116 



BASALTIC ROCKS. 



equal, and ''"morphe^^ form. It is also ascer- 

 tained that some portions of the materials in a crys- 

 tal may not be in perfect chemical combination with 

 the rest, as silex in crystals of carbonate of lime. 



Basalt. — This term is applied to any of the trap 

 rocks which have a black, bluish, or leaden-gray 

 colour, and a uniform, compact texture. Indeed, it 

 may be regarded as an intimate mixture of augite, 

 feldspar, and iron, to which a mineral of an olive- 

 green colour, called olivine^ is often superadded in 

 distinct grains or nodular masses. The iron is usu- 

 ally magnetic, and is often accompanied by another 

 metal called titanium. Augite or hornblende is the 

 predominant mineral, the feldspar being in much 

 the smallest proportions. Other minerals are also 

 found in basalts; and this may pass into any va- 

 riety of trap, especially into greenstone, clinkstone, 

 ?iX\^wacke. Basaltic rocks are often covered with 

 a reddish-brown incrustation, owing to the oxy- 

 genation of the iron from exposure to the atmo- 

 sphere. 



Greenstone, or dolerite, is a granular rock. Or one 

 of a granular structure, the constituent parts of 

 which are hornblende and imperfectly crystallized 

 feldspar ; the feldspar being more abundant than in 

 basalt, and the gr^^ins or crystals of the two miner- 

 als more distinct from each other. When the feld- 

 spar is red, or the rock contains quartz and is of a 

 highly crystalline character, it is then called sye- 

 nitic greenstone. 



Clinkstone. — A trap rock is called clinkstone when 

 the feldspar greatly prevails and the texture be- 

 comes nearly compact, causing it to yield a metal- 

 lic sound when struck. Its prevailing colour is 

 gray, or greenish-gray. When it has a more earthy 

 texture it passes into what is called clay stone. 



Trachyte.-- This is a porphyritic rock of a whitish 

 or greenish colour, composed principally of glassy 

 feldspar, with crystals of the same, generally with 



