206 UNSTRATIFIED OR IGNEOUS ROCKS. 



upon the strata, these exist mostly as masses intruded among the 

 strata. Again, if Plutonics are the great reservoirs and volcanics the 

 outpoured liquid, the intrusives are the fillings of the conduits between. 

 Erosion has subsequently carried away the overflowed portions, and 

 exposed the conduits as dikes. 



Kinds. — In the acidic groups, perhaps the most typical is felsite. 

 This rock is a very compact, fine-grained aggregate of quartz and 

 orthoclase, and therefore light-colored. Chemically it has the same 

 composition as granite, and mineralogically it differs only in the fine- 

 ness of texture and in the absence of mica. When the felsitic rock 

 contains, imbedded in the fine-grained mass, large, well-formed crystals 

 of feldspar, then it is called porpJiyrite. If quartz-crystals are also 

 distinctly visible, then it is called quartz-porpliyry, or elvanite, a 

 mottled rock often mistaken for granite. The word po?phyritic is 

 often applied to any rock in which distinct crystals are visible in a 

 finer ground-mass. Thus, we have porphyritic granite, porphyritic 

 diorite, etc. The porphyritic structure is probably formed thus : The 

 fused magma first cooled slowly until the large crystals separated ; and 

 then was injected into the fissure and the solidification completed. 



Intrusive rocks of the basic sub-group are usually called green- 

 stones or traps. This term, therefore, includes intrusive diorites, dia- 

 bases, aphanites, melaphyrs, etc. These differ from the massive rocks 

 of the same composition only by being finer grained : but the same is 

 true also of felsites as compared with granites. The difference is prob- 

 ably wholly due to rate of cooling. The same fused mass which, if 

 cooled slowly, forms granite, if injected into fissures and cooled more 

 rapidly, would form felsite or quartz-porphyrite. The difference be- 

 tween massive and intrusive diorites is doubtless due to the same cause. 



II. — Yolcaxic ok Eruptive Rocks. 



Texture and Appearance. — The rocks of this group are usually 

 micro-crystalline, or even crypto-crystalline, and therefore in appear- 

 ance are either minutely speckled or evenly grayish, of various shades. 

 But the most important characteristic is, that they are not wholly crys- 

 talline, but consist either of crystals imbedded in an amorphous or 

 glassy paste, or else are wholly amorphous or glassy. This texture 

 shows that, as compared with the rocks of the other groups, they have 

 cooled quickly, for, on account of the extreme viscosity of fused sili- 

 cates (glass), complete crystallization can take place only by very slow 

 cooling. 



Physical Conditions. — All the physical conditions already described 

 (p. 91) as characteristic of recent lavas, viz., the stony, the glassy, the 

 scoriaceous, and the Utfaceous conditions, are found abundantly in the 

 more typical representations of this group. 



