§ 28. 



GRANITE. 859 



of the newest secondary formations. In the British Isles 

 granite appears in Cornwall, Dartmoor, &c. ; and forms the 

 nucleus of Skiddaw, Shapfell, Ben Nevis, and other moun- 

 tain peaks. 



Granite veins. Granite often occurs in dikes and 

 veins which traverse not only other rocks, but also the 

 pre-existing masses of granite ; proving that the form- 

 ation of this mineral has taken place at various and 

 distant periods. Veins are fissures or chasms produced 

 in rocks either by mechanical disturbance, or by contrac- 

 tion of the mass during its consolidation or refrigeration, 

 and which have been filled by subsequent infiltration or 

 sublimation, or by injections of mineral matter in a state of 

 fusion from a subterranean source. Although many me- 

 tallic veins are synchronous with the rocks they traverse, 

 having been formed by segregation during the consolidation 

 of the mass, yet the veins and dikes of volcanic matter are 

 obviously of later origin than the beds in which they are 

 intruded. Thus the granite veins represented in this 

 diagram (Lign. 196, Jig. 1), are newer than the slate rocks 

 through which they are disseminated. 



Granite veins traversing other rocks are themselves 

 sometimes intersected by intrusions of other melted mate- 

 rials. This sketch {IAgn. 196, Jig. 2) represents a mass of 

 schistose rock, which is crossed by granite veins (a, a) in 

 one direction, and again by veins of porphyry (b, b, b), 

 which cut through both the schist and the granite. When 

 gneiss is intersected by granite, it becomes shifted, as 

 in this example, in which the granite veins {Lign. 196, 

 Jig. 3, a, a, a) have displaced the laminae of gneiss (b, b, b). 

 Thus by numerous observations of phenomena of a like 

 nature, it is now clearly established that granite has been 

 ejected during the Cumbrian, Silurian, Carboniferous, 

 Oolitic, Cretaceous, and even Tertiary epochs. 



Where granite has been erupted in a fluid or softened 



