HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 553 



Chemical composition. — The average chemical composition as given by 

 Geikie is as follows : Silica, 72.07; alumina, 14.81 ; potash, 5.11; soda, 

 2.79; lime, 1.G3; magnesia, 0.33 ; iron protoxide, 2.22; loss by ignition, 

 1.09. Total, 100.05 per cent. Specific gravity, 2.66. 



Structure. — The granites are holocrystalline granular rocks without 

 trace of amorphous interstitial matter. Asa rule none of the essential 

 constituents show perfect crystal outlines, though the feldspathic min- 

 erals are often quite perfectly formed. The quartz has always been the 

 last mineral to solidify, and hence occurs only as irregular granules 

 occupying the interspaces. It is remarkable from its carrying innumer- 

 able cavities filled with liquid and gaseous carbonic acid or with saline 

 matter. So minute are these cavities that it has been estimated by 

 Sorby that from one to ten thousand millions could be contained in a 

 single cubic iuch of space. The microscopic structure of a mica granite 

 from Maine is shown in transparency No. 39075 and in Fig. 1, PI. cxx. 



The rocks vary in texture almost indefinitely, presenting all grada- 

 tions from fine evenly granular rocks to coarsely porphyritic forms in 

 which the feldspars, which are'the only constituents porphyritically de- 

 veloped, are several inches or feet in length. Compare specimens 36751 

 and 26815 from Maryland and Egypt, with 26386 and 38763 from Mas- 

 sachusetts and California. Concretionary forms are rare. Specimen 

 No. 38546 (see Fig. 2, PI. cxxvi) is of a granite concretion from Fonni, 

 Sardina, cut in halves. Specimen 70098 from Craftsburg, Vermont, is 

 unique on account of the numerous concretionary masses of black mica 

 it carries. 



Colors. — The prevailing color is some shade of gray, though greenish, 

 yellowish, pink, to deep red, are not uncommon, as may be seen by ref- 

 erence to the collection. The various hues are due to the color of the 

 prevailing feldspar and the abundance and kind of the accessory min- 

 erals. Granites, in which muscovite is the prevailing mica, are nearly 

 always very light gray in color : (Specimen 36164 from the Vosges 

 Mountains). The dark gray varieties are due largely to abundant black 

 mica or hornblende (specimens 36186 and 38762 from Germany and 

 California) ; the greenish and pink or red colors to the prevailing green- 

 ish, pink or red feldspars (specimens 29527, 26386, and 37667 from 

 New Hampshire, Massachusetts, and New Brunswick). 



Classification and nomenclature. — Several varieties are commonly rec- 

 ognized and designated by names dependent upon the predominating 

 accessory mineral. We thus have (1) muscovite granite, (2) biotite 

 granite or granitite, (3) biotite muscovite granite, (4) hornblende 

 granite, (5) hornblende biotite granite, and more rarely (6) pyroxene, 

 (7) tourmaline, and (8) epidote granite. The name protogine has 

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

 by talc (specimen No. 36127 from Mount Blanc). Graphic granite or 

 pegmatite is a granitic rock consisting essentially of quartz and ortho- 

 clase so crystalized together in long parallel columns or shells that a 



