STRUCTURE OF GRANITE. 217 



Jersey the syenitic granite is red. The red rock at Syene, in Egypt, 

 is a syenitic granite. Characteristic syenitic granite is seen at Stron- 

 tian, in Argyleshire. 



Gneissose Granite is granite which has a schistose character. 



Graphic Granite is also schistose, but consists of orthoclase and 

 quartz, so arranged in parallel layers that a transverse fracture 

 exhibits the quartz in forms, suggesting letters of an Oriental lan- 

 guage. It occurs near Ilmenau, and by Limoges, &c. 



Pegmatite is a kind of giant granite, in which the crystals of 

 orthoclase are sometimes a foot long, and the white mica occurs in 

 large flakes. It is only known in other granite, and generally contains 

 tourmaline, garnet, topaz, &c. It is seen near Penig, in Saxony. 

 Sometimes the greater part of the rock is formed in a milk-white 

 quartz. It occurs in Ireland, according to Zirkel, in Carlingford Bay, 

 and is frequently cavernous, with the walls of the cavities covered 

 with crystals. 



Haplite consists almost entirely of orthoclase and quartz, with very 

 little mica. It is seen at Gottleube in Saxony. 



Tourmaline Granite is granite in which the mica is partly replaced 

 by schorl. It is seen in Cornwall, and is also known as Luxulianite. 

 The felspar is flesh-coloured, and there is very little quartz. 



Beresite is a variety of granite, rich in iron-pyrites, and poor in 

 mica. It occurs in dykes at Beresowsk, in the Ural mountains. 



Many other local varieties occur. Thus, in the Fichtelgebirge, at 

 Vordorf, the granite is rich in Epidote, and in Cornwall granite con- 

 tains tin. 



Joints in Granite. Granite is generally characterised by joints 

 or division planes. In Cornwall they usually run from N.jST.W. to 

 S.S.E., or at right angles to the direction in which the granitic masses 

 extend through the country. Near the Land's End, especially off the 

 coast and on the tops of the moors, the granite has a rude columnar 

 structure. In the Harz Mountains there are three principal divi- 

 sion planes, one in the direction in which the rock extends, another 

 at right angles to this direction, and a third which is horizontal and 

 less developed. In the province of Constantine, in Algeria, granite 

 is found in regular columns, with five or six sides, which, at a distance, 

 have the aspect of basalt. 



In the Riesengebirge and Fichtelgebirge granite often has a 

 spheroidal structure, with the spheres ranging in size from 2 inches 

 to 2 feet. The structure is concentric, and the kernel is sometimes 

 formed of crystals of orthoclase. ISTear Oporto concretions in granite 

 are 50 feet in diameter. 



Decomposition of Granite. The decomposition of granite is well 

 seen in Cornwall, especially near St. Austell, and at Cornwood, in 

 Devonshire. It is even more marked at St. Yrieix, south of Limoges. 

 Decomposition results from the solvent action of carbonic acid, dis- 

 solved in water, acting upon the soda, potash, magnesia, lime, iron, 

 or other soluble constituent of the rock, resulting in the production 

 of a friable rock called arkose. 



