CHAPTER XV. 



THE GRANITIC OR PLUTONIC GROUP OF ROCKS. 



FROM a geological point of view it is convenient to associate together 

 all the deep-seated rocks which are completely crystallised, for they 

 occur under similar conditions. The term granitic applied to these 

 rocks, which have a granular texture like granite, merely indicates 

 similar conditions of solidification for the rocks which are associated 

 under that name. From the point of view of evolution it would be 

 more natural to associate the volcanic rocks with their corresponding 

 plutonic representatives. The student in a granitic district will not, 

 however, usually be able to trace the granite into rhyolite, the syenite 

 into trachyte, or diorite into andesite, even if there be some indica- 

 tions of a passage from gabbro into dolerite. Hence, though impor- 

 tant questions of theory are involved in establishing the unity of the 

 plutonic and volcanic series, no practical inconvenience will be found 

 in grouping together the massive igneous rocks, which form axes of 

 upheaval, and have been exposed at the surface by denudation. 



Mineral Composition of Granite. The term granite in its modem 

 sense was first used by Werner. 1 As already indicated, this rock is 

 typically a crystalline mixture of orthoclase (and oligoclase), quartz, and 

 mica, varying in texture with the size of the crystals, which are some- 

 times as fine as mustard- seed, and sometimes as large as the closed 

 fist. When the crystals are large the texture is more irregular. 

 When an un crystalline matrix separates the crystals from each other, 

 the rock is passing into felsite, a term still conveniently used for 

 altered rhyolitic rocks. 



The orthoclase constituent, which usually occurs in twin crystals, 

 cleaves with a pearly fracture and gives granite its characteristic 

 colour, being pink, red, or brownish red, reddish brown, white, 

 yellowish grey, green, or reddish grey, and is even blue in Connecticut 

 and the Pyrenees. 



The oligoclase is less transparent, contains more soda than ortho- 

 clase, is more fusible, and has a grey or greenish tinge. In some 

 granites, these felspars are in about equal quantity. Frequently in 

 Scotch granites the oligoclase surrounds the orthoclase crystals as a 

 rind. Professor Zirkel points out that at Schreibershau, in the 



1 See Zirkel's " Petrographie, " 1866, a general history of plutonic rocks, in- 

 dispensable to the student, from which we have quoted numerous facts. 



