134 
Monpay, Decemper 11TH, 1854. 
LIEUT.-COL. LARCOM, F.R.S., Vicz-PresipEnt, 
in the Chair. 
Tue following account ofa series of analyses, instituted to 
determine the relative quantities of potash and soda in the 
felspar of the Dublin and Wicklow granite, was read by the 
Rey. Joseph A. Galbraith, Erasmus Smith’s Professor of 
Natural and Experimental Philosophy in the University of 
Dublin. 
‘‘ Granite is a rock which generally consists of three con- 
stituents, namely, quartz, mica, and felspar, in varying propor- 
tions. The first of these, quartz or silex, is a mineral of in- 
variable composition ; the second, mica, is a mineral of com- 
plex constitution, admitting of considerable varieties both in 
its chemical composition and in its physical properties. In 
some granites it is replaced either wholly or in part by horn- 
blende; as, for instance, in the syenite of Egypt, and the 
granites of the county of Galway. In some by tale, as in the 
Protogene of Mont Blane, and in parts of Cornwall ; and occa- 
sionally by the remarkable hydrated mineral chlorite, as in 
the granite rocks in the vicinity of Liskeard. The third con- 
stituent, felspar, is a mineral of very definite composition, ad- 
mitting of no important chemical variety, except in its alkaline 
constituents. The nature and extent of this variety will be 
best understood by comparing together the following theo- ~ 
retical analyses, which are extreme cases :— 
Composition of Typical Orthose, 
or Potash Felspar. 
Composition of Typical Albite, 
or Soda-Felspar. 
Silex, 64:81 | Silex, 68°74 
Alumina, 18°38 | Alumina, 19:50 
Potash, . 16°81 | Soda, 11-76 
100-00 | 100-00 
