116 
BASALTIC ROCKS. 
" equal, and " morphe,''^ form. It is also ascer- 
tained tbat 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, 
and 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 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 si/e- 
niiic greenstone. 
Clinkstone. — A trap rock is called clinkstone when 
the feldspar greatly prevails and the texture he- 
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 
