THE MATERIALS OF MOUNTAIN CHAINS. 25 



is also intruded on a smaller scale, forming veins, 

 which penetrate into other rocks, or sometimes 

 cut through the granite itself. The only evidence 

 of the condition and temperature at which the 

 granite was intruded is afforded by its junction 

 with slate. In Cornwall, where the slate near to 

 it has acquired the texture of mica schist and 

 gneiss, there is no evidence to show whether that 

 metamorphism was due to the heat of the granite, 

 or to the pressure which it exerted, or both com- 

 bined. 



A few rocks which are found in mountain 

 regions resemble granite in texture, but differ 



n it in mineral constituents, owing to the 

 original chemical difference of the material out 



.vhich the Crystals are formed. Syenite is well 



known in Charnwood Forest and in Guernsey. 



Syenite is a rock formed commonly of orthoclase 

 felspar, hornblende and black mica. They are a 



variable group, including mica syenites, augite 



iites, nepheline syenites, zircon syenites and 

 many othi 



A third type of granite rock is named gabbro. 

 It is familiarly known in the Cuchullin hills in 

 Skye. Its crystals are as large as those of granite, 

 and similarly arranged. It is formed of a plagio- 



5e felspar like labradorite, associated with some 

 mineral of a brassy or metallic aspect like diallage, 

 and often contains black mica and olivine; and 

 in some localities hornblende. 



These granitic rocks have been termed plu- 

 tonic because they appear to originate in the re- 

 gion which mythology assigns to Pluto, in the 

 interior of the earth, consolidating slowly under 

 great pressure. 



