64 BOCKS FOEMED THROUGH IGNEOUS AGENCIES 



to designate rocks of this class. Greisen is a name applied to 

 a qnartz-mica rock, with accessory topaz, occurring associated 

 with the tin ores of Saxony and regarded as a granite meta- 

 morphosed by exhalations of fluoric acid. Luxullianite and 

 Trowlesworthite are local names given to tourmaline or tour- 

 maline-fluorite granitic rocks occurring at Luxullian and 

 Trowlesworth, in Cornwall, England, The name Uoiahite has 

 been given to an epidotic granite with pink feldspars occurring 

 in the Unaka Mountains, western North Carolina. 



The name granite 'porphyry is made to include a class of rocks 

 placed by Professor Rosenbuseh under the head of dike rocks, 

 and differing from the true granites mainly in structural fea- 

 tures. They consist in their typical forms of orthoelase feldspars 

 and quartzes porphyritically developed in a finer holocrystalline 

 aggregate of the minerals common to the granite group. 



Geological Age and Mode of Occurrence. — The granites are 

 massive rocks, occurring most frequently associated with the 

 older and lower rocks of the earth's crust, sometimes inter- 

 stratified with metamorphic rocks or forming the central por- 

 tions of mountain chains. They are not, as once supposed, the 

 oldest of rocks, but occur in eruptive masses or bosses invading 

 rocks of all ages up to late Mesozoic or Tertiary times. Profes- 

 sor Whitney considered the eruptive granites of the Sierra 

 Nevada to be Jurassic. Zirkel divided the granites described in 

 the reports of the 40th Parallel Survey into three groups: (1) 

 Those of Jurassic age; (2) those of Palaeozoic age, and (3) those 

 of Archaean age. The granites of the eastern United States, on 

 the other hand, have, in times past, been regarded as mainly 

 Archaean, though Dr. Wadsworth has shown that the Quincy, 

 Massachusetts, stone is an eruptive rock of late Primordial or 

 more recent age, while Professor Hitchcock regards the eruptive 

 granites of Vermont as having been protruded during Silurian 

 or perhaps Devonian times. 



The granites are among the most wide-spread and commonest 

 of igneous rocks, and are of great economic importance for 

 structural and monumental work. In the United States they are 

 to be found mainly in the Appalachian region and from the front 

 range of the Eocky Mountains westward to the Pacific coast 



