468 



PORPHYEY — AMYGDALOID. 



[Ch. XXVIII. 



Fig. 622. 



felspars being generally abundant in tbe granites. The volcanic rocks 

 moreover, whether basaltic or trachytic, contain less silica than the gra- 

 nites, in -which last the excess of silica has gone to form quartz. This 

 mineral, so conspicuous in granite, is usually wanting in the volcanic for- 

 mations, and never predominates 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 scattered 

 through an earthy or compact base, the rock is termed a porphyry (see 

 fig. 622). Thus trachyte is porphyritic ; for in it, as in many modern 

 lavas, there are crystals of felspar ; but in some porphyries the crystals 

 are of augite, olivine, or other minerals. If the base be greenstone, 

 basalt, or pitchstone, the rock may be 

 denominated greenstone-porphyry, pitch- 

 stone porphyry, and so forth. The old 

 classical type of this form of rock is the 

 red porphyry of Egypt, or the well-known 

 " Rosso antico." It consists, according to 

 Delesse, of a red felspathic base in which 

 are disseminated rose-colored crystals of 

 the felspar called oligoclase, with some 

 plates of blackish hornblende and grains 

 of oxidized iron-ore (fer oligiste). Red 

 quartziferous porphyry is a much more 

 siliceous rock, containing about 70 or 80 

 per cent, of silex, while that of Egypt has 

 only 62 per cent. 



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

 every variety of composition. It comprehends any rock in which round 

 or almond-shaped nodules of some mineral, such as agate, chalcedony, cal- 

 careous spar, or zeolite, are scattered through a base of wacke, basalt, 

 greenstone, or other kind of trap. It derives its name from the Greek 

 word amygdala, an almond. The origin of this structure cannot be 

 doubted, for we may trace the process of its formation in modern lavas. 

 Small pores or cells are caused by bubbles of steam and gas confined in 

 the melted matter. After or during consolidation, these empty spaces 

 are gradually filled up by matter separating from the mass, or infiltered 

 by water permeating the rock. As these bubbles have been sometimes 

 lengthened by the flow of the lava before it finally cooled, the contents of 

 such cavities have the form of almonds. In some of the amygdaloidal 

 traps of Scotland, where the nodules have decomposed, the empty cells 



Porphyry. 



White crystals of felspar in a dark base 

 of hornblende and felspar. 



