90 



LYELL'S ELEMENTS OF GEOLOGY. 



Greenstone Trachyte Porphyry Amygdaloitl. 



after the remarks above made. Oilier minerals are occasionally 

 found in basalt ; and this rock may pass insensibly into almost 

 any variety of trap, especially into greenstone, clinkstone, and 

 wacke, which will be presently described. 



Greenstone^ or Dolerite, is usually defined as a granular rock, 

 the constituent parts of which are hornblende and imperfectly 

 crystallized felspar; the felspar being more abundant than in 

 basalt, and the grains or crystals of the two minerals more dis- 

 tinct from each other. This name may also be applied when 

 augite is substituted for hornblende (the dolerite of some au- 

 thors), or when albite replaces common felspar, forming the 

 rock sometimes called Andesite. 



Sijenitic greenstone. — The highly crystalline compounds of 

 the same two minerals, felspar and hornblende, having a graniti- 

 form texture, and with occasionally some quartz accompanying, 

 may be called Syenitic greenstone, a rock which frequently 

 passes into ordinary trap, and as frequently into granite. 



Trachyte. — A porphyritic rock of a whitish or greyish colour, 

 composed principally of glassy felspar, with crystals of the 

 same, generally with some hornbfende and some titaniferous 

 iron. In composition it is extremely different from basalt, this 

 being a felspathic, as the other is an augitic, rock. It has a pe- 

 culiar rough feel, whence the name T^i^axvi, trachus, rough. Some 

 varieties of trachyte contain crystals of quartz. 



Porphyry is merely a certain 

 form of rock, very characteristic 

 of the volcanic formations. When 

 distinct crystals of one or more 

 minerals are scattered through an 

 earthy or compact base, the rock is 

 termed a porphyry. (See Fig. 87.) 

 Thus trachyte is porphyritic ; for 

 in it, as in many modern lavas, 

 there are crystals of felspar ; but in 

 some porphjy'ies the crystals are ol 

 augite, olivine, or other minerals. 

 If the base be greenstone, basalt, or 

 Porphyry. pitchstonc, the rock may be denomi- 



Wiite crystals of felspar in a dark natcd sreenstone-porphvrv, pitch- 



oase of hornblende and felspar. , ■, 1^.1 



stone-porphyry, and so forth. 

 Amygdaloid. — This is also another form of igneous rock, ad- 

 mitting of every variety of composition. It comprehends any 

 rock in which round or almond-shaped nodules of some mineral, 

 such as agate, calcedony, calcareous spar, or zeolite, are scat- 

 tered through a base of wacke, basalt, greenstone, or other kind 



