PYROGENIC ROCKS 275 



termediate group with the percentage of sihca (SiOo) between 

 50 * and 68 ; while the fourth and fifth constitute the basic group, 

 with the percentage of SiO^ below 50. That this classification is 

 only approximate appears from the fact that some gabbros have a 

 percentage of SiOo approaching 55, while some granites fall as 

 low as 62 per cent. Each of these groups may be subdivided ac- 

 cording to texture as follows : 



Textural Groups. 



Crystalline D ivision. 



A. HolocrystalUnc, or Granitic. Also called phaneritic, 



with the minerals all crystallized out, and the lead- 

 ing minerals visible magascopically, as in granite. 

 They may occasionally exhibit a rude porphyritic 

 structure. The crystals may be in part idiouiorphic 

 (automorphic), each having its own form, or allo- 

 triomorphic (xenomorphic), having their outline 

 determined by the surrounding crystals, or both. In 

 basic igneous rocks, of either holo- or cryptocrys- 

 talline texture, an opiiitic structure may occur, in 

 which the crystals of pyroxene have the character of 

 large plates, separated and penetrated by fine needles 

 and crystals of feldspar. The pyroxene has crystal- 

 lized around the feldspar, as shown by the fact that 

 the pyroxene is in optical continuity on both sides 

 of the obstruction. Orbicular structure may also 

 occur in basic igneous rocks, and consists of an 

 abundance of spheroidal aggregates ranging in size 

 up to that of a walnut or over, and composed of 

 silicates with radial or concentric arrangements. In 

 the well known napoleonite from Corsica they con- 

 sist of concentric shells with a radial structure and 

 composed of hornblende and feldspar alternately. 

 The Finnish granite, known as Rapakiwi, belongs 

 here. (Harris-13.) 



B. Cryptocrystalline, also called aphanitic, with a consid- 



erable proportion of the mineral constituents not 

 visible magascopically. When larger crystals (pheno- 

 crysts) are scattered through the main mass (ground 

 mass) the rock is said to be porphyritic or porphyry. 



" 55) according to some. 



