216 VARIETIES OF GRANITIC ROCKS. 



and soda varies from a trace to 6 -3 per cent. Water may be absent, 

 and never amounts to more than 2 per cent. 



Chemical Classification of Granites. Haughton proposed in 

 I856 1 to divide granites into a potash group and a soda group, 

 because the percentage of these alkalies not only determines the 

 kind of felspar in the rock, but usually has a relation to the percent- 

 age of silica. Thus in the granite of Croghan Kinshela, in Wexford, 

 the soda amounts to 5-58 as against '4 of potash, while the silica is 

 So per cent. At Baveno the soda amounts to 6-12 and the potash to 

 3 '5 5 per cent., while the silica is 74*82. When the percentage of 

 potash increases, the amount of silica frequently diminishes, though 

 there is no necessary relation between these substances. 



Mineral Varieties of Granite. The percentage of quartz in 

 granites varies, according to Haughton, from 20*0 to 35*0, though 

 Delesse states it as high as 60 'o in a granite of the Vosges. 



The mica varies from 4*0 to 27*0 in Irish granites, while at Tholy, 

 in the Vosges, it is stated to be only one per cent. 



The felspar varies from 40*0 to 69*0 in Irish examples, though the 

 highly quartzose granite of the Vosges, already referred to, has but 

 35-0 per cent. 



So great are the variations in mineral composition of granites that 

 it becomes necessary to recognise the value of adventitious minerals 

 in distinguishing local modifications of the rock. Besides the type, 

 the following varieties may be defined : 



Granitite, formed of red orthoclase, much oligoclase, little quartz, 

 and little dark-green magnesian mica. It is especially distinguished 

 by the absence of potash mica, the predominance of oligoclase, and the 

 reduced importance of the quartz. It contains magnetic iron and 

 titanic iron. Many Irish granites are intermediate between the typical 

 granites and granitite. In British geology the rock has no certain 

 representative, and the term is chiefly used in the Kiesengebirge, 

 Harz, &c. Augite-bearing granitite occurs in dykes in the metamor- 

 phic rocks of Laveline in the Vosges, at Titisee in the Schwarzwald, 

 &c. It is less common than hornblendic granitite. The augite is 

 formed in green prismatic crystals or crystalloids. 



Protogine, or talc granite of the Alps, has the same composition as 

 granite, but contains in addition a pale green talc-like mineral. Its 

 quartz is easily broken. The oligoclase has a greenish tinge, while the 

 orthoclase is grey. The mica is usually in six-sided plates. The talc 

 is only freely developed when the rock becomes schistose. It is well 

 seen in Mont Blanc and the Western Alps. Between Schneeberg and 

 Eibenstock, in the Erzgebirge, it contains no mica, and the felspar is 

 flesh-red. 



Syenitic Granite or hornblendic granite is intermediate between 

 granite and syenite. It contains less quartz than granite, and horn- 

 blende to a large extent takes the place of the mica, which is always 

 dark. It forms the summits and central parts of the Vosges. The 

 accidental minerals are sphene, zircon, chlorite, iron pyrites. In 

 1 Quart. Jour. Geol. Soc , vol. xii. p. 177. 



