278 PRINCIPLES OF STRATIGRAPHY 



I. The Granite iiiaguia, when holocrystalline constitutes granite, 

 when cryptocrystalHne, rhyolite, quartz fclsite (petro- 

 silex) or quart::; porphyry; when glassy, (i) rhyolite ob- 

 sidian, (2) pcrlite, or (3) rhyolite pumice. 

 II. The Syenitic iiiagiua when heconiing- holocrystalHne consti- 

 tutes syenite; when cryptocrystalHne, trachyte felsite, or 

 trachyte porphyry; when glassy, (i) syenite obsidian, (2) 

 pcrlite, or (3) syenite pumice. 



III. The Dioritic magma when becoming holocrystalline consti- 



tutes diorite or quartz diorite (free quartz present) ; when 

 cryptocrystalHne, andesite, andesite felsite, or andesite 

 porphyry and dacite, dacite felsite, or dacite porpJiyry; 

 when glassy, (i) andesite obsidian, or (2) andesite pumice. 



IV. The Gabbroitic magma when holocrystalline constitutes gab- 



bro and olivine gabbro ; when cryptocrystalHne, augite an- 

 desite or augite andesite prophyry, and basalt or basalt 

 porphyry; when glassy, (i) basalt obsidian, or (2) scoria. 

 V. The Ultra basic (non-feldspathic) magmas, when holocrystal- 

 line constitute pyroxenite, and peridotites (with olivine) ; 

 when cryptocrystalHne, augitite and limburgite (with cor- 

 responding porphyrys) ; and, w'hen glassy, basic obsidians 

 and scorias. 



The following may further be noted : 



Dolerite, a field name to be used for a holocrystalline rock, either 

 a diorite or a gabbro, in which the ferromagnesian minerals 

 cannot be determined at sight. (Chamberlin and Salisbury-2 : 



45^-) 



Trap, a field name for a cryptocrystalHne, non-porphyritic, dark 

 rock, of either dioritic or gabbroitic composition. 



Diabase, a holocrystalline rock of gabbroitic composition, but with 

 a diabasic (ophitic) texture, the plagioclase feldspar crystals be- 

 ing lath-shaped and fully crystallized (idiomorphic), while the 

 augite between them is allotriomorphic. It occurs chiefly as a 

 dike rock or in sills. 



Hornblendite, a holocrystalline rock composed of hornblende with- 

 out feldspar. 



Water and Ice as Igneous Rocks. 



Water solidifying into ice by a lowering of temperature must be 

 referred to the igneous rocks. Its texture may be holocrystalline, 

 but is more commonly cryptocrystalHne, or glassy. It must be 



