fort and the Poolbeg Lighthouse, coming, I presume, from Bullock ; 
and judging from the similarity of the appearances presented by 
the basal cleavage faces of these crystals, as compared with those 
recognisable on the similar cleavage face of the sliced crystals of 
microcline, I am disposed to conclude that the granites of Dalkey 
are largely made up of this Feldspar—the physical and chemi- 
eal characteristics of which were so thoroughly determined by 
Descloiseaux, in the original memoir, published in 1876, in the 
“Comptes Rendus,” Vol. 82, No. 12, p. 885, and in the “ Annales de 
Physique et Chimie,” 5™ Série, T, LX., 1876. 
This latter notice is the more extended and complete, contain- 
ing photographic illustrations which render comparison with 
specimens under examination relatively easy and satisfactory. 
_ One of the most remarkable optical characteristics of this 
mineral is’ the chequered transverse banded structure (structure 
quadrillée) presented by the thin sections made parallel to the 
basal cleavage, and which is so markedly apparent in the sections 
from the Dalkey Feldspars (vide Plate 15). The Fig. No. 10, 
p. 436, of the memoir in the Ann. de Phy. et Chim., comes so 
very near in appearance to that of my drawiny, that both 
illustrations might be taken as having been made from the same 
crystal. 
The essential conclusions arrived at by Descloiseaux in his | 
masterly memoir, are :— First, that Microcline Felespar is a dimor- 
phic form of Orthose Feldspar, and triclinic in system. Second, 
that while up to the present Orthose Feldspar has been regarded 
and taken as the essentially potash Feldspar, it is the microcline 
variety which really holds that position, since the highest per- 
centage of soda in the microclines analyzed by Damour and 
Pisani, and selected by Descloiseaux, as published by him in his 
Memoir, had not exceeded 3:95, with a minimum in potash of 
10°95%. On the other hand, it is a well-established fact, that 
certain varieties of orthose contain as much soda as potash, so 
that the limits between the orthose and the albite, as regards 
composition, are much less broadly marked than between the 
microcline and the orthose. 
