248 Scientific Proceedings, Royal Dublin Society. 
Descloiseaux further points out the resistance which the micro- 
cline opposes to decomposition, an observation which quite agrees 
with the state in which the crystals present themselves in the 
Granites of Dalkey Island. 
From the point of view of Geology, this dimorphism of the 
potash Feldspar merits consideration, since it is well established 
that dimorphism in a mineral implies distinct conditions of for- 
mation for the one and the other form. It may be asked is there 
not a distinction to be made between granites rich in Microcline 
Feldspar and those in which either eamere or albite are more 
prominent as a constituent, and may not such a distinction assist 
in better characterizing the granites in general relatively to their 
conditions of formation. 
That the Granites of Dalkey are Microcline Granites would 
seem to result directly from the comparison of the table of ana- 
ee of the Feldspars of the Dublin and Wicklow Granite, made 
y Galbraith, in 1856, with that of the Microclines analyzed by 
eae and Pisani, and published in Descloiseaux’s Memoir; and 
this comparison will also help to prove how important is the de- 
termination of the physical and optical properties of minerals 
relatively to the interpretation of their chemical analysis. The 
comparison of these tables seems, therefore, both interesting and 
even a necessary compliment to this paper, and I accor diaely 
annex it. 
Tables referred to— 
DAMOUR AND PISANI. 
Ann. de-Phy. et Chim, Tom. IX., December 1876. 
fiat Te) fei | TLS |e Ve CV obis: (ABV Neva en eval va. IX | 2 | xy 
ee sy ae | | 
Silica, . . «| 64°30] 64°08) 64°80 .65°75| 65°17) 65°55] 64-97] 64°80 65-43, 65-43] 64°70] 64:90 
‘Alumina, . | 19°70| 20°70) 19°60} 20°90) 17-70] 20°30) 21-47] 19°90, 19°58 19°58] 19°50) 20-92 
Ferric Oxyde, .| 074| - = = 0:50; = = - |. 0-85} O85] = 0-28 
Potash,.  . «| 15°50| 13:75] 13°50] 1320] 13:86] 43°90] 1220) 12°11) 12-45) 12-45) 12°90] 10-95 
Soda, .. . «| 0-48| 4-27] 1-56] 1:60| 1-64] i-60| 41-78) 2-10, 2-31) 2:31] S-40l age 
Times fst pfeil = - - - 56) = - - | - - - = 
Magnesia, . = = = py coe 0:25) —- Os) sae = = = = 
Loss by ignition, .| 0°35] 0-20) 0-20 0-20) 0°65 - 0:81/ 0°30 0°30] “ x 0-20 
| | | — ———_ |__| 
101°17| 100-0! 99°66 101°65| 100-33 101°41|101'55! 99-21 100-12 100-12] 100-50] 101°30 
| ' [a a ee ee eee eee 
Sp. Gravitie, .| 2°54 2S 2 2:54, — | 2:576| 2°47| Jos 2°584| 2:543| 2°58] 2°57 
