IGNEC 



The Igneous rocks, including under this name the plutonic or older igneous, and 

 volcanic or more modern rocks, are extensively distributed over the earth's surface. T 

 form the solid frame- work of the globe, and appear to have once existed in a state of igne 

 fusion, from which they have cooled down. They contain no trace of organic life; 

 many valuable minerals and metallic ores, as tin, lead, silver, copper, &c., are fount 

 them. The igneous rocks are either amorphous or at times exhibit a jointed struct! 

 being separable into cubical or prismatic masses. They are generally crystalline, 

 consist mostly of a mixture of more or less crystalline minerals. 



Granite consists of quartz, felspar, and sometimes mica or hornblende; when ho 

 blende is substituted for mica, the rock is termed Syenite. Granite is of various coloi 

 mostly dependent on the character of the contained felspar or mica. It sometimes prese 

 a cuboidal or rude columnar structure. Granite, although generally considered a primi 

 rock, and the foundation upon which the stratified rocks are placed, appears to be of 

 geological ages, both secondary and tertiary; the latter strata in the Andes having to 

 observed by Mr. Darwin to be traversed by this rock. Protogine is a variety of gran 

 containing the mineral talc; and Pegmatite is another form of granite, in which the fels] 

 and quartz are regularly arranged, whence it is sometimes called graphic granite. 



Serpentine or Ophiolite, principally consists of silicate of magnesia, combined with lii 

 water, and oxide of iron. Diallage is nearly allied to Serpentine. 



Greenstone (Diorite, &c.) consists of hornblende and compact felspar. Hypersthene 

 Augite Greenstone contains an admixture of either hypersthene or augite. 



In Porphyry, crystals of quartz or felspar are imbedded in a granitic or homogenous ba 



Amygdaloid (Almond-stone) contains almond shaped cavities, either empty, encrust 

 half, or completely filled. [Trachy 



Trachyte is a felspathic rock. Pitchstone, pumice, and greystone are varieties 



In Basalt augite predominates over felspar. Where basalt, and indeed also otl 

 igneous rocks exhibit a resemblance to a series of steps, they have received the name 

 trap rock. Under the general name of trap rock are included all basalts and other rocks 

 volcanic origin. Fingal's Cave, presents a beautiful example of columnar basalt. 



Lava is the result of modern volcanic action; it is very variable in its composition a 

 structure, being allied to the basalt and trachytes. 



GRAPHIC GRANITE. 



FINGAL'S CAVE, Isle of Staffa. 



LONDON: PUBLISHED 



