196 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



containing about the same quantity of feldspar, but differing in the 

 other minerals. In fact, it would be easy to assign the conditions 

 under which the first or second variety of granite would be formed, 

 which consist of similar elements, but of different minerals. 



Composition of the Syenites. 



The syenites of Carlingford, which are coarse-grained and fine- 

 grained, and occasionally pass into pure hornblende -rock, are com- 

 posed of two minerals, anorthite and hornblende. As this is the 

 first time that anorthite is noticed as a British mineral, it may be 

 useful to discuss its chemico-mineralogical formula. 



Carlingford Anorthite. 



Per cent. Atoms. 



Silica 45-87 .. ..0-997.... 3 



Alumina 3473 ... 0*676 2 



Lime \\ 



Magnesia 



{;l,^}....0 688....2 



99-25 

 The formula deducible from this composition is 

 2R0, SiOg + 2{Al203, SiOg}. 



It is plain that this feldspar is the anorthite of Vesuvius and 

 Hecla, and is identical with the anorthite of Java, of Columbia, and 

 of meteoric stones. It is essentially a volcanic mineral and is the 

 most basic of all the feldspar-family, as it contains disilicate of lime. 

 It is completely decomposed by muriatic acid, and is harder than 

 ordinary feldspar, striking fire under the hammer freely. 



At Grange Irish, this mineral is formed by the addition of carbo- 

 niferous limestone to the fused granite of the second variety already 

 described. 



I shall now describe the hornblende which forms part of the 

 syenites. 



Carlingford Hornblende. 



Specific gravity =2*923. 



Per cent. Atoms. 



Silica 50*72 1 i-2«4 



Alumina 9-36 j • - ■ • 



Protoxide of iron .... 18*61'1 



Lime 16-96 > 1-243 



Magnesia 2-40J 



Loss by ignition .... 1-52 



99-57 



From the preceding analysis, uniting, as Rammelsberg does, the 

 alumina with the silica, we obtain 



RO; {SiOg, AlA}; 



