Handbook of Paleontology 67 



of gases and water vapor, which are known as mineral- 

 izers. Rock of very coarse texture is formed, especially 

 in fissures along which the gases and vapors escape. 

 They are known as pegmatite dikes or veins from the 

 name given to intergrown masses of quartz, feldspar and 

 mica found where they occur in granite. The more 

 deeply buried the magma, the more slowly will it cool and 

 hence the more coarsely crystalline the resulting rocks. 

 The central part of a magma cools more slowly than the 

 outer part, and a large mass cools more slowly than a 

 small one. In the case of porphyries the large crystals 

 developed while the ground mass, which indicates rapid 

 cooling, was still fluid. 



Magmas and hence the rocks composing them are made 

 up predominantly of eight oxides: silica (Si0 2 ) ; alumina 

 (A1 2 3 ) ; iron oxides (ferric, Fe 2 O s ; ferrous, Fe O) ; 

 magnesia (MgO) ; lime (CaO) ; soda (Na 2 0) and pot- 

 ash (K 2 0). Chief among these oxides is silica which 

 upon solidification of the magma unites with the other 

 oxides to form various silicate minerals. Magmas vary 

 from a composition in which silica is most abundant 

 (sometimes 75 per cent of the mass), with alumina and 

 potash next, to a composition which shows an increase 

 in the oxides of sodium, calcium, magnesium and iron 

 and a decrease in silica (50 per cent or less) together 

 with a reduction in the oxides of potassium and alumi- 

 num. The former are termed the acid magmas and from 

 the acid portion of an igneous magma are formed rocks 

 in which light-weight and light-colored minerals predomi- 

 nate; the latter are termed the basic magmas and from 

 the basic portion of an igneous magma are derived dark- 

 colored and heavy minerals. 



