116 



BASALTIC ROCKS. 



" isos," 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 witl 

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



Basalt. — This term is applied to any of the tra% 

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

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

 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 ir 

 distinct grains or nodular masses. The iron is usu- 

 ally magnetic, and is often accompanied byanothe] 

 metal called titanium. Augite or hornblende is tht 

 predominant mineral, the feldspar bein£ in mud 

 the smallest proportions. Other minerals are alsc 

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

 riety of trap, especially into greenstone, clinkstone 

 and wacke. Basaltic rocks are often covered witl 

 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 I 

 basalt, and the grains 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 claystone. 



Trachyte.— This is a porphyritic rock of a whitish 

 or greenish colour, composed principally of glassy 

 feldspar, with crystals of the same, generally with 



