GEOLOGY. 83 



varieties of granite, viz: graphic granite, syenitic granite, 

 talcose granite, porphyritic granite, eurite and pegmatite: 

 these varieties have various proportions of each element, and 

 also various colors and crystaline arrangement. 



SYENITE. 



Syenite is composed of quartz, feldspar and hornblende : it 

 is sometimes called syenitic granite, it has received this name 

 from the ancient quarries at Syene in Egypt. It has the 

 appearance of granite, but its composition is different, horn- 

 blende being substituted for the mica in granite. Syenite, 

 according to Lyell, frequently loses its quartz and passes insen- 

 sibly into syenitic greenstone. 



PORPHYRY. 



Rocks of a homogeneous, compact structure, containing some 

 other crystaline mineral, of the same age with the base, are 

 called porphyry. The base, or principal mass of the rock, may 

 be greenstone, claystone, basalt, or other rock containing crys- 

 tals of feldspar, augite, olivine, <fec. 



" True classical porphyry, [says Dr. Hitchcock,] such as was 

 most commonly employed by the ancients, has a base of com- 

 pact feldspar, with embedded crystals of feldspar." The term 

 porphyry is indefinite, and does not belong to any particular 

 rock. The term is of Greek origin, and signifies purple, but 

 this rock is of a variety of colors, and is the hardest and most 

 durable of all rocks. 



GREENSTOXE. 



This is a granular rock composed of feldspar and hornblende ; 

 the felsdpar is imperfectly crystalized : greenstone sometimes 

 contains augite and iron also. The hornblende predominates 

 in quantity over the other ingredients. 



TRACHYTE. 



Trachyte is a porphyritic rock of a grayish or whitish color, 



