
132 | GEOGNOSY. ~~ [Boox Il. 
ferred that the rock has a thoroughly crystalline structure, with no 
macroscopic ground-mass, nor microscopic base of any kind between 
the crystals or crystalline individuals. More recent and exhaustive 
study of the subject, however, has led to the conclusion that though 
nothing like a vitreous or even porphyritic ground-mass can be 
detected, there is yet discernible an analogous kind of entirely 
crystalline magma, in which the crystals or crystalline débris of 
the rock are embedded, and in which they are partially dissolved. 
Having regard to the relations between this magma and its enclosed 
* minerals, M. Michel-Lévy has observed that microscopic examina- 
tion points to a distinction between granites in which the quartz is 
more recent than the other constituents and has consolidated at 
once, and those in which there are remains of earlier bi-pyramidal 
quartz. He distinguishes these two series as (A) Ancient granites, _ 
composed of black mica, hornblende, oligoclase, and orthoclase, 
forming a crystalline debris embedded in a more recent crystalline 
magma of orthoclase and quartz. (B) Porphyroid granites, generally 
finer in grain than the preceding, and further distinguished by 
the occurrence of bi-pyramidal crystals of quartz (which made 
their appearance between the old felspar and the recent ortho- 
clase), and of a notable quantity of white mica (rare among the 
ancient granites) posterior in advent even to the more recent 
uartz." 
: Among the component minerals of granite, the quartz presents 
special interest under the microscope. It is often found to be 
full of cavities containing liquid, sometimes in such numbers as to 
amount to a thousand millions in a cubic inch. The liquid in these 
cavities appears usually to be water containing sodium and potassium 
chlorides, with sulphates of these metals and of calcinm (p. 96). 
The mean of eleven analyses of granites made by Dr. Haughton 
gave the following average composition: silica, 72°07; alumina, 
14°81; peroxide of iron, 2°22; potash, 5:11; soda, 2°79; lime, 1:63; 
magnesia, 0°33; loss by ignition, 1:09; total, 100-05, with a mean 
specific gravity of 2°66. 7 
Most large masses of granite present differences of texture in dif- 
ferent parts of their area. In particular they are apt to be traversed 
by veins, sometimes due to a segregation of the surrounding 
minerals in rents of the original pasty magma, sometimes to a pro- 
trusion of a less coarsely crystalline (felsitic) part of the granitic mass 
into fissures of the main rock (Fig. 21). Some of the more important 
of these varieties are distinguished by special names. Thus, where the 
component minerals assume large proportions, as they are specially 
apt to do in segregation veins, the rock is termed Pegmatite, the quartz 
and felspar having crystallized together in masses often larger than 
a man’s head, the mica also assuming the shape of plates several 
inches or even feet in diameter. Such coarse-grained varieties may 
be found here and there in venous or cavernous spaces in the heart of 
' Bull. Soc. Géol. France, 3rd ser. iii. (1875) p. 199. 
