THE GEANITE-LIPAEITE GROUP 



63 



structure of a mica granite from Maine is shown in Fig. 3 

 and in Fig. 1, PL 5. 



The granites vary in texture almost indefinitely, presenting all 

 gradations from fine evenly granular rocks to coarsely porphy- 

 ritie forms in which the 

 feldspars, which are the 

 only constituents porphy- 

 ritically developed, are 

 several inches in length. 



Colors. — The prevail- 

 ing color is some shade of 

 gray, though greenish, 

 yellowish, pink, to deep 

 red, are not uncommon. 

 The various hues are due 

 to the color of the prevail- 

 ing feldspar and the 

 abundance and kind of 

 the accessory minerals. 

 Granites in which mus- 

 covite is the prevailing 

 mica, are nearly always 

 very light gray in color. The dark gray colors are due largely to 

 abundant black mica or hornblende, the greenish and pink or 

 red to the prevailing greenish, pink, or red feldspars. 



Classification and Nomenclature. — Several varieties are com- 

 monly recognized and designated by names dependent upon the 

 predominating accessory mineral. We thus have (1) musco- 

 vite granite, (2) lioUte granite ov graniiite, (3) hiotite-muscovite 

 granite^ (4) hornllende granite, (5) hornhlende-'biotite granite, 

 and more rarely (6) pyroxene, (7) tourmaline and (8) epidote 

 granite. The name protogine, not now very generally used, has 

 been given to a granite in which the mica is in part or wholly 

 replaced by talc. 



Graphic granite, or pegmatite, is a granitic rock consisting 

 essentially of quartz and orthoelase so crystallized together in 

 long parallel columns or shells that a cross-section bears a crude 

 resemblance to Hebrew writing. Aplit is a name used by the 

 Germans for a granite very poor in mica and consisting essen- 

 tially of quartz and feldspar only. 



The names granitell and Unary granite have also been used 



Fig. 3. — ^Microstrueture of museovite-bio- 

 tite granite, Hallowell, Maine. 



