118 



LYELL'S ELEMENTS OF GEOLOGY. 



Plutonic Rocks. 



CHAPTER IX. 



PLUTONIC ROCKS GRANITE. 



General aspect of granite — Decomposing into spherical masses — Rude colum- 

 nar structure — Analogy and difference of volcanic and plutonic formations — 

 Minerals in granite, and their arrangement — Graphic and porphyritic granite — 

 Occasional minerals — Syenite — Syenitic, talcose, and schorly granites — Eurite — 

 Passage of granite into trap — Examples near Christiania and in Aberdeenshire — ■ 

 Analogy in composition of trachyte and granite — Granite veins in Glen Tilt, 

 Cornwall, the Valorsine, and other countries — Different composition of veins 

 from main body of granite — Metalliferous veins in strata near their junction with 

 granite — Apparent isolation of nodules of granite — Quartz veins — Whether plu- 

 tonic rocks are ever overlying — Their exposure at the surface due to denuda- 

 tion. 



The plutonic rocks may be treated of next in order, as they 

 are most nearly allied to the volcanic class, already considered. 

 I have described, in the first chapter, these plutonic rocks as the 

 unstratified division of the crystalline or hypogcne formations, 

 and have endeavoured to point out in the Frontispiece, at D, the 

 position which they occupy, when first formed, relatively to the 

 volcanic formations, B. 



By some writers all the rocks now under consideration have 

 been comprehended under the name of granite, which is, then, 

 understood to embrace a large family of crystalline and com- 



Fig. 105. 



Mass of granite near the Sharp Tor, Cornwall. 



pound rocks, usually found underlying all other formations; 

 whereas we have seen that trap very commonly overlies strata 

 of different ages. Granite often preserves a very uniform char- 

 acter throughout a wide range of territory, forming hills of a pe- 



