600 



PORPHYRY. 



[Ch. xxvm. 



trachyte, which has 66 per cent, of silica, has a sp. gr. of only 2*68 ; 

 trachytic porphyry, containing 69 per cent, of silica, a sp. gr. of only 

 2 '5 8. If we then take a rock of intermediate composition, such as 

 that prevailing in the Peak of Teneriffe, which Abich calls Trachyte- 

 dolerite, its proportion of silica being intermediate, or 58 per cent., it 

 weighs 2-78, or more than trachyte, and less than basalt.* The basalts 

 are generally dark in color, sometimes almost black, whereas the tra- 

 chytes are gray, and even occasionally white. As compared with the 

 granitic rocks, basalts and trachytes contain both of them more soda 

 in their composition, the potash-felspars being generally abundant in 

 the granites. The volcanic rocks moreover, whether basaltic or tra- 

 chytic, contain less silica than the granites, in which last the excess of 

 silica has gone to form quartz. This mineral, so conspicuous in gran- 

 ite, is usually wanting in the volcanic formations, and never predomi- 

 nates in them. 



The fusibility of the igneous rocks generally exceeds that of other 

 rocks, for the alkaline matter and lime which commonly abound in 

 their composition serve as a flux to the large quantity of silica, which 

 would be otherwise so refractory an ingredient. 



We may now pass to the consideration of those igneous rocks, the 

 characters of which are founded on their form rather than their com- 

 position. 



Porphyry is one of this class, and very characteristic of the volcanic 

 formations. When distinct crystals of one or more minerals are scat- 

 tered through an earthy or compact base, the rock is termed a por- 

 phyry (see fig. 675). Thus trachyte is porphyritic; for in it, as in 



Fig. 675. 



Fig. 670. 



Porphyry. 



White crystals of felspar in a dark 



base of hornblende and felspar. 



Scoriaceons lava in part converted into 



an amygdaloid. 



Montagne de la Veille, Department of 



Puy de Dome, France. 



many modem lavas, there are crystals of felspar; but in some porphy- 

 ries the crystals are of augite, olivine, or other minerals. If the base 



Dr. Daubeny on Volcanoes, 2d ed. pp. 14, 15. 



