42 STRUCTURAL AND FIELD GEOLOGY 
A few varieties of granite may be mentioned—only the essential 
minerals being named :—/Vormal or Muscovite-granite =felspar + quartz 
+muscovite+biotite ; Granttite or Biotite-granite (Plate [V. 3)=felspar 
+ quartz + biotite ; Hornblende-granite = felspar + quartz + hornblende, 
and usually some biotite ; Zousmaline or Schorl-granite=felspar + quartz 
+schorl; Graphic granite=felspar+quartz, which have crystallised 
together, the quartz assuming the form of successive irregular columnar 
shells, arranged in parallel positions, and enclosed in the felspar 
(see Plate XI. 1). This is known as fegmatitic structure ; when seen in 
cross-section it has some resemblance to Hebrew writing, hence the 
name graphic. While this structure often occurs megascopically, 
especially in coarsely crystalline veins associated with granite, it is 
sometimes only to be detected under the microscope=micropegmatite 
(Plate VIII. 3). Gzant-grantte or Pegmatite=any very coarse-grained 
granite—granites of this kind very frequently show pegmatitic structure ; 
Aplite or Haplite=a fine-grained granite containing little or no mica, 
met with as veins ; Grvezsen=a granite with little or no felspar, occurring 
as veins in normal granite; Porphyritic granite=a rock showing large 
phenocrysts* of felspar, disseminated through a relatively fine-grained 
granitoid matrix ; Granite-porphyry or Microgranite=a rock consisting 
of a microgranitic or micropegmatitic (gvanophyric) groundmass, with 
phenocrysts of felspar, quartz, pyroxene, and occasionally amphibole ; it 
occurs sometimes forming a part of a large mass of ordinary granite: at 
other times it forms dykes and veins proceeding from granite ; Granife- 
gneiss =a granite in which the minerals have in whole or in part a rudely 
parallel arrangement, giving to the rock a coarsely banded structure. 
Structures in granite :—Geodes and drusy cavities; these are irregular 
cavities which often occur sporadically in granite, and are usually lined 
with finely crystallised, well-formed examples of the essential and accessory 
ingredients of the rock (see Plate XI. 2). The felspars and the quartz are 
generally conspicuous, and with these mica and one or more of the 
‘accessory ingredients, as sphene, apatite, zircon, topaz, beryl, etc. 
Secretions ; these are of two kinds—basic and acid. The daszc secretions 
are dark masses of very irregular form and varying size, rich in ferro- 
magnesian minerals (biotite, hornblende), sphene, and iron ores. They 
often resemble fragments broken from some other rock, and subsequently 
enclosed in the granite. Possibly they may be fragments of massive 
aggregates of basic ingredients which may have crystallised out from the 
magma at an early stage in the process of consolidation, and become 
broken up during subsequent movements of the slowly cooling and 
consolidating plutonic mass. The aczd secretions are light-coloured, 
* The origin of such phenocrysts is not yet understood. The 
explanation which is supposed to account for the formation of phenocrysts 
in lava-form rocks (see supra, p. 35) can hardly apply to the phenocrysts 
of plutonic rocks, which cooled at great depths, and therefore under the | 
continuous pressure of heavy overlying rock masses, 

