PLUTONIC OR MASSIVE ROCKi 



203 



1. — Plutonic or Massive Rocks. 



General Appearance. — The rocks of this group are characterized by 

 a coarse-grained, mottled, or speckled appearance, arising from the 

 fact that they are composed of an aggregation of distinct crystals of 

 diiferent colors and of considerable size (macro-crystalline) ; and, what 

 is much more important, the rock is usually wholly made up of an ag- 

 gregation of such crystals, without any paste or ground-mass, either 

 amorphous or glassy, between them. 



The constituent minerals of this group are mainly quartz, feldspar, 

 mica, and hornblende. In the speckled mass the opaque, white, or red- 

 dish or greenish crystals with glistening surface are feldspar, the trans- 

 parent bluish glassy spots are quartz, and the black specks are usually 

 hornblende. The mica can be easily detected as glistening scales of 

 various shades. 



Principal Kinds — Granite. — This rock, which may be regarded as 

 the type of the group, consists of quartz, feldspar, and mica, or else of 

 these, together with horn- 

 blende. Sometimes the 

 mica and hornblende are 

 wanting, and the quartz 

 exists in the form of bent 

 plates imbedded in feld- 

 spar, so that on cross- 

 section they look like He- 

 brew or Arabic characters 

 (Fig. 181, A and B). The 



rock is /then called graphic granite, or pegmatite. Sometimes the feld- 

 spar is in large, well-formed crystals in a finer but still crystalline 

 ground-mass : then it is called porphyritic granite. Sometimes all 

 the crystals are small, and the mass is evenly granular ; then it is called 

 eurite, or granulite. 



Syenite. — English and many American writers use this term to 

 designate a granitic rock in which mica is replaced by hornblende ; 

 and, when both hornblende and mica are present, they use the term 

 syenitic granite. But on the Continent of EurOpe the term syenite is 

 applied to a rock consisting essentially of feldspar and hornblende, 

 and when, in addition, quartz is present (English syenite), they call 

 the rock quartz-syenite. The general aspect of the rock is similar to 

 granite. 



In the rocks thus far mentioned the feldspar is an orthic, ox pot- 

 ash-feldspar (orthoclase) — i. e., is a double silicate of alumina and 

 potash. 



Diorite. — This is a dark, speckled, greenish-gray rock, consisting of 



Fig. 181. — Graphic Granite: A, cross-section; B. longitudin- 

 al section. 



