PLUTONIC OR IGNEOUS ROCKS. 



649 



represented in the preceding figure. D'Aubuisson mentions, that in a hollow way which 

 had been only six years blasted through granite, the rock was entirely decomposed to the 



deptli of three inches. He also states, that the granite country of Auvergne the 



Vivarais and the eastern Pyrenees is frequently so much decomposed, that the tra- 

 veller may imagine himself on large tracts of gravel. But this is owing to a cause 

 peculiar to the district, the large development of carbonic acid gas, which produces 

 the granite-rot, styled by Dolomieu la maladie du granite, the rock subjected to its 

 action crumbling to pieces in the hand. Nevertheless, great durability is the prevailing 

 characteristic of granite, and hence its extensive employment in the construction of our 

 docks and bridges, where that quality is of the first importance. 



Various minerals occur in granite besides those which enter into the composition of 

 the rock, either traversing it in veins, or imbedded in it as crystals as tourmaline, 



emerald, berge, topaz, garnet, and fluor-spar. Large 

 curved crystalline prisms of tourmaline are found in the 

 granite at Portsoy in Bariffshire, in Norway, the Tyrol, 

 and Saxony ; a village in the latter country, where it is 

 met with, giving to the mineral its other name of schorl. 

 The Mourne granite mountains in Ireland contain topaz, 

 associated with berge ; and large and magnificent crystals 

 of the former are furnished by the district of Cairngorm 

 in Aberdeenshire. They are common also with emerald 

 in the Uralian and Altain ranges. Among the metal- 

 liferous minerals, tin chiefly occurs in Cornwall and 

 Saxony, but iron is found in the Piedmontese Alps and 

 the Pyrenees, and occasionally silver. 

 Granite Vein in Grauwacke. The fact of granite traversing the stratified formations 



in veins of fantastic description has already been noticed. This is a proof of the fused 

 and melted condition of the rock when erupted, in which state it flowed into cracks 

 and fissures of the superincumbent strata. Above is a striking example, given by Sir 



James Hall, of the Loch Dee granite, in Wigton shire, 

 invading the grauwacke, near the top of the hill of 

 Lauren. Several other junctions occur in the same 

 neighbourhood, the granite meeting the nearly vertical 

 strata at various angles, sometimes in veins nearly a 

 hundred yards wide, but gradually thinning out. An 

 instance, from the upper basin of the Spey, of granite 

 veins penetrating the gneiss in a direction parallel 

 Granite vein in Gneiss. to tne strataj an d joining together, producing appa- 



rent alternations of the two rocks, is here given. In such cases the posterior formation 

 of the granite to the invaded strata is very evident; and in fact, whilst the granite goes 



back to the earliest stage of the earth's history of which 

 we can catch a glimpse, the oldest stratified rocks, the 

 gneiss, and mica schist systems having been formed out of 

 its disintegrated material, we have numerous examples 



of its having erupted at a comparatively modern date, 

 instances of granite formations in every subsequent geo- 

 logical epoch, down to the tertiary period, being found. 



In this cut, an example is presented from the Alps, of 

 beds belonging to the upper secondary strata, kindred to 

 our magnesian limestone, lias, and oolite, uplifted and turned back by the intruding 



